Directional display apparatus

Abstract

To provide a privacy display that brings about image visibility for main users located typically at an on-axis position and reduces visibility of image content for snoopers located typically at an off-axis position.SOLUTION: A switchable privacy display comprises a spatial light modulator, and switchable liquid crystal retarder arranged between crossed quarter-wave plates and polarisers. In a privacy mode of operation, on-axis light from the spatial light modulator is directed without loss, whereas off-axis light has reduced luminance to reduce visibility of the display to off-axis snoopers. In a public mode of operation, liquid crystal retardance is adjusted so that off-axis luminance and reflectivity are unmodified. The display may also be operated to switch between day-time and night-time operation, for example for use in an automotive environment.SELECTED DRAWING: Figure 1A

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE This disclosure relates generally to illumination from light modulating devices, and more specifically to optical stacks for controlling illumination for use in displays, including privacy displays and nighttime displays. [Background technology]

[0002] Privacy displays provide image visibility to the primary user, typically in an on-axis position, and reduce the visibility of image content to prying eyes, typically in an off-axis position. Privacy functionality can be provided by microlouver optical films that transmit some light from the display in the on-axis direction and have low brightness in off-axis positions. However, such films are lossy for frontal illumination, and the microlouvers can cause moiré due to pixel beating of the spatial light modulator. The pitch of the microlouvers must be selected according to the panel resolution choice, which can increase inventory and costs.

[0003] A switchable privacy display can be provided by control of the off-axis light output.

[0004] The control may be provided by, for example, brightness reduction by a switched backlight of a liquid crystal display (LCD) spatial light modulator. Display backlights generally employ a waveguide and edge-emitting light source. Certain imaging directional backlights have the additional function of directing illumination through a display panel to a viewing window. An imaging system may be formed between a number of light sources and the respective window images. One example of an imaging directional backlight is a light valve that may employ a folded optical system, which may therefore also be an example of a folded imaging directional backlight. Light may propagate substantially losslessly in one direction through the light valve, while counter-propagating light may be extracted by a reflective inclined facet, as described in U.S. Pat. No. 9,519,153, the entirety of which is incorporated herein by reference. Summary of the Invention

[0005] According to a first aspect of the present disclosure, a method for manufacturing a display device includes a spatial light modulator arranged to output light along an output direction, a display polarizer arranged on the side of the spatial light modulator, an additional polarizer arranged on the same side of the spatial light modulator as the display polarizer, the additional polarizer being arranged to pass respective linearly polarized polarization states, and first and second quarter wave plates arranged between the additional polarizer and the display polarizer, the first quarter wave plate being arranged on the input side of the second quarter wave plate and arranged to pass respective linearly polarized polarization states. A display device is provided, comprising first and second quarter-wave plates, the first and second quarter-wave plates being arranged to convert a linearly polarized polarization state passing through one of the display polarizer and the additional polarizer on the input side of the first quarter-wave plate to a circularly polarized polarization state, and the second quarter-wave plate on the output side being arranged to convert a circularly polarized polarization state incident thereon to a linearly polarized polarization state passing through the other of the display polarizer and the additional polarizer on the output side of the second quarter-wave plate, and at least one retarder arranged between the pair of quarter-wave plates.

[0006] Advantageously, a privacy or low stray light display can be provided that can operate in landscape and portrait modes. Advantageously, the modification of the output light can be independent of azimuth angle, and therefore can provide symmetry with a degree of circular symmetry. Additionally, displays for automotive applications can be operated to reduce reflections from windshields and side windows during nighttime operation.

[0007] The pair of quarter wave plates may each comprise a passive quarter wave plate including a layer of oriented uniaxially birefringent material. Advantageously, the retarder is low cost and can be conveniently manufactured in large quantities and large areas.

[0008] The pair of quarter wave plates may have crossed optical axes. Each of the pair of quarter wave plates may have an optical axis arranged at 45 degrees to the electric vector transmission direction of the adjacent display polarizer or additional polarizer. Each of the pair of quarter wave plates may have a retardance in the range of 110 nm to 175 nm, preferably 130 nm to 140 nm, for light of wavelength 550 nm. Advantageously, light may be transmitted with high efficiency in the on-axis direction. Advantageously, the color variation of the brightness roll-off may be reduced.

[0009] The retarder may comprise a liquid crystal retarder comprising a layer of liquid crystal material. Advantageously, the retardance of the retarder can be increased to achieve improved polar variation of the transmission and reflectance.

[0010] The liquid crystal retarder may be a switched liquid crystal retarder. The liquid crystal retarder may further include electrodes arranged to apply a voltage to switch the liquid crystal material layer. Advantageously, the display may be switched between a narrow angle mode for privacy and other low stray light applications and a wide angle mode for public viewing by multiple users, or a public mode. Further display brightness uniformity may be improved in the public operating mode.

[0011] The liquid crystal retarder may include two surface alignment layers disposed adjacent to and on either side of a layer of liquid crystal material. The two surface alignment layers may each be arranged to provide homogeneous alignment within the adjacent liquid crystal material. The liquid crystal material may have positive dielectric anisotropy. Advantageously, the display may be resistant to the visibility of flow of material under compression.

[0012] Each of the two surface alignment layers may have an alignment direction arranged at 45 degrees to the electric vector propagation direction of at least one of the display polarizer and the additional polarizer, which may advantageously improve the chromaticity of the display.

[0013] The layer of liquid crystal material may have a twist. Advantageously, the display may be switchable between a wide angle mode and a narrow angle mode with a degree of circular symmetry in transmission and reflectance.

[0014] The twist is (90+m * 180) degrees, where m is zero or a positive integer. Advantageously, the variation in transmission with polar angle through the retarder can be tailored.

[0015] The liquid crystal material layer may have a 90 degree twist and a retardance in the range of 420nm to 550nm, most preferably 460nm to 480nm, for light of a wavelength of 550nm. The liquid crystal material layer may have a 270 degree twist and a retardance in the range of 650nm to 800nm, most preferably 700nm to 720nm, for light of a wavelength of 550nm. The liquid crystal material layer may have a 450 degree twist and a retardance in the range of 820nm to 1000nm, most preferably 880nm to 920nm, for light of a wavelength of 550nm. The liquid crystal material layer may have a 360 degree twist and a retardance in the range of 1100nm to 1400nm, most preferably 1150nm to 1300nm, for light of a wavelength of 550nm. The liquid crystal material layer may have a twist of 225 degrees and a retardance in the range of 750nm to 1200nm, most preferably in the range of 900nm to 1100nm, for light of a wavelength of 550nm. Advantageously, the degree of symmetry of the narrow angle mode can be adjusted by selection of the twist and retardance of the liquid crystal material layer.

[0016] The first and second quarter wave plates and the at least one retarder may be arranged to introduce a phase shift in the polarization components of light passing through one of the display polarizer and the additional polarizer at an input side of the at least one retarder, the at least one retarder reducing the intensity of light passing through the other one of the display polarizer and the additional polarizer along an axis tilted relative to the optical axis, and the liquid crystal retarder may be arranged to introduce a phase shift in the polarization components of light passing through one of the display polarizer and the additional polarizer at an input side of the at least one retarder, the at least one retarder reducing the intensity of light passing through the other one of the display polarizer and the additional polarizer along an axis tilted relative to the optical axis, the liquid crystal retarder being arranged to introduce a phase shift in the polarization components of light passing through the other one of the display polarizer and the additional polarizer at an input side of the at least one retarder, the liquid crystal retarder being arranged to introduce a phase shift in the polarization components of light passing through the other one of the display polarizer and the additional polarizer along an axis tilted relative to the optical axis, the liquid crystal retarder being arranged to introduce a phase shift in the polarization components of light passing through the other one of the display polarizer and the additional polarizer |R|=-0.02947 * θ^3+5.81385 * θ^2-397.950 * θ+10090 and having a total retardance that is within 10% of the value R in nanometers given by where θ is the outer angle of the tilt axis at which the maximum reduction in brightness occurs relative to a tilt axis with an elevation angle of 0 degrees. The liquid crystal retarder may have a retardance for light of a wavelength of 550 nm that is within 4% of the value R. Advantageously, in the privacy mode of operation, the polar position of minimum transmission can be selected.

[0017] The at least one retarder may include at least one passive retarder. Advantageously, the cost, thickness, and complexity of a privacy display or other low stray light display may be reduced.

[0018] The at least one passive retarder may include a passive retarder having an optic axis perpendicular to a plane of the passive retarder Advantageously, the thickness of the passive retarder stack may be reduced.

[0019] The at least one passive retarder may include multiple passive retarders having optic axes parallel to the plane of the intersecting passive retarders. Advantageously, multiple passive retarders can be provided at low cost.

[0020] The at least one passive retarder may comprise a layer of liquid crystal material having a retardance in the range of -400nm to -1000nm, or +500nm to +1200nm, preferably in the range of -750nm to -850nm, or preferably in the range of +950nm to +1050nm, for light of a wavelength of 550nm. Advantageously, the thickness can be reduced and a desirable field of view for the narrow angle mode can be provided.

[0021] The at least one passive retarder may comprise a pair of passive retarders with optic axes parallel to the plane of the crossed passive retarders. The pair of passive retarders may have slow axes extending at 45° and 135°, respectively, with respect to an electric vector transfer direction parallel to the electric vector transfer of the display polarizer. The display device may further comprise an additional pair of passive retarders disposed between the first mentioned pair of passive retarders. The additional pair of passive retarders may have slow axes in the plane of the crossed passive retarders. The additional pair of passive retarders may have slow axes each extending at 0° and 90°, respectively, with respect to an electric vector transfer direction parallel to the electric vector transfer of the display polarizer. Advantageously, the passive retarders can be provided at low cost. Color variations in transmission and reflection may be reduced. Symmetry of transmission and reflection with a degree of circular symmetry may be provided in a thin structure.

[0022] The first and second quarter wave plates and the at least one retarder may be arranged to introduce a phase shift in the polarization components of light passing through one of the display polarizer and the additional polarizer at an input side of the at least one retarder, the at least one retarder reducing the intensity of light passing through the other one of the display polarizer and the additional polarizer along an axis tilted relative to the optical axis, and the at least one passive retarder providing, for light of a wavelength of 550 nm, a phase shift according to the formula: |R|=-0.02947 * θ^3+5.81385 * θ^2-397.950 * θ+10090 and having a total retardance that is within 10% of the value R in nanometers given by where θ is the outer angle of the tilt axis at which the greatest reduction in brightness occurs relative to a tilt axis with an elevation angle of 0 degrees. The at least one passive retarder may have a retardance for light of a wavelength of 550 nm that is within 4% difference from the value R. Advantageously, the polar position of minimum transmission may be selected.

[0023] The display polarizer and the additional polarizer may have electric vector transmission directions that are parallel. Advantageously, high efficiency can be achieved in the public mode and for output in the normal direction.

[0024] The at least one retarder may include a retarder including an oriented uniaxial birefringent material, which can advantageously provide low cost.

[0025] The at least one retarder may be arranged to introduce a phase shift along an axis tilted relative to the optical axis of the retarder to the polarization components of the light passing through one of the display polarizer and the additional polarizer and the quarter wave plate at the input side of the at least one retarder. Advantageously, the transmission and reflectance can vary with polar angle.

[0026] The at least one retarder may comprise at least one switchable liquid crystal retarder. The at least one retarder may be arranged to introduce a phase shift, in a switchable mode of the switchable liquid crystal retarder, along an axis tilted relative to the optical axis of the retarder to the polarization components of light passing through one of the display polarizer and the additional polarizer, and a quarter wave plate at the input side of the at least one retarder. Advantageously, the display may be switched into a mode in which the transmittance and reflectance may vary with polar angle.

[0027] The at least one retarder may be arranged such that it does not introduce a phase shift along an axis along the optical axis of the retarder to the polarization components of the light that has passed through one of the display polarizer and the additional polarizer, and the quarter wave plate on the input side of the at least one retarder. Advantageously, the front transmission may have a high efficiency.

[0028] The display polarizer may be an output polarizer located at the output side of the spatial light modulator. The additional polarizer may be located at the output side of the display polarizer. One of the display polarizer and the additional polarizer at their input side may be the display polarizer. Advantageously, thickness and efficiency can be optimized.

[0029] The reflective polarizer may be disposed between the display polarizer and the first quarter wave plate. The display polarizer and the reflective polarizer may be disposed to pass the same linearly polarized polarization state. Advantageously, the display may have enhanced reflectance for off-axis polar positions. Advantageously, the off-axis visual security level is enhanced in ambient lighting.

[0030] A further additional polarizer may be arranged at the output side of the first mentioned additional polarizer, the further additional polarizer being arranged to pass a linearly polarized polarization state, and at least one further retarder may be arranged between the first mentioned additional polarizer and the further additional polarizer. Advantageously, the off-axis brightness may be further reduced in a privacy mode of operation to increase the visual security level.

[0031] The further additional polarizer may be a reflective polarizer and the at least one further retarder may comprise a switchable liquid crystal retarder comprising a layer of liquid crystal material and electrodes arranged to apply a voltage to switch the layer of liquid crystal material. Advantageously, a high off-axis reflectance may be provided in a privacy mode of operation to increase the level of visual security and a low off-axis reflectance may be provided in a public mode of operation to increase image visibility.

[0032] The at least one retarder disposed between the pair of quarter wave plates may comprise at least one passive retarder having a retardance in the range of -100 nm to -400 nm, or +200 nm to +600 nm, preferably in the range of -200 nm to -300 nm, or preferably in the range of +300 nm to +500 nm, for light of wavelength 550 nm. Such retarders may provide the spatial light modulator with high brightness at off-axis viewing angles that may otherwise achieve undesirable visual security levels. Advantageously, off-axis brightness is reduced with low visibility of color changes. An increased visual security level in privacy mode may provide acceptable off-axis image visibility in public mode. The emissive display may have a high visual security level.

[0033] The display polarizer may be an input polarizer arranged at the input side of the spatial light modulator. The additional polarizer may be arranged at the input side of the display polarizer. One of the display polarizer and the additional polarizer at their input side may be an additional polarizer. Advantageously, the contrast of the display may be increased compared to an arrangement with an additional front component. Furthermore, the visibility of material flow in at least one liquid crystal layer may be reduced.

[0034] The output polarizer may be arranged at the output side of the spatial light modulator, and the further polarizer may be arranged at the output side of the output polarizer, the output polarizer and the further polarizer arranged to pass respective linearly polarized polarization states, and the at least one further retarder may be arranged between the output polarizer and the further polarizer. The display device may further comprise a reflective polarizer arranged between the output polarizer and the at least one further retarder, the output polarizer and the reflective polarizer arranged to pass the same linearly polarized polarization state, and the at least one further retarder may comprise a switchable liquid crystal retarder comprising a layer of liquid crystal material and electrodes arranged to apply a voltage to switch the layer of liquid crystal material. Advantageously, the number of front components can be reduced.

[0035] The at least one retarder disposed between the pair of quarter wave plates may comprise at least one passive retarder having a retardance in the range of -100 nm to -400 nm, or +200 nm to +600 nm, preferably in the range of -200 nm to -300 nm, or preferably in the range of +300 nm to +500 nm, for light of a wavelength of 550 nm. Advantageously, off-axis brightness is reduced with a low visibility of color shift.

[0036] The additional polarizer when crossed with a second conceptual polarizer of the same material may have a transmittance at wavelengths between 520 nm and 560 nm that is less than the transmittance at wavelengths between 450 nm and 490 nm. The transmittance at wavelengths between 450 nm and 490 nm may be greater than 1%, preferably greater than 2%, and most preferably greater than 3%. The transmittance at wavelengths between 520 nm and 560 nm may be less than 3%, preferably less than 2%, and most preferably less than 1%. Advantageously, off-axis color variations may be reduced compared to broadband polarizers. The transmission efficiency may be increased, the reflectance in privacy mode may be increased, and a higher level of visual security may be provided.

[0037] The first embodiment may exclude cases where the pair of quarter-wave plates have intersecting slow axes and the retarder further includes an additional pair of passive retarders disposed between the quarter-wave plates and having slow axes in the plane of the intersecting passive retarders.

[0038] The first embodiment may exclude the case where one of the pair of quarter-wave plates has a slow axis extending at least 40° and at most 50° relative to an electric vector transfer direction that is parallel to the electric vector transfer of the display polarizer, and the other of the pair of quarter-wave plates has a slow axis extending at least 130° and at most 140° relative to an electric vector transfer direction that is parallel to the electric vector transfer of the display polarizer, and the retarder further comprises an additional pair of passive retarders disposed between the quarter-wave plates, one of the additional pair of passive retarders having a slow axis in the plane of the passive retarder extending at least -10° and at most 10° relative to the electric vector transfer direction that is parallel to the electric vector transfer of the display polarizer, and the other of the additional pair of passive retarders having a slow axis in the plane of the passive retarder extending at least -10° and at most 10° relative to the electric vector transfer direction that is parallel to the electric vector transfer of the display polarizer.

[0039] According to a second aspect of the present disclosure, there is provided a viewing angle control optical element for application to an output side of a display device for use in ambient lighting comprising a spatial light modulator arranged to output light, the spatial light modulator comprising an output polarizer, and the viewing angle control optical element comprising an additional polarizer, first and second quarter wave plates, and at least one retarder arranged between the first and second quarter wave plates.

[0040] Advantageously, the viewing angle control optics can be attached to an existing display to provide a retrofit privacy control element to form a display device capable of operating in landscape and portrait modes. Advantageously, the modification of the output light can be independent of azimuth angle and therefore can provide symmetry with a degree of circular symmetry.

[0041] The viewing angle control element may further include a reflective polarizer. The first and second quarter wave plates and the retarder may be disposed between the reflective polarizer and the additional polarizer. Advantageously, the modification of the display reflectance in ambient lighting can enhance the reflectance symmetrically with some degree of circular symmetry.

[0042] Any of the aspects of the present disclosure may be applied in any combination.

[0043] The embodiments of the present disclosure may be used in a wide variety of optical systems. Some embodiments may include or work with a variety of projectors, projection systems, optical components, displays, microdisplays, computer systems, processors, self-contained projector systems, visual and / or audiovisual systems, and electrical and / or optical devices. Aspects of the present disclosure may be used in virtually any device related to optical and electrical devices, optical systems, presentation systems, or any device that may include any type of optical system. Thus, embodiments of the present disclosure may be employed in optical systems, optical devices used in visual and / or optical presentations, visual peripherals, and the like, as well as in many computer environments.

[0044] Before proceeding to the disclosed embodiments in detail, it should be understood that the disclosure is not limited to the details of the particular arrangements shown in the application or construction, as other example embodiments are possible. Moreover, aspects of the disclosure may be described in various combinations and arrangements to define unique embodiments. Additionally, the terminology used herein is for purposes of description and not of limitation.

[0045] These and other advantages and features of the present disclosure will become apparent to those of ordinary skill in the art from reading this disclosure in its entirety. [Brief description of the drawings]

[0046] By way of example, embodiments are illustrated in the accompanying drawings in which like reference symbols refer to similar parts and in which:

[0047] [Figure 1A] FIG. 1 is a schematic diagram showing a side perspective view of a switchable privacy display for use in ambient lighting, including a transmissive spatial light modulator, a reflective polarizer, a switchable retarder positioned between a quarter-wave plate, and an additional polarizer positioned on the output side of the spatial light modulator.

[0048] [Figure 1B] FIG. 1B is a schematic diagram illustrating the orientation of optical layers within the optical stack of FIG. 1A in a front view.

[0049] [Figure 1C] FIG. 13 is a schematic diagram illustrating the orientation of the optical layers at the output of a twisted nematic spatial light modulator in a front view to achieve a vertical output polarization state.

[0050] [Diagram 2] FIG. 1 is a schematic diagram showing a perspective side view of an arrangement of multiple retarder layers including a passive positive C-plate compensation retarder disposed between parallel polarizers and a 270 degree super twist switchable liquid crystal retarder disposed between quarter wave plates.

[0051] [Figure 3A] 1B is a schematic diagram illustrating, in a side view, the propagation of output light from the spatial light modulator through the optical stack of FIG. 1A in a public mode of operation.

[0052] [Figure 3B] FIG. 1B is a schematic diagram illustrating, in a side view, the propagation of light rays from an ambient light source through the optical stack of FIG. 1A in a public mode of operation.

[0053] [Figure 3C] 1B is a schematic diagram illustrating, in a side view, the propagation of output light from the spatial light modulator through the optical stack of FIG. 1A in a privacy mode of operation.

[0054] [Figure 3D] FIG. 1B is a schematic diagram illustrating, in a side view, the propagation of light rays from an ambient light source through the optical stack of FIG. 1A in a privacy mode of operation.

[0055] [Figure 4A] 3B is a schematic graph showing the variation in transmittance with respect to the polarity direction of the transmitted light beam in FIG. 3A.

[0056] [Figure 4B] 3C is a schematic graph showing the variation in reflectance with respect to the polarity direction of the transmitted light beam of FIG. 3B.

[0057] [Figure 4C] 3D is a schematic graph showing the variation of transmittance versus polarity direction of the transmitted light beam of FIG. 3C.

[0058] [Figure 4D] 3E is a schematic graph showing the variation in reflectance with respect to the polarity direction of the transmitted light beam in FIG. 3D.

[0059] [Figure 5A] FIG. 1B is a schematic diagram showing an external view of the display of FIG. 1A operating in a public mode in a front perspective view.

[0060] [Figure 5B]FIG. 1B is a schematic diagram showing an external view of the display of FIG. 1A operating in privacy mode, in a front perspective view.

[0061] [Figure 6A] 1B is a schematic diagram showing the luminance appearance of a mobile device in public mode including the display of FIG. 1A in perspective view, the appearances being, from the top left clockwise, front-on landscape, front-on portrait, downward portrait, and right-side landscape.

[0062] [Figure 6B] 1B is a schematic diagram showing, in perspective view, the brightness appearance of a mobile device in privacy mode including the display of FIG. 1A, which shows, clockwise from the top left, front-on landscape, front-on portrait, downward portrait, and right-side landscape.

[0063] [Figure 6C] 1B is a schematic diagram showing perspective views of the reflectivity of a mobile device in privacy mode including the display of FIG. 1A, with the views shown clockwise from the top left: front-on landscape, front-on portrait, downward portrait, and right-side landscape.

[0064] [Figure 7A] FIG. 1 is a schematic diagram illustrating a top view of a motor vehicle having a switched directional display disposed within a passenger compartment in a nighttime mode of operation.

[0065] [Figure 7B] FIG. 1 is a schematic diagram illustrating a side view of a motor vehicle having a switched directional display disposed within a passenger compartment in a nighttime mode of operation.

[0066] [Figure 8A] FIG. 1 is a schematic diagram showing a side perspective view of a switchable privacy display for use in ambient lighting, including an emissive spatial light modulator, a compensating switched liquid crystal retarder between a quarter-wave plate, a reflective polarizer, and an additional polarizer.

[0067] [Figure 8B]FIG. 1 is a schematic diagram showing in a side perspective view a viewing angle control element including a reflective polarizer, a switchable liquid crystal retarder between a quarter wave plate, and an additional polarizer.

[0068] [Figure 9] FIG. 1 is a schematic diagram illustrating a retarder layer arrangement in perspective side view, comprising a 270 degree super-twist switchable liquid crystal retarder disposed between parallel polarizers and disposed between quarter wave plates.

[0069] [Figure 10A] 10 is a schematic graph showing the variation of transmittance with respect to the polar direction of the transmitted light in the public mode in the arrangement of FIG. 9 with a 270 degree twist.

[0070] [Figure 10B] 10 is a schematic graph showing the variation of reflectivity with respect to the polar direction of the reflected light beam in the public mode in the arrangement of FIG. 9 with a 270 degree twist.

[0071] [Figure 10C] 10 is a schematic graph showing the variation of transmittance versus polar direction of transmitted light in privacy mode for the arrangement of FIG. 9 with a 270 degree twist.

[0072] [Figure 10D] 10 is a schematic graph showing the variation of reflectance versus polar direction of reflected light in privacy mode for the arrangement of FIG. 9 with a 270 degree twist.

[0073] [Figure 11A] 10 is a schematic graph showing the variation of transmittance with respect to the polar direction of the transmitted light in the public mode in the arrangement of FIG. 9 with a 450 degree twist.

[0074] [Figure 11B] 10 is a schematic graph showing the variation of reflectivity with respect to the polar direction of the reflected light beam in the public mode in the arrangement of FIG. 9 with a 450 degree twist.

[0075] [Figure 11C] 10 is a schematic graph showing the variation of transmittance versus polar direction of transmitted light in privacy mode for the arrangement of FIG. 9 with a 450 degree twist.

[0076] [Figure 11D] 10 is a schematic graph showing the variation of reflectance versus polar direction of reflected light in privacy mode for the arrangement of FIG. 9 with a 450 degree twist.

[0077] [Figure 12A] FIG. 1 is a schematic diagram illustrating a perspective side view of a switchable liquid crystal retarder arrangement including a twisted nematic liquid crystal retarder with a 90 degree twist disposed between quarter wave plates.

[0078] [Figure 12B] 12B is a schematic graph showing the variation in transmittance with respect to the polarity direction of a transmitted light beam in the public mode in the arrangement of FIG. 12A.

[0079] [Figure 12C] 12B is a schematic graph showing the variation in reflectance with respect to the polarity direction of a reflected light ray in the public mode in the arrangement of FIG. 12A.

[0080] [Figure 12D] 12B is a schematic graph showing the variation in transmittance with respect to the polar direction of the transmitted light in the privacy mode in the arrangement of FIG. 12A.

[0081] [Figure 12E] 12B is a schematic graph showing the variation in reflectance with respect to the polar direction of the reflected light in privacy mode for the arrangement of FIG. 12A.

[0082] [Figure 13A] FIG. 1 is a schematic diagram illustrating a perspective side view of a switchable liquid crystal retarder arrangement including a twisted nematic liquid crystal retarder with a 90 degree twist and a positive C-plate compensation retarder disposed between quarter wave plates.

[0083] [Figure 13B] 13B is a schematic graph showing the variation in transmittance with respect to the polarity direction of a transmitted light beam in the public mode in the arrangement of FIG. 13A.

[0084] [Figure 13C] 13B is a schematic graph showing the variation in reflectance with respect to the polarity direction of a reflected light ray in the public mode in the arrangement of FIG. 13A.

[0085] [Figure 13D] 13B is a schematic graph showing the variation in transmittance with respect to the polarity direction of the transmitted light in the privacy mode in the arrangement of FIG. 13A.

[0086] [Figure 13E] 13B is a schematic graph showing the variation in reflectance with respect to the polar direction of the reflected light in privacy mode for the arrangement of FIG. 13A.

[0087] [Figure 14A] FIG. 1 is a schematic diagram illustrating a perspective side view of a switchable liquid crystal retarder arrangement including a twisted nematic liquid crystal retarder with a 360 degree twist disposed between quarter wave plates.

[0088] [Figure 14B] 14B is a schematic graph showing the variation in transmittance with respect to the polarity direction of a transmitted light beam in the public mode in the arrangement of FIG. 14A.

[0089] [Figure 14C] 14B is a schematic graph showing the variation in transmittance with respect to the polar direction of the transmitted light in the privacy mode in the arrangement of FIG. 14A.

[0090] [Figure 15A] FIG. 1 is a schematic diagram illustrating a perspective side view of a switchable liquid crystal retarder arrangement including a twisted nematic liquid crystal retarder with a 225 degree twist disposed between quarter wave plates.

[0091] [Figure 15B] 15B is a schematic graph showing the variation in transmittance with respect to the polarity direction of a transmitted light beam in the public mode in the arrangement of FIG. 15A.

[0092] [Figure 15C] 15B is a schematic graph showing the variation in transmittance with respect to the polarity direction of the transmitted light in the privacy mode in the arrangement of FIG. 15A.

[0093] [Figure 16] 1 is a schematic graph showing the variation of retarder retardance at null polar angle for a retarder layer disposed between quarter wave plates;

[0094] [Figure 17A] FIG. 1 is a schematic diagram illustrating in perspective side view an arrangement of a negative C-plate retarder disposed between quarter wave plates and a reflective polarizer disposed between parallel polarizers.

[0095] [Figure 17B] 17B is a schematic graph showing the variation in transmittance with respect to the polar direction of a transmitted light beam in the arrangement of FIG. 17A.

[0096] [Figure 17C] 17B is a schematic graph showing the variation in reflectance with respect to the polar direction of a reflected light beam in the arrangement of FIG. 17A.

[0097] [Figure 18A] FIG. 1 is a schematic diagram illustrating in perspective side view an arrangement of a negative C-plate retarder disposed between a quarter-wave plate that is disposed between a display polarizer and a reflective polarizer.

[0098] [Figure 18B] 18B. FIG. 18C is a schematic graph showing the variation of transmittance versus polarity direction for the arrangement of FIG. 18A, which has a different retarder value than the arrangement of FIG. 18B.

[0099] [Figure 18C] FIG. 1 is a schematic diagram illustrating in perspective side view an arrangement of a positive C-plate retarder disposed between quarter wave plates and a reflective polarizer disposed between parallel polarizers.

[0100] [Figure 18D] 18D is a schematic graph showing the variation in transmittance with respect to the polarity direction of a transmitted light beam in the arrangement of FIG. 18C.

[0101] [Figure 18E] 18D is a schematic graph showing the variation in reflectance with respect to the polar direction of a reflected light beam in the arrangement of FIG. 18C.

[0102] [Figure 19A] FIG. 1 is a schematic diagram illustrating in perspective side view an arrangement of a negative C-plate retarder disposed between a quarter-wave plate that is disposed between a display polarizer and a reflective polarizer.

[0103] [Figure 19B] 19B is a schematic graph showing the variation of transmittance versus polarity direction for the arrangement of FIG. 19A, which has a different retarder value than the arrangement of FIG. 18D;

[0104] [Figure 20A] FIG. 1 is a schematic diagram showing a side perspective view of a switchable privacy display including a backlight, an additional polarizer, a first liquid crystal retarder positioned between a first pair of quarter wave plates, a spatial light modulator 48, and a second liquid crystal retarder positioned between a second pair of quarter wave plates and a further additional polarizer.

[0105] [Figure 20B] A schematic diagram showing a side perspective view of a switchable privacy display including a backlight, a further plurality of retarders arranged between a reflective recirculating polarizer and a transmissive spatial light modulator, a reflective polarizer, a plurality of retarders, and an additional polarizer.

[0106] [Figure 21] FIG. 3 is a schematic diagram showing a side perspective view of a viewing angle control element including a passive compensation retarder between quarter wave plates, a reflective polarizer, and a switchable liquid crystal retarder 301 between the quarter wave plates arranged between the reflective polarizer and an additional polarizer.

[0107] [Figure 22] FIG. 3 is a schematic diagram showing a side perspective view of a viewing angle control element including a passive compensation retarder, a switchable liquid crystal retarder, and a further additional polarizer and a switchable liquid crystal retarder 301 arranged between the further additional polarizer and a quarter wave plate arranged between the further additional polarizer.

[0108] [Figure 23A] FIG. 13 is a schematic diagram illustrating an exemplary arrangement of additional retarders in a side perspective view, including a passively controlled retarder, a passive compensation retarder, and a switchable liquid crystal retarder.

[0109] [Figure 23B] 23B is a schematic graph showing the variation of transmittance versus polarity direction of transmitted light in the arrangement of FIG. 23A in privacy mode.

[0110] [Figure 23C] 23B is a schematic graph showing the variation in transmittance with respect to the polarity direction of a transmitted light beam in the arrangement of FIG. 23A in the public mode.

[0111] [Figure 23D] FIG. 23B is a schematic graph showing the variation in reflectivity versus polarity direction of the reflected light beam in privacy mode for an arrangement similar to that of FIG. 23A, where the output polarizer is provided by an additional polarizer and the input polarizer is provided by a reflective polarizer.

[0112] [Figure 24A]FIG. 17D is a schematic diagram showing a side view of a switchable privacy display for use in ambient lighting, comprising a transmissive spatial light modulator including a recirculating backlight, a switchable retarder positioned between the reflective polarizer and an additional polarizer at the output side of the spatial light modulator, and a passive retarder between the quarter wave plate arrangement of FIG. 17C positioned between the reflective polarizer and the output polarizer of the spatial light modulator.

[0113] [Figure 24B] 24B is a schematic graph showing the variation of output luminance with outer viewing angle for various components of the stack of FIG. 24A.

[0114] [Figure 24C] 24B is a schematic graph showing the variation of output luminance with outer viewing angle for the arrangement of FIG. 24A.

[0115] [Figure 24D] FIG. 24B is a schematic graph showing the variation of output luminance with outer viewing angle for an arrangement similar to FIG. 24A, omitting the passive retarder located between the quarter wave plates.

[0116] [Figure 25A] 1 is a schematic graph showing the variation of output luminance with wavelength for a broadband absorbing polarizer and a leaky absorbing polarizer.

[0117] [Figure 25B] 23A is a schematic graph showing the variation of output color versus polarization direction for transmitted light for the broadband absorptive polarizer of FIG. 25A and the arrangement of FIG. 23A in the public mode of multiple retarders 300 of FIGS. 18A-18B.

[0118] [Figure 25C] 23A is a schematic graph showing the variation of output color versus polarity direction for transmitted light for the leaky absorbing polarizer of FIG. 25A and the arrangement of FIG. 23A in the public mode of multiple retarders 300 of FIGS. 18A-18B.

[0119] [Figure 25D] 23A is a schematic graph showing the variation of output color versus polarization direction for transmitted light for the broadband absorptive polarizer of FIG. 25A and the arrangement of FIG. 23A in the public mode of multiple retarders 300 of FIGS. 19A-19B.

[0120] [Figure 25E] 23A is a schematic graph showing the variation of output color versus polarization direction for transmitted light for the leaky absorbing polarizer of FIG. 25A and the arrangement of FIG. 23A in the public mode of multiple retarders 300 of FIGS. 19A-19B.

[0121] [Figure 26A] FIG. 1 is a schematic diagram showing a side view of a switchable privacy display for use in ambient lighting, comprising an emissive spatial light modulator, a switchable retarder arranged between a reflective polarizer, i.e. an additional polarizer and a further additional polarizer, a passive retarder between the reflective polarizer and a quarter-wave plate arranged between the output polarizer of the spatial light modulator, an air gap and a diffuser.

[0122] [Figure 26B] FIG. 1 is a schematic diagram showing a side view of a switchable privacy display for use in ambient lighting, including a transmissive spatial light modulator, a recycling backlight, a switchable retarder positioned between a reflective polarizer and an additional polarizer, a passive retarder between a quarter-wave plate positioned between the reflective input polarizer and the input polarizer of the spatial light modulator, an air gap, and a diffuser.

[0123] [Figure 27A] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27B] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27C]1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27D] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27E] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27F] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27G] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Fig. 27H] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27I] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27J] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27K] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27L] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27M] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27N] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27O]1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27P] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27Q] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27R] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27S] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27T] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Fig. 27U] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27V] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Fig. 27W] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Fig. 27X] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27AA] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Diagram 27AB] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Fig. 27AC]1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Fig. 27AD] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Figure 27AE] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator. [Fig. 27AF] 1A-1C are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with a transmissive or emissive spatial light modulator.

[0124] [Figure 28A] FIG. 1 is a schematic diagram showing a side perspective view of a viewing angle control element including a passive compensation retarder stack, a reflective polarizer, and a switchable liquid crystal retarder between a quarter-wave plate positioned between the reflective polarizer and an additional polarizer.

[0125] [Figure 28B] FIG. 2 is a schematic diagram illustrating a side perspective view of an example of a passive retarder stack including a passive controlled retarder that includes a series of four aligned A-plates.

[0126] [Figure 28C] 28C is a schematic graph showing the variation in transmittance with respect to the polar direction of a transmitted light beam in the arrangement of FIG. 28B.

[0127] [Figure 29A] FIG. 13 is a schematic diagram illustrating the operation of an imaging waveguide in a narrow angle mode of operation in a rear perspective view.

[0128] [Figure 29B] 29B is a schematic graph showing the viewing luminance plot of the output of FIG. 29A when used in a display device that does not have a switchable liquid crystal retarder.

[0129] [Diagram 30]FIG. 2 is a schematic diagram illustrating a backlight including a collimating waveguide in a side view.

[0130] [Diagram 31] 31 is a schematic graph showing the viewing luminance plot of the output of FIG. 30 when used in a display device without a switchable liquid crystal retarder.

[0131] [Figure 32A] FIG. 1 is a schematic diagram illustrating illumination of a retarder layer with off-axis light in a perspective view.

[0132] [Figure 32B] FIG. 2 is a schematic diagram illustrating, in a perspective view, illumination of a retarder layer with off-axis light of a first linear polarization state at 0 degrees.

[0133] [Figure 32C] FIG. 1 is a schematic diagram illustrating, in a perspective view, illumination of a retarder layer with off-axis light of a first linear polarization state at 90 degrees.

[0134] [Fig. 32D] FIG. 1 is a schematic diagram illustrating, in a perspective view, illumination of a retarder layer with off-axis light of a first linear polarization state at 45 degrees.

[0135] [Figure 33A] FIG. 1 is a schematic diagram illustrating illumination of a C-plate retarder with off-axis polarized light with a positive elevation angle in a perspective view.

[0136] [Figure 33B] FIG. 1 is a schematic diagram illustrating illumination of a C-plate retarder with off-axis polarized light having a negative outer angle in a perspective view.

[0137] [Figure 33C] FIG. 1 is a schematic diagram illustrating, in a perspective view, illumination of a C-plate retarder with off-axis polarized light with positive elevation angle and negative outer angle.

[0138] [Figure 33D]FIG. 1 is a schematic diagram showing a perspective view of illumination of a C-plate retarder with off-axis polarized light having a positive elevation angle and a positive outer angle.

[0139] [Diagram 34] 33A to 33D. FIG. 33C is a schematic graph showing the variation in output transmittance with respect to the polarity direction of the transmitted light beam in FIG.

[0140] [Figure 35A] FIG. 1 is a schematic diagram illustrating, in a perspective view, illumination of a crossed A-plate retarder layer with off-axis polarized light having a positive elevation angle.

[0141] [Figure 35B] FIG. 1 is a schematic diagram illustrating, in a perspective view, illumination of a crossed A-plate retarder layer with off-axis polarized light having a negative outer angle.

[0142] [Figure 35C] FIG. 1 is a schematic diagram illustrating, in a perspective view, illumination of a crossed A-plate retarder layer with off-axis polarized light having a positive elevation angle and a negative outer angle.

[0143] [Figure 35D] FIG. 1 is a schematic diagram illustrating, in a perspective view, illumination of a crossed A-plate retarder layer with off-axis polarized light having a positive elevation angle and a positive outer angle.

[0144] [Diagram 36] 35A to 35D. FIG. 35B is a schematic graph showing the variation in output transmittance with respect to the polarity direction of the transmitted light beam in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0145] For purposes of this disclosure, a terminology relating to optical retarders will now be explained.

[0146] In a layer containing uniaxially birefringent material, there is a direction that dominates the optical anisotropy, while all directions perpendicular to that direction (or at a given angle to that direction) have equivalent birefringence.

[0147] The optic axis of an optical retarder refers to the direction of propagation of light rays in a uniaxially birefringent material where no birefringence occurs, which is distinct from the optic axis of an optical system, which may be, for example, parallel to a line of symmetry or perpendicular to the display surface along which the chief ray propagates.

[0148] For light propagating perpendicular to the optic axis, the optic axis is the slow axis when linearly polarized light with a vector direction parallel to the slow axis travels slowest. The slow axis direction is the direction with the highest refractive index at the design wavelength. Similarly, the fast axis direction is the direction with the lowest refractive index at the design wavelength.

[0149] In a positive dielectric anisotropy uniaxial birefringent material, the slow axis direction is the extraordinary axis of the birefringent material. In a negative dielectric anisotropy uniaxial birefringent material, the fast axis direction is the extraordinary axis of the birefringent material.

[0150] The terms "half wavelength" and "quarter wavelength" refer to the operation of the retarder at a design wavelength λ, which may typically be between 500 nm and 570 nm. In this exemplary embodiment, exemplary retardance values ​​are provided for a wavelength of 550 nm unless otherwise specified.

[0151] A retarder introduces a phase shift between two perpendicularly polarized components of a light wave incident on it and is characterized by the amount of relative phase it imparts to the two polarization components, which is related to the birefringence Δn and the retarder thickness d. Γ=2.π.Δn.d / λ0 Equation 1

[0152] In Equation 1, Δn is defined as the difference between the extraordinary and ordinary refractive indices, i.e. Δn=n e -n o formula 2 As follows.

[0153] For a half-wave retarder, the relationship between d, Δn, and λ0 is selected such that the phase shift between the polarization components is Γ=π. For a quarter-wave retarder, the relationship between d, Δn, and λ0 is selected such that the phase shift between the polarization components is Γ=π / 2.

[0154] The term "half-wave retarder" herein typically refers to light propagating perpendicular to the retarder and perpendicular to the spatial light modulator.

[0155] We now describe some aspects of the propagation of light through a transparent retarder between a pair of polarizers.

[0156] The state of polarization (SOP) of a beam of light is described by the relative amplitude and phase shift between any two orthogonally polarized components. A transparent retarder does not change the relative amplitude of these orthogonally polarized components, but only affects their relative phase. Providing a net phase shift between the orthogonally polarized components changes the SOP, while maintaining the net relative phase maintains the SOP. In this specification, the SOP may be referred to as the polarization state.

[0157] A linear SOP has a polarization component with a non-zero amplitude and an orthogonal polarization component with zero amplitude.

[0158] A linear polarizer transmits a unique linear SOP with a linear polarization component parallel to the linear polarizer's electric vector transmission direction, and attenuates light with a different SOP.

[0159] An absorptive polarizer is a polarizer that absorbs one polarization component of incident light and transmits the second, orthogonal polarization component. An example of an absorptive linear polarizer is a dichroic polarizer.

[0160] A reflective polarizer is a polarizer that reflects one polarization component of incident light and transmits a second orthogonal polarization component. Examples of reflective polarizers that are linear polarizers include multilayer polymer film stacks such as DBEF™ or APF™ from 3M, or wire grid polarizers such as ProFlux™ from Moxtek. Reflective linear polarizers may further include a cholesteric reflective material and a quarter wave plate arranged in series.

[0161] A retarder placed between a linear polarizer and a parallel linear analyzing polarizer that introduces no relative net phase shift will transmit light completely except for any residual absorption in the linear polarizer.

[0162] A retarder that provides a relative net phase shift between the orthogonal polarization components changes the SOP and is attenuated in the analyzing polarizer.

[0163] In this disclosure, "A-plate" refers to an optical retarder that uses a layer of birefringent material with its optic axis parallel to the plane of the layer.

[0164] "Positive A-plate" refers to a positively birefringent A-plate, i.e., an A-plate that has a positive Δn.

[0165] In this disclosure, a "C-plate" refers to an optical retarder that uses a layer of birefringent material with its optic axis perpendicular to the plane of the layer. A "positive C-plate" refers to a positive birefringent C-plate, i.e., a C-plate with positive Δn. A "negative C-plate" refers to a negative birefringent C-plate, i.e., a C-plate with negative Δn.

[0166] "O-plate" refers to an optical retarder that uses a layer of birefringent material whose optic axis has a component parallel to the plane of the layer and a component perpendicular to the plane of the layer. "Positive O-plate" refers to a positively birefringent O-plate, i.e., an O-plate with positive Δn.

[0167] An achromatic retarder may be provided in which the retarder material has a retardance Δn.d that varies with wavelength λ as follows: Δn.d / λ=κ Equation 3

[0168] where κ is substantially constant.

[0169] An example of a suitable material is modified polycarbonate from Teijin Films. In this embodiment, an achromatic retarder may be provided to advantageously minimize color change between polar angle viewing directions with less luminance falloff and polar angle viewing directions with more luminance falloff, as described below.

[0170] Various other terms used in this disclosure with respect to retarders and liquid crystals are now explained.

[0171] A liquid crystal cell has a retardance given by Δn.d, where Δn is the birefringence of the liquid crystal material in the liquid crystal cell and d is the thickness of the liquid crystal cell independent of the orientation of the liquid crystal material within the liquid crystal cell.

[0172] Homogeneous alignment refers to the alignment of liquid crystals in a switchable liquid crystal display where the molecules are aligned substantially parallel to the substrate. Homogeneous alignment is sometimes also called planar alignment. Homogeneous alignment may typically comprise a small pretilt, such as 2 degrees, such that the molecules at the surface of the alignment layer of the liquid crystal cell are slightly tilted, as described below. The pretilt is configured to minimize degeneracy in switching the cell.

[0173] In this disclosure, homeotropic alignment is the state in which rod-like liquid crystal molecules are aligned substantially perpendicular to the substrate. Discotic liquid crystal homeotropic alignment is defined as the state in which the axis of the column structure formed by the disk-like liquid crystal molecules is aligned perpendicular to the surface. In homeotropic alignment, the pretilt is the tilt angle of the molecules close to the alignment layer, which is typically close to 90 degrees, for example 88 degrees.

[0174] A twisted liquid crystal layer has a twisted structure (also known as a helical structure or helix) of nematic liquid crystal molecules. The twist can be achieved by non-parallel alignment of the alignment layers. Additionally, cholesteric dopants may be added to the liquid crystal material to break the degeneracy of the twist direction (clockwise or counterclockwise) and further control the pitch of the twist in the relaxed (typically undriven) state. A supertwisted liquid crystal layer has a twist of more than 180 degrees. Twisted nematic layers used in spatial light modulators typically have a twist of 90 degrees.

[0175] Liquid crystal molecules with positive dielectric anisotropy can be switched from a homogeneous alignment (such as an A-plate retarder alignment) to a homeotropic alignment (such as a C-plate or O-plate retarder alignment) by an applied electric field.

[0176] Liquid crystal molecules with negative dielectric anisotropy can be switched from a homeotropic alignment (such as a C-plate or O-plate retarder alignment) to a homogeneous alignment (such as an A-plate retarder alignment) by an applied electric field.

[0177] Rod-like molecules have positive birefringence. Therefore, as described in Equation 2, n e >n0. Discotic molecules have negative birefringence. Therefore, n e <n0である。

[0178] Positive retarders, such as A-plates, positive O-plates, and positive C-plates, can typically be provided by stretched films or rod-like liquid crystal molecules. Negative retarders, such as negative C-plates, can typically be provided by stretched films or discotic liquid crystal molecules.

[0179] Parallel liquid crystal cell alignment refers to the alignment direction of the homogeneous alignment layer being parallel or, more typically, antiparallel. In the case of pretilted homeotropic alignment, the alignment layer may have components that are substantially parallel or antiparallel. A hybrid alignment liquid crystal cell may have one homogeneous alignment layer and one homeotropic alignment layer. A twisted liquid crystal cell may be provided by alignment layers that do not have parallel alignment, for example, oriented at 90 degrees to each other.

[0180] The transmissive spatial light modulator may further comprise a retarder between the input display polarizer and the output display polarizer, for example as disclosed in U.S. Pat. No. 8,237,876, the entirety of which is incorporated herein by reference. Such a retarder (not shown) is in a different location than the passive retarder of this embodiment. Such a retarder compensates for the degradation of contrast at off-axis viewing positions, which has a different effect on the brightness reduction at off-axis viewing positions of this embodiment.

[0181] The privacy mode of operation of a display is one in which the viewer can view with such low contrast sensitivity that they cannot clearly see the image. Contrast sensitivity is a measure of the ability to distinguish between different levels of luminance in a static image. Inverse contrast sensitivity can be used as a measure of visual security, in that a high visual security level (VSL) corresponds to low image visibility.

[0182] For a privacy display that presents an image to a viewer, visual security can be obtained as follows. VSL=(Y+R) / (YK) Equation 4

[0183] where VSL is the visual security level, Y is the luminance of the white state of the display at the peeper's viewing angle, K is the luminance of the black state of the display at the peeper's viewing angle, and R is the luminance of the reflected light from the display.

[0184] The panel contrast ratios are as follows: C=Y / K formula 5

[0185] In a high contrast optical LCD mode, the white state transmittance remains substantially constant with viewing angle. In the reduced contrast LCD mode of the present embodiment, the white state transmission typically decreases as the black state transmission increases, Y+K~PL Equation 6 It will look like this.

[0186] Then the visual security level can be further obtained as follows:

number

[0187] The off-axis relative luminance, P, is typically defined as a percentage of the frontal luminance, the viewer's angle L for which the display may have an image contrast ratio C, and the surface reflectance ρ.

[0188] The off-axis relative luminance P is sometimes referred to as the privacy level. However, such privacy level P describes the relative luminance of a display at a given polar angle compared to the frontal luminance, and is not a measure of the appearance of privacy.

[0189] The display may be illuminated by a Lambertian ambient illuminance, I. Thus, in a completely dark environment, a high contrast display has a VSL of approximately 1.0. As ambient illumination increases, the perceived image contrast decreases, the VSL increases, and a private image is perceived.

[0190] For a typical LCD display, the panel contrast C is greater than 100:1 for nearly all viewing angles, allowing visual security levels to be approximated as follows:

[0191] VSL=1+I.ρ / (π.PL) Equation 8 Compared to privacy displays, wide-angle displays are desirably easily viewable in standard ambient lighting conditions. One measure of image visibility is given by contrast sensitivity, such as the Michelson contrast given by: M=(I max -I min ) / (I max +I min ) Equation 9

[0192] therefore, M=((Y+R)-(K+R)) / ((Y+R)+(K+R))=(YK) / (Y+K+2.R) Equation 10

[0193] Therefore, the visual security level (VSL) is comparable (but not identical) to 1 / M. In this study, for a given off-axis relative luminance P, the wide-angle image visibility W is approximated as follows: W=1 / VSL=1 / (1+I.ρ / (π.PL)) Equation 11

[0194] In this study, the desired white point (u w ',v w Output color from (u w '+Δu', v w The color variation Δε of Δv′ (Δε′+Δv′) may be determined by the CIELUV color difference index, assuming a typical display spectrum illuminant, Δε = (Δu' 2 +Δv' 2 ) 1 / 2 Formula 12 can be obtained by

[0195] The structure and operation of various switchable display devices will now be described. In this description, common elements will have common reference numerals. It should be noted that the disclosure of any element applies to each device in which the same or corresponding element is provided. Therefore, for the sake of brevity, such disclosure will not be repeated.

[0196] It would be desirable to provide a switchable privacy display that has a degree of circular symmetry.

[0197] 1A is a schematic diagram illustrating a side perspective view of a switchable privacy display for use in ambient lighting 604, including a transmissive spatial light modulator 48, a reflective polarizer 302, a plurality of retarders 300 including a switchable retarder 301 and a passive retarder 330 disposed between the quarter wave plates 296A and 296B, and an additional polarizer 318 disposed at the output side of the spatial light modulator 48. The switchable privacy display is an example of a display device 100.

[0198] A display apparatus 100 for use in ambient lighting 604 includes a spatial light modulator 48 arranged to output light along an output direction, a display polarizer arranged on the side of the spatial light modulator 48, an additional polarizer 318 arranged on the same side of the spatial light modulator 48 as the display polarizer, the additional polarizer 318 arranged such that the display polarizer and the additional polarizer 318 pass their respective linearly polarized polarization states, and a first quarter wave plate 296A and a second quarter wave plate 296B arranged between the additional polarizer 318 and the display polarizer, the first quarter wave plate 296A being arranged to polarize the input of the second quarter wave plate 296B. The display polarizer and the additional polarizer 318 include a first quarter-wave plate 296A and a second quarter-wave plate 296B arranged on the input side and arranged to convert a linearly polarized polarization state that passes through one of the display polarizer and the additional polarizer 318 on the input side of the first quarter-wave plate 296A to a circularly polarized polarization state, and the output side second quarter-wave plate 296A is arranged to convert a circularly polarized polarization state incident thereon to a linearly polarized polarization state that passes through the other of the display polarizer and the additional polarizer 318 on the output side of the second quarter-wave plate 296A, and at least one retarder arranged between the pair of quarter-wave plates 296A, 296B.

[0199] The present disclosure modifies the transmission phase of circularly polarized light by at least one retarder between the quarter wave plate 296A and the quarter wave plate 296B. The phase modification is independent of the azimuth angle, and therefore provides a degree of circular symmetry. In comparison, the arrangement shown in FIG. 23A provides primarily better control of the phase in the lateral direction, but less control in the elevation direction. Advantageously, a privacy display can be provided that can operate in landscape and portrait modes.

[0200] Additionally, the display device 100 achieves a reduced viewing cone angle in two dimensions, enabling night mode operation in the automotive vehicle 600, for example, to reduce reflections from the windshield and reduce stray light onto other occupants and interior surfaces of the vehicle 600.

[0201] The pair of quarter wave plates 296A, 296B may each comprise a passive quarter wave plate comprising a layer of oriented uniaxially birefringent material as described above. A passive quarter wave plate is a plate formed from a material that exists in an unaltered state during operation of the display device 100. The orientation of the molecules of the layer of material can be considered the state of the material. This is in contrast to a layer comprising a switchable material, which can be switched between at least a first and a second state during operation of the display device 100.

[0202] The pair of quarter wave plates 296A, 296B may each have a retardance in the range of 110 nm to 175 nm, preferably 130 nm to 140 nm, for light with a wavelength of 550 nm. Such quarter wave plates are first order quarter wave plates. Advantageously, passive quarter wave plates can be manufactured in high volume, large area, and at low cost.

[0203] The display polarizer may be the output polarizer 218 of the display device 100. The display device 100 may further include a reflective polarizer 302 arranged between the plurality of retarders 300 and the output polarizer 218, where the output polarizer 218 and the reflective polarizer 302 are arranged to pass the same linearly polarized polarization state. In this case, the electric vector transfer direction 303 of the reflective polarizer 302 is parallel to the electric vector transfer direction 319 of the additional polarizer 318. The electric vector transfer direction 303 of the reflective polarizer 302 may be parallel to the electric vector transfer direction 219 of the output polarizer 218.

[0204] A display device 100 for use in ambient lighting 604 may include a spatial light modulator 48 arranged to output light 400. In this disclosure, the spatial light modulator 48 may include a liquid crystal display including an input polarizer 210, an output polarizer 218 having substrates 212, 216, a liquid crystal layer 214, and red, green, and blue pixels 220, 222, 224. A backlight 20 is arranged to illuminate the spatial light modulator 48 and includes an input light source 15, a waveguide 1, a back reflector 3, and an optical stack 5 including diffusers, light turning films, and other known optical backlight structures. An asymmetric diffuser, which may comprise, for example, asymmetric surface relief features, may be provided in the optical stack 5 to increase diffusion in the elevation direction compared to the lateral direction. Advantageously, image uniformity may be improved.

[0205] In this embodiment, the backlight 20 may be arranged to provide an angular light distribution with reduced luminance for off-axis viewing positions compared to the front luminance. A typical wide angle backlight has a higher angular roll-off such that the full width at half maximum of the relative luminance may be preferably greater than 40°, more preferably greater than 60°, and most preferably greater than 80°.

[0206] Backlight 20 can provide a brightness at polar angles greater than 45 degrees relative to the normal to spatial light modulator 48 that is up to 30% of the brightness along the normal to spatial light modulator 48, preferably up to 20% of the brightness along the normal to spatial light modulator 48, and most preferably up to 10% of the brightness along the normal to spatial light modulator 48. In the exemplary embodiment of FIG. 1A, the brightness at polar angles greater than 45 degrees relative to the spatial light modulator may be up to 18%.

[0207] The backlight 20 may further include a switched backlight arranged to switch the output angular brightness profile to reduce off-axis brightness in a privacy mode of operation and increase off-axis brightness in a public mode of operation. In a narrow angle mode, such as a privacy display mode, such a directional backlight reduces some off-axis brightness and advantageously increases frontal efficiency, reducing display visibility and stray light for off-axis positions. In a public mode of operation, such as the public mode of a switched privacy display, such a switched backlight increases brightness in off-axis viewing directions. Advantageously, image visibility is increased for off-axis users and image uniformity is increased for on-axis users.

[0208] Exemplary embodiments of the backlight 20 are described below with reference to FIGS. 28A-31.

[0209] The display may further include a reflective recirculating polarizer 208 disposed between the backlight 20 and the spatial light modulator 48. The reflective recirculating polarizer 208 is different from the reflective polarizer 302 of this embodiment. The reflective recirculating polarizer 208 reflects polarized light from the backlight having a polarization orthogonal to the electric vector transmission direction of the dichroic input polarizer 210. The reflective recirculating polarizer 208 may not reflect ambient light 604 to an observer.

[0210] An output polarizer 218 may be positioned at the output of the spatial light modulator 48, and an additional polarizer 318 may be positioned at the output of the output polarizer 218, such that the one of the output polarizer 218 and the additional polarizer at the input of the display device is the output polarizer 218. The output polarizer 218 may be positioned to provide high extinction for light from the pixels 220, 222, 224 of the spatial light modulator 48 and to prevent back reflection from the reflective polarizer 302 back into the pixels 220, 222, 224.

[0211] An additional polarizer 318 may be disposed on the output side of the output polarizer 218. One or more of the polarizers 210, 218, 318 may be a dichroic polarizer.

[0212] 1A, the multiple retarders 300 include a pair of quarter wave plates 296A, 296B, a passive retarder 330, and a switchable liquid crystal retarder 301, but may generally be replaced by other configurations of at least one retarder disposed between the quarter wave plates 296A and 296B. The retarder 300 does not affect the intensity of light passing through the reflective polarizer 302, the retarder 300, and the additional polarizer 318 along an axis 199 perpendicular to the plane of the retarder 300, but the retarder 300 reduces the intensity of light passing along an axis tilted perpendicular to the plane of the retarder 300, at least in one of the switchable states of the switchable retarder 301. The principle behind this effect is explained in more detail below with reference to Figures 32A-36, and results from the presence or absence of a phase shift introduced by the passive retarder 300 to light along an axis that is at a different angle to the liquid crystal material of the retarder 300. A similar effect is achieved in all of the devices described below.

[0213] The substrates 312, 316 of the switchable liquid crystal retarder 301 may include electrodes arranged to provide a voltage across the layer 314 of liquid crystal material 414. A control system 352 may be arranged to control the voltage applied across the electrodes of the switchable liquid crystal retarder 301 by the voltage driver 350.

[0214] Any of the layers discussed herein may be bonded by adhesives, such as pressure sensitive adhesives and optically clear adhesives. Advantageously, image uniformity may be improved. Part of the interface may be provided with an air gap. Advantageously, assembly yield may be increased and costs may be reduced. Additional frontal reflection may be enhanced, increasing the visual security level.

[0215] FIG. 1B is a schematic diagram illustrating the orientation of the optical layers in the optical stack of FIG. 1A in a front view.

[0216] The electric vector transfer directions of the polarizers in the stack are shown. Any two of the electric vector transfer direction 219 of the outer polarizer 218, the electric vector transfer direction 303 of the reflective polarizer 302, and the electric vector transfer direction 319 of the additional polarizer 318 may be parallel to each other. The electric vector transfer direction 211 of the input polarizer 210 and the electric vector transfer direction 207 of the reflective recirculating polarizer 208 may be perpendicular to the electric vector transfer direction of the other polarizers in the stack.

[0217] 1B, an input electric vector transmission direction 211 at the input polarizer 210 can be transformed by the liquid crystal layer 214 to result in an output polarization state determined by an electric vector transmission direction 219 of the output polarizer 218. This arrangement may be appropriate when the spatial light modulator 48 is a liquid crystal display. The passive retarder 330 may include a retardation layer formed from material 430, the switchable liquid crystal retarder 301 may include a layer 314 of liquid crystal material 414, and the first and second quarter wave plates 296A, 296B may include retardation layers formed from materials 496A and 496B, respectively.

[0218] The pair of quarter-wave plates 296A, 296B may have crossed optical axes, meaning that the optical axis of the first quarter-wave plate 296A is arranged at 90 degrees to the optical axis of the second quarter-wave plate when viewed in a direction parallel to the viewing plane of the display 100. Each of the pair of quarter-wave plates 296A, 296B may have an optical axis arranged at 45 degrees to the electric vector propagation direction of the adjacent output polarizer 218 or additional polarizer 318.

[0219] Light rays transmitted through the retarders 301, 330 in a front-on direction along the axis 199 have their polarization state unchanged by the retarders 301, 330. In such on-axis operation, the crossed quarter wave plates 296A, 296B are positioned to leave the polarization state from the output polarizer 218 unchanged, as that polarization state is transmitted by the additional polarizer 318. Advantageously, high efficiency is achieved for the on-axis light viewing direction.

[0220] If the display device 100 further includes a reflective polarizer 302, the first quarter wave plate 296A adjacent to the reflective polarizer 302 can have an optical axis arranged at 45 degrees to the electric vector propagation direction 303 of the reflective polarizer 302. If the electric vector propagation direction 303 of the reflective polarizer 302 is parallel to the electric vector propagation direction 219 of the output polarizer 218, the first quarter wave plate 296A may have an optical axis arranged at 45 degrees to the electric vector propagation direction of the output polarizer 218 and the reflective polarizer 302.

[0221] In the example shown in Figure 1B, the electric vector transmission directions of the polarizers are shown parallel to the x and y directions that define the vertical and horizontal directions of the display. However, the arrangement is not limited to this orientation and the entire stack can be rotated to any particular angle, for example 90 degrees, so long as the relative angles of the various electric vector transmission directions of the polarizers and the optic axes of the retarders are maintained.

[0222] The spatial light modulator 48 may be, for example, an in-plane switching (IPS) type liquid crystal display, a fringe field switching (FFS) type liquid crystal display, or a vertically aligned nematic (VA) type liquid crystal display. Such spatial light modulators have an output electrical vector direction 219 that is typically horizontal or vertical.

[0223] In comparison, a twisted nematic (TN) liquid crystal display may have an output polarizer 218 electric vector transmission direction 219 that is typically oriented at 45 degrees to the horizontal. It would be desirable to provide a TN-LCD display with a vertical output electric vector transmission direction 219, such that in at least one orientation it is vertical so that it can be used with polarized sunglasses (which typically have a vertical electric vector transmission direction).

[0224] FIG. 1C is a schematic diagram illustrating, in front view, the orientation of the optical layers at the output of twisted nematic spatial light modulator 48 to achieve a vertical output polarization state.

[0225] The layers that define the spatial light modulator 48 are not shown, although the layers and their properties may be the same as the spatial light modulator 48 described above. In the example shown in Figure 1C, the electrical transfer vector direction 219 of the output polarizer 218 is oriented at 45 degrees to the horizontal and vertical directions of the display when viewed perpendicular to the viewing plane of the display device 100. The optical axis of the first quarter wave plate 496A is oriented at 45 degrees to the electrical transfer vector direction 219 of the adjacent output polarizer 218. The orientation of the remaining layers shown in Figure 1C is unchanged from the arrangement shown in Figure 1B.

[0226] Thus, the electrical transmission vector direction 219 of the output polarizer 218 is no longer parallel to the electrical transmission vector direction 319 of the additional polarizer 318, but instead is oriented at a 45 degree angle. Thus, the optical axes of the pair of quarter-wave plates 296A and 296B are not crossed in the example shown in FIG. 1C, but are instead arranged at a 45 degree angle with respect to each other. However, the arrangement shown in FIG. 1C can still function similarly to the arrangement shown in FIGS. 1A and 1B to reduce the transmission of light 400 in off-axis directions. This arrangement may be particularly suitable for sunglasses applications. Further TN-LCDs can be advantageously manufactured at lower cost than other LCD types and are suitable for operation in high temperature range environments, such as the interior of an automobile.

[0227] In other embodiments, the additional polarizer 318 may have a transmission direction 319 parallel to the electric vector transmission direction 219 of the output polarizer 218. Advantageously, the display can be viewed with polarized sunglasses in both landscape and portrait modes of operation.

[0228] FIG. 2 is a schematic diagram illustrating an arrangement of multiple retarder layers 300 in a perspective side view, including a passive positive C-plate compensation retarder 330 disposed between parallel polarizers 218, 302, 318, and a 270 degree super twist switchable liquid crystal retarder 301 disposed between quarter wave plates 296A and 296B.

[0229] FIG. 2 shows an example of the above embodiment. At least one retarder includes a liquid crystal retarder 301 including a layer of liquid crystal material 414. The switchable liquid crystal retarder 301 includes two surface alignment layers 413, 415 adjacent to the liquid crystal material 414 on either side thereof and arranged to provide homogeneous alignment in the adjacent liquid crystal material 414. The alignment directions 417A, 417B of the alignment layers (which are disposed on the electrodes 413, 415 but are not shown separately) are perpendicular to each other. Each of the quarter wave plates 296A, 296B is formed from a material having an axial direction 497A, 497B. As mentioned above, the optical axes of both quarter wave plate layers are parallel to the plane of the layers and define angles 298A, 298B with respect to the electric vector transmission directions of the output polarizer 218, the additional polarizer 318 and the reflective polarizer 302, respectively. In the example shown in Figure 2, the optical axis direction 497A of the first quarter wave plate 296A defines a 45 degree angle 298A with the electric vector transfer directions 219, 303 of the polarizers 218, 302, respectively. The optical axis direction 497B of the second quarter wave plate 296B defines a 135 degree angle 298B with the electric vector transfer directors defined by the polarizers. Thus, the optical axis directions 497A, 497B of the first and second quarter wave plates 296A, 296B are disposed at 90 degrees relative to each other and thus intersect.

[0230] The quarter wave plates 296A, 296B may have crossed optical axes 497A, 497B to minimize color variation in the light 400 output by the display device 100. Because of the on-axis operation, the crossed quarter wave plates 296A, 296B do not change the polarization state from the output polarizer 218 such that the on-axis light rays are efficiently transmitted through the additional polarizer 318. Furthermore, the chromaticity of the first quarter wave plate 497A may be compensated by the chromaticity of the second quarter wave plate 497B in at least one of the operational modes.

[0231] In this embodiment, "crossed" refers to an angle of substantially 90° between the optical axis 497A and the optical axis 497B of the quarter wave plate in the plane of the quarter wave plate. To reduce the cost of the retarder material, it is desirable to provide a material with some variation in retarder orientation, for example due to stretching errors during film manufacturing, and for example due to film orientation during assembly. Variations in retarder orientation away from the preferred direction can reduce the front brightness and increase the minimum transmission. Preferably, the angle 497A is at least 35° and at most 55°, more preferably at least 40° and at most 50°, and most preferably at least 42.5° and at most 47.5°. Preferably, the angle 497B is at least 125° and at most 145°, more preferably at least 130° and at most 135°, and most preferably at least 132.5° and at most 137.5°.

[0232] The quarter wave plates 296A, 296B may be arranged to provide a circular output polarization. Δn.d=(2q+1).λ / 4 Equation 13 and where m is (zero or) an integer and λ is the design wavelength as above. In one example, one or both of the quarter wave plates 296A, 296B may each have a retardance light of 550 nm, in the range of 110 nm to 175 nm, preferably 130 nm to 140 nm. Such quarter wave plates 296A, 296B are examples of zero order quarter wave plates, where q=0 in Equation 13.

[0233] Higher order waveplates (q>0) may also be used, allowing adjustment of the polar characteristics of the luminance and reflectance variations as described elsewhere herein.

[0234] At least one retarder 300 may be a switchable liquid crystal retarder 301 and may further include electrodes 413, 415 configured to apply a voltage V to switch the material 414 of the switchable liquid crystal retarder 301. The material 414 of the switchable liquid crystal retarder 301 may be switchable between two states. In a first state, where a first voltage, which may be zero volts, is applied by the electrode layers 413, 415, the material 414 may be in a state where the molecules of the material 414 of the layer 314 are aligned parallel to the plane of the layer 314 defined by the switchable liquid crystal retarder 301. An example of this state is a twisted nematic state or a super twisted nematic state.

[0235] In a second state, where a second voltage different from the first voltage is applied by the electrode layers 413, 415, the molecules of material 414 of the switchable liquid crystal retarder 301 may be reoriented to a vertical orientation, where the molecules of material 414 of layer 314 are oriented perpendicular to the plane of layer 314 defined by the switchable liquid crystal retarder 301. In this case, the liquid crystal material 414 has a positive dielectric anisotropy as described above.

[0236] Light 400 entering the multiple retarders 300 acquires a particular linear polarization state from either the output polarizer 218 or the additional polarizer 318. When the light 400 passes through either the first quarter wave plate 296A or the second quarter wave plate 296B, the linear polarization is converted to a circular polarization state. When the light 400 passes through at least one retarder 301, 330, the circular polarization of the light 400 may undergo retardation of one of the polarization components depending on the state of the material of the at least one retarder 301, 330. The retardation may vary based on the angle of incidence of the light 400 with respect to the axis 199 perpendicular to the viewing surface of the display device 100. In particular, the retardation of the polarization caused by on-axis light 410 passing through at least one retarder may be different from the retardation caused by off-axis light 420, as will be discussed in detail below.

[0237] The at least one retarder may include a retarder including an oriented uniaxially birefringent material. The at least one retarder may be arranged at the input side of the multiple retarders 300 to introduce a phase shift along an axis tilted with respect to the optical axis of the at least one retarder to the polarization components of the light 400 passing through the output polarizer 218 and one of the additional polarizers 318 and the quarter wave plates 296A, 296B.

[0238] The light 400 then passes through the second quarter wave plate 296B and the first quarter wave plate 296A, and the retarded circular polarization state is converted to a linear polarization state. However, depending on the retardation of the light 400 as it passes through the at least one retarder, the resulting linearly polarized state may vary. The intensity of the light 400 passed by the additional polarizer 318 or the other of the output polarizer 218 may then vary depending on the retardation of the light 400 as it passes through the at least one retarder. As discussed in more detail below, this arrangement may be used to control the intensity of the on-axis 410 and off-axis 420 light passing through the multiple retarders 300.

[0239] At least one retarder 301, 330 may be positioned at the input side of the multiple retarders 300 such that the output polarizer 218 and one of the additional polarizers 318 and quarter wave plates 296A, 296B do not introduce a phase shift to the polarization components of the light 400 passing therethrough along an axis along the optical axis of the at least one retarder.

[0240] When the at least one retarder includes at least one switchable liquid crystal retarder 301, the at least one retarder may be arranged at the input side of the multiple retarders 300 to introduce a phase shift to the polarization components of the light 400 passing through the output polarizer 218 and one of the additional polarizers 318 and quarter wave plates 296A, 296B along an axis tilted with respect to the optical axis of the at least one retarder in the switchable mode of the switchable liquid crystal retarder 301.

[0241] As described above, the layer 314 of liquid crystal material 414 may have a twist. The twist τ may be determined by Equation 14: where m is zero or a positive integer. τ=(90+M * 180)° Equation 14

[0242] The twist of the liquid crystal material may be defined as the total angle of rotation of the molecules of the material of the liquid crystal material layer 301 across the thickness defined by the liquid crystal material layer 301. The twist of the liquid crystal material layer 301 in the embodiment shown in Figure 2 is 270 degrees. The twist of the liquid crystal material layer 301 may also be any one of 90 degrees or 450 degrees.

[0243] The plurality of retarders 300 may further include at least one passive retarder 330. The at least one passive retarder 330 may include a passive retarder having an optical axis 431 perpendicular to a plane of the passive retarder. The passive retarder 300 may be a C-plate retarder.

[0244] The presence of at least one passive retarder 330 in addition to the switched liquid crystal retarder 301 can secondarily improve the output of the display device 100. The presence of at least one passive retarder 330 in addition to the switched liquid crystal retarder 301 can improve the viewing angle of the output of the display device 100 and can compensate for chromaticity variations with polarity and azimuth.

[0245] As discussed above, the switchable liquid crystal retarder 301 has a thickness that is optimized for a particular wavelength of light 400 output by the display device 100. Thus, the performance of the multiple retarders 300 is most optimized at this wavelength, and not optimized at wavelengths of light 400 output by the display device 100 that are different from this wavelength.

[0246] The passive retarder 330 can advantageously increase the size of the polarized region where the luminance is reduced and the reflectance is increased.

[0247] FIG. 3A is a schematic diagram illustrating, in side view, the propagation of output light from spatial light modulator 48 through the optical stack of FIG. 1A in the public mode of operation.

[0248] In the public mode of operation, light rays 400, 402 emitted in an on-axis direction by the backlight 20 have a polarization state 360 ​​parallel to the electric vector transfer direction 219 of the output polarizer 218 exiting the spatial light modulator 48. The on-axis light rays 400 then traverse the multiple retarder layers 300 where the polarization is transformed by the first quarter-wave plate 296A, pass through the switchable liquid crystal retarder 301 and the passive retarder 330, and are then transformed from a circular polarization state to a linearly polarized polarization state 362 by the first quarter-wave plate 296B. In the public mode, the switchable liquid crystal retarder 301 is typically (but not necessarily) in an off state with no voltage applied. If the switchable liquid crystal retarder 301 has a positive dielectric anisotropy, the switchable liquid crystal retarder 301 behaves similarly to an A-plate retarder in the first state.

[0249] Thus, the circular polarization state of the on-axis light ray 400 is retarded when passing through the switchable liquid crystal retarder 301. However, the retardation of the switchable liquid crystal layer 314 of the liquid crystal retarder 301 may be configured such that the components of the circularly polarized light are retarded by the same amount. If the passive retarder 330 is a C-plate retarder, the on-axis light ray 400 propagates in a direction substantially parallel to the optical axis of the passive retarder 330. Thus, the on-axis light ray 400 experiences minimal retardation when passing through the passive retarder 330. The combined effect from the multiple retarders 300 causes the on-axis light ray 400 to exit the multiple retarders 300 with the same or similar linearly polarized polarization state 362, resulting in the linear polarization state 360 ​​that the on-axis light ray 400 enters the multiple retarders 300. This linear polarization state 362 is parallel to the electric vector propagation direction 319 of the additional polarizer 318, and therefore the on-axis light ray 400 exits the display device 100 with a relatively unvarying brightness.

[0250] In the public mode, off-axis light ray 402 traverses the multiple retarders 300 in the same manner as the on-axis light ray 400. Thus, when the switchable liquid crystal retarder 301 is in the first of its two states, the multiple retarders 300 do not fully convert the polarization states 360, 361 of light ray 400 passing through the switchable compensation retarder 300 perpendicular to the plane of the switchable retarder, or light ray 402 passing through the switchable compensation retarder 300 at an acute angle perpendicular to the plane of the switchable retarder 301.

[0251] Polarization state 362 is substantially identical to polarization state 360, and polarization state 364 is substantially identical to polarization state 361. Thus, the angular transmission profile is substantially uniform over a wide range of polarization regions.

[0252] In other words, when the layer 314 of liquid crystal material 414 is in the first of the two orientation states, the multiple retarders 300 impart no overall retardance to light passing through the multiple retarders 300 perpendicular to the plane of the retarder or at an acute angle to the perpendicular to the plane of the retarder 300.

[0253] Advantageously, the change in display luminance with the viewing angle of the first state is substantially unchanged. Multiple users can advantageously view the display from a wide range of viewing angles.

[0254] FIG. 3B is a schematic diagram illustrating, in a side view, the propagation of light rays from an ambient light source 604 through the optical stack of FIG. 1A in the public mode of operation.

[0255] An on-axis ray 410 of the ambient light 604 traverses the retarders 300 in a manner similar to the on-axis ray 400 emitted from the backlight 20 described above. The on-axis ray 410 traverses the retarders 300 in a reverse direction relative to the on-axis ray 400 emitted from the backlight 20, however, as described above with respect to light emitted from the backlight 20, the reverse traversal of the retarders 300 may not change the effect of the retarders 300 on the light ray. Thus, the on-axis ray 410 reaches the backlight 20 and may be absorbed or recycled as the light 400 emitted from the display device 100.

[0256] Similarly, the off-axis light ray 402 does not undergo a total polarization state conversion when passing through the multiple retarders 300. The ambient light 604 is unpolarized, and the off-axis light ray does not initially have a polarization 370. The additional polarizer 318 passes the polarization component 372 that is parallel to the electric vector transmission direction 319 of the additional polarizer. The additional polarizer 318 absorbs most of the polarization state 372 that is perpendicular to the electric vector transmission direction 319 of the additional polarizer 318. Some light is reflected from the front surface of the polarizer 318 by Fresnel reflection at the outer air interface. After traversing the multiple retarders 300, the linear polarization state 374 of the off-axis light ray 402 is thereby parallel to the electric vector transmission direction 303 of the reflective polarizer 302, and the off-axis light ray is not reflected, but instead passes through the reflective polarizer 302 to the spatial light modulator 448 where it may be absorbed by the input polarizer 210 or transmitted into the backlight 20.

[0257] Advantageously, display reflectance in the public mode is reduced over a wide range of viewing angles, allowing multiple users to conveniently view the display from a wide range of viewing angles with high image contrast.

[0258] FIG. 3C is a schematic diagram illustrating, in side view, the propagation of output light from spatial light modulator 48 through the optical stack of FIG. 1A in a privacy mode of operation.

[0259] In the privacy mode, the switchable liquid crystal retarder 301 is in an on-state with a voltage applied across the liquid crystal layer 314. Thus, the switchable liquid crystal retarder 301 may be in the second of the two states. If the switchable liquid crystal retarder 301 has a positive dielectric anisotropy, the switchable liquid crystal retarder 301 behaves in the second state similarly to a C-plate retarder. This means that the optical axis of the switchable liquid crystal retarder 301 is substantially parallel to the propagation direction of the on-axis light ray 400 through at least a part of the thickness of the layer 314. Thus, the on-axis light ray 400 does not experience any retardation when passing through the switchable liquid crystal retarder 301 in the second state, and thus the linear polarization state 360 ​​of the on-axis light ray 400 before traversing the multiple retarders 300 is the same as the linear polarization state 362 after traversing the multiple retarders 300. Thus, the on-axis light ray 400 exits the display through the additional polarizer 318 and has a brightness that does not vary much in the privacy mode of operation.

[0260] The off-axis light ray 402 emitted from the backlight 20 undergoes a polarization conversion when passing through the material of the switchable liquid crystal retarder 301. This is due to the acute angle of the off-axis light ray 402, as will be discussed in more detail below. The off-axis light ray 402 therefore reaches the additional polarizer 318, which has a linear polarization state 364 that is at least partially rotated compared to the linear polarization state 361. The linear polarization state 364 has at least some perpendicular component with respect to the electric vector propagation direction 319 of the additional polarizer 318, and therefore the brightness of the off-axis light ray 402 is reduced compared to the on-axis light ray 400.

[0261] Advantageously, the display brightness at wide viewing angles can be reduced in the second state, thus preventing peepers from seeing the image emitted by the display device 100 at wide viewing angles. Stray light can be reduced in nighttime operation, while a frontal user can still see the image.

[0262] FIG. 3D is a schematic diagram illustrating, in a side view, the propagation of light rays from an ambient light source 604 through the optical stack of FIG. 1A in a privacy mode of operation.

[0263] In privacy mode operation, the on-axis light ray 410 from the ambient light source 604 traverses the retarders 300 in a similar manner as the on-axis light ray 400 emitted from the backlight 20, as described in connection with FIG. 3C. The on-axis light ray 410 traverses the retarders 300 in the opposite direction to the on-axis light ray 400 emitted from the backlight 20, but as discussed for light emitted from the backlight 20, the traverse direction of the retarders 300 into or out of the display does not change the effect of the retarders 300 on the light ray. Thus, the on-axis light ray 410 reaches the backlight 20 and can be absorbed or recycled as light 400 emitted from the display device 100.

[0264] In contrast, off-axis light rays 412 emitted from the backlight 20 undergo a polarization transformation when passing through the material 414 of the switchable liquid crystal retarder 301. This is due to the acute angle of the off-axis light rays 412, as discussed in more detail below. The off-axis light rays 412 therefore reach the reflective polarizer 302 with a linear polarization state 374 that is at least partially rotated compared to the linear polarization state 372. The linear polarization state 374 has at least some perpendicular state to the electric vector transmission direction 303 of the reflective polarizer 302, and is therefore at least partially reflected by the reflective polarizer 302. The light rays 412 then traverse the retarders 300 in the opposite direction, reversing the polarization transformation from the first pass through the retarders 300 and resulting in a polarization state 376 that is parallel to the electric vector transmission direction of the additional polarizer 318. The off-axis light rays 412 therefore leave the display device 100 with a polarization state 378, resulting in a stack that appears mirror-like when viewed from a wide angle. The additional polarizer 318 absorbs most of the polarization state 372 that is perpendicular to the additional polarizer's electric vector propagation direction 319 , but may reflect some of the perpendicular state 404 .

[0265] Advantageously, the reflectance at wide viewing angles can be increased in the second state, such that an onlooker can be prevented from seeing the image emitted by the display device 100 at wide viewing angles by the reflected light rays reducing the contrast of the image emitted by the display device, and thus increasing the reflectance R increases the visual security level VSL, as described in equation 4 above.

[0266] Figure 4A is a schematic graph showing the variation of transmittance with respect to the polarity direction of the transmitted light beam in Figure 3A. Figure 4B is a schematic graph showing the variation of reflectance with respect to the polarity direction of the transmitted light beam in Figure 3B. Figure 4C is a schematic graph showing the variation of transmittance with respect to the polarity direction of the transmitted light beam in Figure 3C, and Figure 4D is a schematic graph showing the variation of reflectance with respect to the polarity direction of the transmitted light beam in Figure 3D.

[0267] Thus, the effect of the operation of the privacy mode of the display device 100 can be seen. In the public mode shown in FIG. 4A, the transmittance is high over a wide range of both outside and elevation angles. In comparison, as shown in FIG. 4C when the display device 100 is operating in the privacy mode, the transmittance remains high in a narrow range of outside and elevation angles around 0 degrees, but drops sharply as both the outside and elevation angles increase, as shown in FIG. 4C. The drop in transmittance is substantially symmetric around the azimuth angle. Thus, the display device 100 can function as a privacy display in both horizontal and vertical directions, since peepers at wide angles do not receive light transmitted by the display device.

[0268] The effect of the privacy mode of the display device 100 can also be seen through a comparison of the reflectance from the display device 100 in Figures 4B and 4D. In the privacy mode shown in Figure 4D, the reflectance remains low in a narrow range of outer and elevation angles around 0 degrees, but increases rapidly as both the outer and elevation angles increase. The increase in reflectance is substantially symmetric around the azimuth angle. Thus, the display device 100 can function as a privacy display in both the horizontal and vertical directions, and a peeper cannot distinguish the image transmitted by the display device at wide angles because he or she receives more light reflected from the display device 100.

[0269] The schematic graphs shown in Figures 4A-4D were obtained using several retarders 300 having the parameters set forth in Table 1 A. The desired ranges of retardation and other properties of the layers have been established by simulation of the retarder stack and experimentation with display optics stacks. [Table 1] [Table 2]

[0270] In the embodiment shown in Table 1A, the layer 314 of liquid crystal material may have a twist of 270 degrees and a retardance in the range of 650 nm to 800 nm, and most preferably in the range of 700 nm to 720 nm, for light of a wavelength of 550 nm. An advantageously desired symmetric viewing angle profile may be achieved in transmission, and can be achieved for reflectance if a reflective polarizer 302 is provided.

[0271] FIG. 5A is a schematic diagram showing an external view of the display device 100 of FIG. 1A operating in the public mode, from a front perspective view.

[0272] FIG. 5B is a schematic diagram showing an external view of the display device 100 of FIG. 1A operating in privacy mode, from a front perspective view.

[0273] 5A, display device 100 is viewable from orientations corresponding to upper viewing quadrants 530, 532, lower viewing quadrants 534, 536, side viewing locations 526, 528, and lower central viewing regions 522, 524, and on-axis location 520, without significant variation in brightness of output light 400 from display 100. Thus, the image emitted by display device 100 as viewed from on-axis region 520 will be visible from all orientations, as indicated by the white color representing the image emitted by display device 100.

[0274] In the privacy mode shown in FIG. 5B, the field of view of the on-axis region 520 of the display device 100 is not affected by the change in mode, and therefore the image emitted by the display device 100 in the on-axis region is visible. However, in other viewing regions, the change in mode changes what is visible in these regions. As described above, in the privacy mode, the brightness of the transmitted off-axis light 402 is reduced. Thus, the image visibility W of the image emitted by the display device 100 is reduced when viewed from an orientation corresponding to the upper viewing quadrants 530, 532, and when viewed from an orientation corresponding to the side viewing positions 526, 528 and the lower central viewing regions 522, 524 of the lower viewing quadrants 534, 536. In addition, as described above, in the privacy mode, the ambient light 604 is reflected from the reflective polarizer 302 and exits the display device 100 as off-axis rays 402, increasing the brightness of the ambient light reflection in the off-axis orientation. Thus, the output light 400 from the display device as seen by a spectator viewing the display device 100 from an off-axis orientation is determined by the light reflected by the reflective polarizer 302 and the output transmission, such that the display device 100 appears mirror-like in some off-axis orientations.

[0275] FIG. 6A is a schematic diagram showing, in perspective view, the brightness status of a mobile device in public mode, including the display device 100 of FIG. 1A, showing, clockwise from the top left, front-on landscape, front-on portrait, downward portrait, and right-side landscape.

[0276] The viewing direction 199 of the display device 100 is perpendicular to the viewing surface of the display device 100. In all orientations shown, in the public mode, the image emitted by the playing device 100 is visible to the viewer, as represented by the white color representing the image emitted by the display device 100. The image is viewable in all of the on-axis landscape and portrait orientations 520, the face-down portrait orientation 522, and the right-side landscape orientation 528.

[0277] FIG. 6B is a schematic diagram showing the brightness status of a mobile device in privacy mode including a display device 100 having the appearance shown in FIG. 1A, and shows, from the top left clockwise, front-on landscape, front-on portrait, downward portrait, and landscape as viewed from the right.

[0278] In the privacy mode, the field of view for the observer remains the on-axis landscape and portrait orientations 520, and the image emitted by the display device 100 is visible to the observer. However, in face-down portrait orientation 522 and right-view landscape orientation 528, the image emitted by the display device is no longer visible. A peeper looking from a wide angle will instead see a mirror-like surface provided by the reflective polarizer 302 described above.

[0279] FIG. 6C is a schematic diagram showing the reflectivity status of a mobile device in privacy mode including a display device 100 having the appearance shown in FIG. 1A, and shows, from the top left clockwise, front-on landscape, front-on portrait, downward portrait, and landscape as viewed from the right.

[0280] In the privacy mode, the field of view to the observer remains unchanged in on-axis landscape and portrait orientations 520 due to minimal reflectance from the additional polarizer 318. However, the downward portrait orientation 522 and the reflection in the right-looking landscape orientation 528 from the surface of the additional polarizer 318 can result in a frontal reflection as described above, desirably increasing the visual security level VSL.

[0281] FIG. 7A is a schematic diagram illustrating a top view of a motor vehicle 600 having a switched directional display 100 disposed within a passenger compartment 602 in a nighttime mode of operation.

[0282] The night mode of the switchable directional display 100 may correspond to the privacy mode described above. The light cone 620 (e.g., representing a cone of light that is greater than 50% of the peak brightness emitted by the switchable directional display 100) may indicate a range of angles over which the image emitted by the switchable directional display 100 is perceptible. As shown in FIG. 7A, when the switchable directional display 100 is in the night mode, the driver 604 is contained within the area horizontally defined by the light cone 620, and therefore the image emitted by the switchable directional display 100 is perceptible to the driver 604. In contrast, high angle light rays 622 that fall horizontally outside the light cone 620 are reduced in brightness, and therefore the image emitted by the switchable directional display 100 may not be perceptible to the passengers 608 in the vehicle 600. This may be advantageous if the occupants are trying to sleep or relax at night.

[0283] In such an arrangement, if some image visibility to other users is acceptable, the reflective polarizer 302 can be omitted. Advantageously, display efficiency can be increased and visibility of stray light can be reduced.

[0284] FIG. 7B is a schematic diagram illustrating a side view of a motor vehicle 600 having a switched directional display 100 disposed within a passenger compartment 602 in a nighttime mode of operation.

[0285] In night mode, the volume of space 606 occupied by the driver's face falls within an area vertically defined by the light cone 620, and the image emitted by the switchable directional display 100 is therefore perceptible to the driver within the volume of space 606. However, for high angle light rays 622 that fall vertically outside of the light cone 620, there may be a reduction in brightness in night mode operation. The brightness of the high angle light rays 622 that may reflect off the windshield 618 of the motor vehicle 600 may be reduced. This may advantageously reduce the reflection of the display 100 perceived on the windshield 618 by the driver within the volume of space 606.

[0286] FIG. 8A is a schematic diagram showing a side perspective view of a switchable privacy display 100 for use in ambient lighting 604, including an emissive spatial light modulator 48, a switchable liquid crystal retarder 301 between quarter-wave plates 296A and 296B, a reflective polarizer 302, and an additional polarizer 318.

[0287] 8A shows an embodiment of a display device 100 in which the spatial light modulator 48 is provided by a display type that provides output light 400 by emitting light, such as an organic LED display (OLED) having an output polarizer 218. The output polarizer 218 can reduce the brightness of ambient light reflected from the OLED pixel plane by one or more retarders 518 inserted between the output display polarizer 218 and the OLED pixel plane. The one or more retarders 518 may be quarter wave plates and are different from any of the retarders 300 of the present disclosure. Features of the embodiment of FIG. 8A that are not discussed in further detail can be assumed to correspond to features having the same reference numbers described above, including any potential variations of the features.

[0288] Another type of emissive spatial light modulator 48 may be provided by micro LEDs formed as an addressable array of light-emitting pixels. The micro LEDs may be inorganic semiconductors and may have a size smaller than the respective pixel area. Different semiconductor materials, such as GaN and InP, as well as electroluminescent quantum dot and quantum rod structures, may be used to provide different emission wavelengths. The micro LED semiconductors may also be formed from the same semiconductors that can directly emit, for example, blue or UV light. The directly emitted light may be converted to other wavelengths by color conversion materials such as phosphors or quantum dot materials. Two or more emission wavelengths in a color band may be provided. Multiple emission wavelengths per color band may increase the color gamut of the display. Additionally, a yellow emitter may be provided to increase the color gamut of the display.

[0289] The micro LEDs may be addressed by addressing electrodes and drive circuitry that may be located at or near the micro LED pixels themselves, or may be located at the periphery of the array. The drive and control circuitry may be partitioned between the local micro LED pixel area and the ends of the rows and columns. The drive and addressing circuitry may also be located in a plane (e.g., a backplane) in a different plane than the plane of the micro LEDs.

[0290] 1A, the compensation retarder 330 is omitted. This omission changes the field profile. Advantageously, thickness, cost and complexity can be reduced.

[0291] FIG. 8B is a schematic diagram illustrating a side perspective view of a viewing angle control element 260 including a reflective polarizer 302, a switchable liquid crystal retarder 301 between quarter wave plates 296A and 296B, and an additional polarizer 318.

[0292] The viewing angle control optical element 260 is applied to the output side of the display device 100 for use in ambient lighting 604 and includes a spatial light modulator 48 arranged to output light, the spatial light modulator 48 including an output polarizer 218 arranged on the output side of the spatial light modulator 48, the viewing angle control optical element 260 including a spatial light modulator 48 including an additional polarizer 318, first and second quarter wave plates 296A and 296B, and at least one retarder arranged between the first quarter wave plate 296A and the second quarter wave plate 296B.

[0293] The viewing angle control optical element 260 may further include a reflective polarizer 302 arranged between the output polarizer 218 and an additional polarizer 318 when the viewing angle control optical element 260 is applied to the display device 100, and the retarder 300 including the first and second quarter-wave plates 296A and 296B, and at least one retarder may be arranged between the reflective polarizer 302 and the additional polarizer 318.

[0294] In use, the viewing angle control optical element 260 may be assembled separately from other components forming the display device 100. The viewing angle control optical element 260 may be attached to the polarizing output spatial light modulator 48 by a user or at the factory. The viewing angle control optical element 260 may be provided as a flexible film for curved and bent displays. The viewing angle control optical element 260 may be provided on a rigid substrate, such as a glass substrate.

[0295] The viewing angle control optical element 260 can advantageously provide a retrofit privacy and / or stray light control element that does not need to match the pixel resolution of the panel to avoid moiré artifacts. The viewing angle control optical element 260 of Figure 8B can be attached to an existing display device to form a display device 100 as shown in Figures 1A-1C.

[0296] FIG. 9 is a schematic diagram showing a perspective side view of an arrangement of multiple retarders 300 including a 270 degree super twist switched liquid crystal retarder 301 disposed between parallel polarizers 218 and 318 and between quarter wave plates 296A and 296B.

[0297] Figure 9 shows a different arrangement of multiple retarders 300 than that shown in Figure 2. The arrangement of multiple retarders 300 is the same as that shown in Figure 2, except that this arrangement does not include a passive retarder 330. Features of the arrangement of Figure 9 that are not discussed in further detail may be assumed to correspond to features having like reference numbers as described above, including any potential variations of the features.

[0298] 10A is a schematic graph showing the variation of the transmittance of the transmitted light beam in the public mode with respect to the polar direction in the arrangement of FIG. 9 with a 270 degree twist; FIG. 10B is a schematic graph showing the variation of the reflectance of the reflected light beam in the public mode with respect to the polar direction in the arrangement of FIG. 9 with a 270 degree twist; FIG. 10C is a schematic graph showing the variation of the transmittance of the transmitted light beam in the privacy mode with respect to the polar direction in the arrangement of FIG. 9 with a 270 degree twist; and FIG. 10D is a schematic graph showing the variation of the reflectance of the reflected light beam in the privacy mode with respect to the polar direction in the arrangement of FIG. 9 with a 270 degree twist.

[0299] Thus, it can be seen that the desired effects of increasing reflectivity as both the outer and elevation angles increase, as well as decreasing transmission as both the outer and elevation angles increase, can be advantageously achieved without the presence of a passive retarder 330 in the multiple retarders 300. Advantageously, thickness, complexity, and cost can be reduced.

[0300] The schematic graphs shown in Figures 10A-10D were obtained using several retarders 300 with the parameters listed in the first column of Table 2A. The desired ranges of retardation and other properties of the layers have been established by simulation of the retarder stack and experimentation with display optics stacks.

[0301] 11A is a schematic graph showing the variation of transmittance versus polarity direction of a transmitted light beam in the arrangement of FIG. 9 with a 450 degree twist in a public mode. FIG. 11B is a schematic graph showing the variation of reflectance versus polarity direction of a reflected light beam in the arrangement of FIG. 9 with a 450 degree twist in a public mode. FIG. 11C is a schematic graph showing the variation of transmittance versus polarity direction of a transmitted light beam in the arrangement of FIG. 9 with a 450 degree twist in a privacy mode. FIG. 11D is a schematic graph showing the variation of reflectance versus polarity direction of a reflected light beam in the arrangement of FIG. 9 with a 450 degree twist in a privacy mode.

[0302] Thus, it can be seen that the desired effect of increasing reflectance as both the outer and elevation angles increase, as well as decreasing transmittance as both the outer and elevation angles increase, can be advantageously achieved using a liquid crystal material with a 450° twist. Compared to Figures 10C-10D, the polar profile of Figures 11C-11D achieves a more rapid roll-off of transmittance and reflectance with polar angle. Advantageously, a high visual security level, VSL, can be achieved at smaller polar angles. Pryors cannot see the displayed image at polar angles closer to the user, and an increased privacy effect can be achieved.

[0303] 11A-11D were obtained using a number of retarders 300 having the parameters listed in the second column of Table 2A. The desired ranges of retardation and other properties of the layers have been established by simulation of the retarder stack and experimentation with display optics stacks. [Table 3]

[0304] In an embodiment corresponding to the first row of Table 2A, the layer 314 of liquid crystal material 414 may have a twist of 270 degrees and a retardance in the range of 650 nm to 800 nm, most preferably in the range of 700 nm to 720 nm, for light of a wavelength of 550 nm.

[0305] In an embodiment corresponding to the second row of Table 2A, the layer 314 of liquid crystal material 414 may have a twist of 450 degrees and a retardance in the range of 820 nm to 1000 nm, most preferably in the range of 880 nm to 920 nm, for light of a wavelength of 550 nm.

[0306] FIG. 12A is a schematic diagram showing an oblique side view of an arrangement of multiple retarders 300 including a 90° twisted nematic switchable liquid crystal retarder 301 disposed between parallel polarizers 218 and 318 and between quarter wave plates 296A and 296B.

[0307] Figure 12A shows a different arrangement of the multiple retarders 300 than that shown in Figure 9. The arrangement of the multiple retarders 300 is the same as that shown in Figure 9, except that the switchable liquid crystal retarder 301 is a twisted nematic layer with a 90 degree twist. Features of the arrangement of Figure 12A that are not discussed in further detail may be assumed to correspond to features having like reference numbers as described above, including any potential variations of the features.

[0308] Fig. 12B is a schematic graph showing the variation of transmittance with respect to the polarity direction of a transmitted light beam in the public mode in the arrangement of Fig. 12A. Fig. 12C is a schematic graph showing the variation of reflectance with respect to the polarity direction of a reflected light beam in the public mode in the arrangement of Fig. 12A. Fig. 12D is a schematic graph showing the variation of transmittance with respect to the polarity direction of a transmitted light beam in the privacy mode in the arrangement of Fig. 12A. Fig. 12E is a schematic graph showing the variation of reflectance with respect to the polarity direction of a reflected light beam in the privacy mode in the arrangement of Fig. 12A.

[0309] Thus, it can be seen that the desired effect of increasing reflectivity as both the outside and elevation angles increase, as well as decreasing transmittance as both the outside and elevation angles increase, can be advantageously achieved using liquid crystal material having a 90 degree twist. Compared to the super twisted embodiment described above, the narrowest polar profile is provided in elevation and lateral directions. Such a profile can advantageously reduce lateral and vertical stray light when used, for example, in automotive applications where reduction of stray light to windshields and side windows is desired.

[0310] The schematic graphs shown in Figures 12B-12E were obtained using multiple retarders 300 with the parameters listed in the first row of Table 2B. The desired ranges of retardation and other properties of the layers have been established by simulation of the retarder stack and experimentation with display optics stacks.

[0311] FIG. 13A is a schematic diagram showing a perspective side view of an arrangement of multiple retarders 300 including a twisted nematic liquid crystal retarder 301 with a 90 degree twist disposed between parallel polarizers 218 and 318, and a positive C-plate compensation retarder 330 disposed between quarter wave plates 296A and 296B.

[0312] Figure 13A shows a different arrangement of the multiple retarders 300 from that shown in Figure 12A. The arrangement of the multiple retarders 300 is the same as that shown in Figure 12A, except that this arrangement includes a passive retarder 330. The passive retarder 330 may include any of the features discussed for the passive retarder 330 included in the arrangement of Figure 2 above. Features of the arrangement of Figure 13A that are not discussed in further detail may be assumed to correspond to features having the same reference numbers as described above, including any potential variations of the features.

[0313] Fig. 13B is a schematic graph showing the variation of transmittance with respect to the polarity direction of a transmitted light beam in the public mode in the arrangement of Fig. 13A. Fig. 13C is a schematic graph showing the variation of reflectance with respect to the polarity direction of a reflected light beam in the public mode in the arrangement of Fig. 13A. Fig. 13D is a schematic graph showing the variation of transmittance with respect to the polarity direction of a transmitted light beam in the privacy mode in the arrangement of Fig. 13A. Fig. 13E is a schematic graph showing the variation of reflectance with respect to the polarity direction of a reflected light beam in the privacy mode in the arrangement of Fig. 13A.

[0314] 2, the passive retarder 330 in this arrangement can provide a quadratic improvement in the output of the display device 100. The presence of at least one passive retarder 330 in addition to the switched liquid crystal retarder 301 can provide a quadratic improvement in the chromaticity of the output of the display device 100.

[0315] The schematic graphs shown in Figures 13B-13E were obtained using multiple retarders 300 with the parameters listed in the second row of Table 2B. The desired ranges of retardation and other properties of the layers have been established by simulation of the retarder stack and experimentation with display optics stacks. [Table 4]

[0316] In embodiments corresponding to the first and second rows of Table 2B, the liquid crystal material layer may have a 90 degree twist and a retardance in the range of 420 nm to 550 nm, most preferably in the range of 460 nm to 480 nm, for light of a wavelength of 550 nm. In embodiments corresponding to the second row, the plurality of retarders 300 further includes a passive retarder 330.

[0317] The compensation retarder 330 can advantageously increase the visual security level at smaller polar angles than the uncompensated structure of Figure 12 A. Privacy performance is improved.

[0318] In the privacy mode, it may be desirable to achieve a minimum transmittance and for the reflective polarizer 302 to achieve a maximum reflectance at a lower polar angle than that shown in, for example, Figures 10C-10D.

[0319] 14A is a schematic diagram illustrating in perspective side view a switchable liquid crystal retarder 301 arrangement including a twisted nematic liquid crystal retarder with a 360 degree twist disposed between quarter wave plate 296A and quarter wave plate 296B. Features of the arrangement of FIG. 14A that are not discussed in further detail may be assumed to correspond to features having like reference numbers as described above, including any potential variations of the features.

[0320] FIG. 14B is a schematic graph showing the variation in transmittance versus polarity direction for a transmitted light ray in the public mode for the arrangement of FIG. 14A, and FIG. 14C is a schematic graph showing the variation in transmittance versus polarity direction for a transmitted light ray in the privacy mode for the arrangement of FIG. 14A, where FIGs. 14B-14C were obtained using multiple retarders 300 having the parameters listed in the first row of Table 3. [Table 5]

[0321] The liquid crystal material layer has a 360 degree twist and a retardance in the range of 1100nm to 1400nm, most preferably 1150nm to 1300nm, for light of wavelength 550nm. Advantageously, minimum brightness can be achieved at a polar angle of 55 degrees in the landscape direction and 50 degrees in the elevation direction. Such a display may be provided to achieve different polar angles for optimal privacy in landscape and portrait operation, for example FIG. 14C shows operation in a landscape orientation. Advantageously, in the landscape mode, enhanced uniformity in the landscape direction can be achieved, while in the portrait mode, maximum privacy is achieved.

[0322] For example, in fixed orientation displays such as desktop monitor displays and notebook displays, it may be desirable to provide some rotational symmetry in the brightness falloff. Embodiments may have a different twist than that defined by Equation 14.

[0323] Figure 15A is a schematic diagram illustrating in perspective side view a switchable liquid crystal retarder arrangement including a twisted nematic liquid crystal retarder with a 225 degree twist disposed between quarter wave plates. Features of the arrangement of Figure 15A that are not discussed in further detail can be assumed to correspond to features having like reference numbers as described above, including any potential variations of the features.

[0324] FIG. 15B is a schematic graph showing the variation in transmittance versus polarity direction for the transmitted light beam in the public mode for the arrangement of FIG. 15A, and FIG. 15C is a schematic graph showing the variation in transmittance versus polarity direction for the transmitted light beam in the privacy mode for the arrangement of FIG. 15A, and FIGS. 14B-14C were obtained using multiple retarders 300 having the parameters listed in the second row of Table 3.

[0325] The liquid crystal material layer may have a twist of 225 degrees and a retardance in the range of 750 nm to 1200 nm, most preferably in the range of 900 nm to 1100 nm, for light of a wavelength of 550 nm. Such a display may be provided to achieve different polar angles for optimal privacy in landscape and portrait operation, for example Figure 15C may show the operation in a portrait orientation. Advantageously, in the landscape mode, improved lateral uniformity may be achieved, while in the portrait mode, maximum privacy is achieved.

[0326] In the privacy mode of operation, it is desirable to provide a transmission minimum at a desired polar angle.

[0327] FIG. 16 is a schematic graph illustrating the variation of retarder retardance R with polar angle null θ for retarder 301 disposed between quarter wave plate 296A and quarter wave plate 296B.

[0328] The first and second quarter wave plates 296A, 296B and the at least one retarder 301 may be arranged to introduce a phase shift to the polarization components of light passing through one of the display polarizer 218 and the additional polarizer 318 on the input side of the at least one retarder 301, such that the at least one retarder 301 reduces the intensity of light passing through the other one of the display polarizer 218 and the additional polarizer 318 along an axis tilted with respect to the optical axis 199, such that the liquid crystal retarder 301, for light of a wavelength of 550 nm, is expressed by the equation: |R|=-0.02947 * θ 3 +5.81385 * θ 2 -397.950 * θ+10090 Equation 15 8. The profile 800 has a total absolute retardance value that is within 10% difference from the value R in nanometers as shown by the profile 800 obtained by:

[0329] The null polar angle θ is the outer angle of the tilt axis where the maximum reduction in brightness occurs relative to a tilt axis with an elevation angle of 0 degrees. The liquid crystal retarder may further have a retardance within 4% of the value R for light of a wavelength of 550 nm, as shown by profiles 802, 804.

[0330] In other words, profile 800 shows the desired optimum retardance R for a given null angle θ, where null angle θ is the polar angle (tilt from normal 199) for minimum transmission between parallel polarizer 218 and parallel polarizer 318. Profiles 802 and 804 represent the desired maximum and minimum retardances to achieve the desired polar angle of the null.

[0331] 16 further illustrates the desired retardance for various retarder twist angles described elsewhere herein. In one exemplary embodiment, a twist angle τ of 270 degrees provides a null position at a polar angle of approximately 63° with layer 301 having a nominal retardance of 710 nm.

[0332] The desired twist angle may not increase monotonically with increasing desired retardance, as illustrated elsewhere herein: the twist may correspond to Equation 14, or may have other values, for example 225 degrees in an exemplary embodiment with a null position close to 52 degrees.

[0333] The desired null position θ can be provided by selection of the retardance R and twist angle τ of the liquid crystal retarder 301. Advantageously, in the privacy mode of operation, a polar position θ of minimum transmission may be selected. The roll-off of visual security level with viewing angle can be arranged to provide the desired appearance of privacy.

[0334] It would be desirable to reduce the cost and thickness of privacy displays while achieving a high level of visual security against off-axis peepers in both landscape and portrait display orientations.

[0335] FIG. 17A is a schematic diagram showing an oblique side view of an arrangement of multiple retarders 300 including a negative C-plate passive retarder 330 disposed between the parallel polarizers 218 and 318, a quarter wave plate 296A disposed between the quarter wave plate 296B, and a reflective polarizer 303 disposed between the parallel polarizers 218 and 318.

[0336] Any features of the retarders 300 of FIG. 17A that are not discussed in further detail may be assumed to correspond to features with the same reference numbers described above, including any potential variations of the features. The arrangement of FIG. 17A differs from the arrangement of the retarders 300 already discussed in that the retarders 300 do not include a switchable retarder 301. Thus, the arrangement of FIG. 17A is a passive arrangement. A passive arrangement may be an arrangement in which the retarder layers in the retarders 300 remain in a particular orientation state that does not vary during operation of the device. Thus, a passive arrangement may not require power to operate. Thus, a passive arrangement can continue to operate in a privacy mode.

[0337] The at least one retarder may include at least one passive retarder. The at least one passive retarder may include any of the features of the passive retarder 330 described above. The at least one passive retarder may have an optical axis 431 perpendicular to a plane of the passive retarder. The at least one passive retarder may include a retarder including an oriented uniaxial birefringent material.

[0338] Thus, the material 430 of the C-plate retarder may have a negative dielectric anisotropy. The C-plate may include transparent birefringent materials such as polycarbonate or reactive mesogens, Zeonex™ cycloolefin polymers (COP), discotic polymers, and Nitto Denko™ double-stretched polycarbonate cast onto a substrate to provide homeotropic alignment.

[0339] The first and second quarter-wave plates 296A, 296B and at least one retarder 330 may be arranged to introduce a phase shift to the polarization components of light passing through one of the display polarizer 218 and the additional polarizer 318 on the input side of the at least one retarder, and the at least one retarder 330 reduces the intensity of light passing through the other one of the display polarizer and the additional polarizer along an axis tilted relative to the optical axis 199, and the at least one passive retarder 330 has a retardance R in nm given by Equation 15 for light with a wavelength of 550 nm.

[0340] Advantageously, the cost, complexity, and thickness of a privacy or low stray light display can be reduced compared to the active liquid crystal retarder embodiment of Figure 1 A. As will be further described below, further off-axis brightness can be reduced in displays with or without the liquid crystal retarder 301 by providing additional polarizers.

[0341] The magnitude and desired range of the at least one passive retarder 330 may be provided by Figure 16 and Equation 15 depending on the desired polar null angle θ. Advantageously, a desired angular roll-off for a privacy display can be provided.

[0342] Figure 17B is a schematic graph showing the variation of transmittance with respect to the polarity direction of a transmitted light beam in the arrangement of Figure 17A. Figure 17C is a schematic graph showing the variation of reflectance with respect to the polarity direction of a reflected light beam in the arrangement of Figure 17A.

[0343] As shown in Figure 17B, the transmittance remains high for a narrow range of outer and elevation angles around the 0 degree angle, but drops off sharply as both the outer and elevation angles increase. The drop in brightness is substantially symmetric around the azimuth angle. Thus, a display device 100 including the arrangement of multiple retarders 300 shown in Figure 17A can function as a privacy display in both horizontal and vertical directions, since a peeper does not receive light transmitted by the display device 100.

[0344] As shown in FIG. 17C, the reflectance remains high at a narrow range of outer and elevation angles around the 0 degree angle, but increases rapidly as both the outer and elevation angles increase. The increase in reflectance is substantially symmetric around the azimuth angle. Thus, a display device 100 including the display device arrangement of the multiple retarders 300 shown in FIG. 17A can function as a privacy display in both horizontal and vertical directions, and a peeper cannot distinguish the image transmitted by the display device 100 at wide angles because he receives more light reflected from the display device 100. Additionally, a display device 100 including multiple retarders 300, all of which are passive retarders, can reduce power consumption, thickness, and cost compared to an arrangement including a switchable layer.

[0345] The schematic graphs shown in Figures 17B-17C were obtained using several retarders 300 with the parameters listed in the first column of Table 4A. The desired ranges of retardation and other properties of the layers have been established by simulation of the retarder stack and experimentation with display optics stacks. [Table 6]

[0346] In an embodiment corresponding to the first column of Table 4A, the at least one passive retarder comprises a layer of liquid crystal material having a retardance in the range of -400nm to -1000nm, preferably in the range of -750nm to -850nm, for light with a wavelength of 550nm.

[0347] In an embodiment corresponding to the second column of Table 4A, the at least one passive retarder comprises a layer of liquid crystal material having a retardance in the range of -100 nm to -400 nm, preferably -200 nm to -300 nm, for light with a wavelength of 550 nm.

[0348] For the embodiments of Figures 24A-24D and 25A-25E, it may be desirable to provide passive reduction of off-axis brightness, as described further below.

[0349] Figure 18A is a schematic diagram showing in perspective side view an arrangement of a negative C-plate retarder 330 disposed between a quarter wave plate 296A and a quarter wave plate 296B which are disposed between the display polarizer 218 and a reflective polarizer 302. In comparison to the arrangement of Figure 17A, the multiple retarders 300 are arranged to provide only brightness reduction and do not change the reflective properties of the display.

[0350] FIG. 18B is a schematic graph showing the variation of transmittance versus polarity direction for the arrangement of FIG. 18A with the parameters listed in the second row of Table 4A.

[0351] 18A-18B in combination with a wide angle backlight and a transmissive or emissive display provides some off-axis brightness reduction, but does not provide the desired level of visual security without further brightness reduction and / or reflective enhancing elements. Advantageously, the public mode of operation provides the desired off-axis image visibility.

[0352] The combination of additional off-axis brightness reduction and optional reflection advantageously increases the level of visual security desired in the privacy mode of operation.

[0353] In another configuration, multiple retarders 300 may be placed between the reflective recycling polarizer 208 and the input display polarizer 210 of Figure 1A, as illustrated in the embodiments of Figures 19A, 27G, 27H, 27K, and 27L. Advantageously, the thickness of the screen can be reduced.

[0354] In another configuration, multiple retarders 300 may be disposed between an additional absorbing polarizer 318A and the input polarizer 210 of the transmissive SLM 48, as shown in Figures 27M, 27N, 27Q, and 27R below.

[0355] 15A-15C, the brightness roll-off with polar angle is reduced due to the lower retardance of retarder 330, and there is no variation in reflectance with polar angle. Advantageously, high angular visual security levels can be achieved when the output from the backlight or emissive spatial light modulator has high brightness at high polar angles.

[0356] Features of the arrangements of Figures 18A-18B that are not discussed in further detail can be assumed to correspond to features having like reference numbers as described above, including any potential variations of the features.

[0357] FIG. 18C is a schematic diagram showing an oblique side view of an arrangement of multiple retarders 300 including a passive positive C-plate retarder 330 disposed between the parallel polarizers 218 and 318, and between the quarter wave plates 296A and 296B, and a reflective polarizer 303 disposed between the parallel polarizers 218 and 318.

[0358] Figure 18C shows a different arrangement of the multiple retarders 300 from that shown in Figure 17A. The arrangement of the multiple retarders 300 is the same as that shown in Figure 17A, except that in this arrangement, the material 430 of the C-plate retarder 330 may have a positive dielectric anisotropy with the optic axis 431 perpendicular to the plane of the retarder 330. Features of the arrangement of Figure 18C that are not discussed in further detail may be assumed to correspond to features with like reference numbers as described above, including any potential variations of the features.

[0359] Figure 18D is a schematic graph showing the variation of transmittance with respect to the polarity direction of a transmitted light beam in the arrangement of Figure 18C. Figure 18E is a schematic graph showing the variation of reflectance with respect to the polarity direction of a reflected light beam in the arrangement of Figure 18C.

[0360] It can thus be seen that the desired effect of increasing reflectivity as both the outer and elevation angles increase, as well as decreasing transmission as both the outer and elevation angles increase, can be advantageously achieved using a passive retarder with positive dielectric anisotropy. In comparison with the negative C-plate of FIG. 17A, the positive C-plate 330 of FIG. 18C can be provided by a liquid crystal material, such as a reactive mesogenic material, that has a higher retardance compared to the stretched films typically used to provide negative C-plates. The area over which a desired level of visual security can be provided to off-axis peepers can be advantageously increased.

[0361] The schematic graphs shown in Figures 19A-19B were obtained using several retarders 300 with the parameters listed in Table 4B. The desired ranges of retardation and other properties of the layers have been established by simulation of the retarder stack and experimentation with display optics stacks. [Table 7]

[0362] In an embodiment corresponding to the first column of Table 4B, the at least one passive retarder comprises a layer of liquid crystal material having a retardance in the range of +500 nm to +1200 nm, preferably +800 nm to +1100 nm, for light with a wavelength of 550 nm.

[0363] In an embodiment corresponding to the second column of Table 4B, the at least one passive retarder comprises a layer of liquid crystal material having a retardance in the range of +200 nm to +600 nm, preferably +300 nm to +500 nm, for light with a wavelength of 550 nm.

[0364] For the embodiments of Figures 24A-25C, it may be desirable to provide passive reduction of off-axis brightness, as described further below.

[0365] Figure 19A is a schematic diagram showing in perspective side view an arrangement of a positive C-plate retarder 330 disposed between a quarter wave plate 296A and a quarter wave plate 296B which are disposed between the display input polarizer 210 and the reflective recirculating polarizer 208. In comparison to the arrangement of Figure 18C, the multiple retarders 300 are positioned to only reduce brightness and not perturb the reflective properties of the display.

[0366] FIG. 19B is a schematic graph showing the variation of transmittance versus polarity direction for the arrangement of FIG. 19A with the parameters listed in the second row of Table 4B.

[0367] Thus, the retarder 300 of Figures 19A-19B provides some off-axis brightness reduction that does not achieve the desired level of visual security when combined with a wide angle backlight or emissive display, but when combined with further off-axis brightness reduction and optionally increased reflectance, achieves the desired level of visual security in a privacy mode of operation.

[0368] In another arrangement (not shown), multiple retarders 300 may be disposed between the reflective polarizer 302 and the output display polarizer 218 of Figure 1A, as described in the exemplary embodiment of Figure 18A. Advantageously, for emissive displays, a desired level of privacy can be provided.

[0369] 18D-E, the brightness roll-off with polar angle is reduced due to the lower retardance of the retarder 330, and there is no variation in reflectance with polar angle. Advantageously, the cooperation of multiple retarders arranged between the additional polarizer (which may be the reflective polarizer 302) and the further additional polarizer 318 allows a reduced level of high angle visual security in a display with increased brightness at high angles.

[0370] Features of the arrangements of Figures 19A-19B that are not discussed in further detail can be assumed to correspond to features having like reference numbers as described above, including any potential variations of the features.

[0371] It would be desirable to achieve a further increase in the level of visual security for display 100 operating in privacy mode.

[0372] FIG. 20A is a schematic diagram showing a side perspective view of a switchable privacy display device 100 including a backlight 20, an additional polarizer 318, a first liquid crystal retarder 301 disposed between a first pair of quarter wave plates 296A, 296B, a spatial light modulator 48, a second liquid crystal retarder 301A disposed between a second pair of quarter wave plates 296AA, 296AB, and a further additional polarizer 318A.

[0373] The display polarizer may be the display polarizer 210 disposed at the input side of the spatial light modulator 48. A further additional polarizer 308A may be disposed at the input side of the display polarizer. One of the display polarizers and the one at the input side of the further 308A polarizer may be an additional polarizer.

[0374] FIG. 20A illustrates another embodiment for the display device 100. Features of the embodiment of FIG. 20A that are not discussed in further detail may be assumed to correspond to features with like reference numbers described above, including any potential variations of the features. The embodiment shown in FIG. 20A is an example of an arrangement that includes a further retarder 300A at the input side of the spatial light modulator 48. Also, although the embodiment includes a further retarder 300 at the output side of the spatial light modulator, it is not necessary to include a further retarder 300 at the output side, and only a further retarder 300A may be included in the display 100 to achieve the privacy effect as described above.

[0375] The further retarders 300A may include any of the features of the further retarders 300, as described above. In the embodiment shown in FIG. 20A, the further retarders 300A include further first and second quarter wave plates 296AA, 296AB and a further switchable retarder 301A. The further switchable retarder 301A includes a further liquid crystal material layer 314A having substrates 312A, 316A. In the case of the further switchable retarder, the input polarizer 210 of the spatial light modulator 48 acts as the display polarizer. The reflective polarizer 302 may not be included in the embodiment shown in FIG. 20A. The passive retarder 330 may not be included in the embodiment shown in FIG. 20A.

[0376] The further retarders 300A may act to reduce the brightness of transmitted light at wide viewing angles, similar to the retarders 300 described above. Thus, by including further retarders 300A in the display device in addition to the retarders 300, the brightness of light 400 at wide angles through the display device 100 when operating in privacy mode can be further reduced, thus improving the effectiveness of the privacy mode of the display device 100.

[0377] 20B is a schematic diagram illustrating a side perspective view of a switchable privacy display 100 including a backlight 20, a further plurality of retarders 1330 disposed between the reflective recirculating polarizer 208 and the transmissive spatial light modulator 48, a reflective polarizer 302, a plurality of retarders 300, and an additional polarizer 318. Features not discussed in further detail may be assumed to correspond to features having like reference numbers as described above, including any potential variations of the features.

[0378] Further retarders 1300 may be arranged between the reflective polarizer 302 and the additional polarizer 318. In the embodiment of Fig. 20B, the multiple retarders 1300 include a passive retarder 1330 and a switchable liquid crystal retarder 1301, but may generally be replaced by other configurations of at least one retarder, some examples of which are described above. The retarder 1300 does not affect the intensity of light passing through the reflective polarizer 302, the retarder 1300 and the additional polarizer 318 along an axis perpendicular to the plane of the retarder 1300, but the retarder 1300 reduces the intensity of light passing through the reflective polarizer 302, the retarder 1300 and the additional polarizer 318 along an axis perpendicular to the plane of the retarder 1300, at least in one of the switchable states of the switchable retarder 1301, along an axis inclined perpendicular to the plane of the retarder 1300. The principle behind this effect is explained in more detail below with reference to Figures 32A-33D and results from the presence or absence of a phase shift introduced by the passive retarder 1300 to light along an axis that is at a different angle to the liquid crystal material of the retarder 1300. The substrates 1312, 1316 of the switchable liquid crystal retarder 1301 include electrodes arranged to provide a voltage across the layer 1421 of liquid crystal material.

[0379] The embodiment shown in Figure 20A is an example of an arrangement that includes a plurality of retarders 300, including first and second 296A, 296B quarter wave plates, at the output of the spatial light 48, as well as a further plurality of retarders 1300 at the input of the spatial light modulator 48. Thus, as discussed above, an increased privacy effect is advantageously obtained by using the further plurality of retarders 1300 in addition to the plurality of retarders 300.

[0380] It may be desirable to increase the luminance roll-off with circular symmetry.

[0381] FIG. 21 is a schematic diagram showing a side perspective view of a viewing angle control element 260 including a passive compensation retarder 330BB between the quarter wave plates 296AB and 296BB, a reflective polarizer 302, and a switchable liquid crystal retarder 301A between the quarter wave plates 296AA and 296BA arranged between the reflective polarizer 302 and an additional polarizer 318.

[0382] The viewing angle control element 260 of Figure 21 represents an embodiment of the viewing angle control element 260 shown in Figure 8B. Features not discussed in further detail can be assumed to correspond to features having the same reference numbers as described above, including any potential variations of the features.

[0383] The viewing angle control optical element 260 is applied to the output side of a display device for use in ambient lighting 604 and includes a spatial light modulator 48 arranged to output light, the spatial light modulator 48 including an output polarizer 218 arranged on the output side of the spatial light modulator 48, the viewing angle control optical element 260 including an additional polarizer 318, a retarder 300A, and a further plurality of retarders 300B arranged between the output polarizer 218 and the additional polarizer 318.

[0384] In use, the viewing angle control optics 260 may be user or factory attached to the polarized output spatial light modulator 48. The viewing angle control optics 260 may be provided as a flexible film for curved and bent displays. The viewing angle control optics 260 may be provided on a rigid substrate, such as a glass substrate.

[0385] Advantageously, a retrofit privacy and / or stray light control element may be provided that combines the effect of the retarders 300A as described above with further retarders 300B. The viewing angle control optics 260 may further be provided for factory installation on the spatial light modulator 48. The retarders 300B may, for example, be similar to the retarders 300 shown in FIG. 18C.

[0386] By attaching the viewing angle control optical element 260 of FIG. 21 to an existing display device, a display device as shown in FIG. 20B can be formed.

[0387] In display 100, a wide-angle backlight 20 of a transmissive spatial light modulator 48 may be provided to illuminate structure 260. Backlight 20 may be manufactured with reduced tolerances compared to the narrow-angle backlights of Figures 28A-31 described below, and advantageously, costs may be reduced.

[0388] In operation within the display 100, an emissive spatial light modulator 48 may be provided to illuminate the structure 260. The retarder 300B provides a reduction in off-axis brightness, advantageously increasing the level of visual security. The thickness of the display may be reduced compared to a transmissive spatial light modulator 48 with a backlight 20.

[0389] We now describe an arrangement for providing lateral brightness falloff that does not have substantially circular symmetry.

[0390] 22 is a schematic diagram showing a viewing angle control element 260 in a side perspective view, including a reflective polarizer 1302, a passive compensation retarder 1330, a switchable liquid crystal retarder 1301, a further additional polarizer 1318, and a switchable liquid crystal retarder 301 between a quarter wave plate 296A and a quarter wave plate 296B arranged between the further additional polarizer 1318 and the additional polarizer 318. The viewing angle control element 260 of FIG. 22 represents an embodiment of the viewing angle control element 260 shown in FIG. 8B. Features not discussed in further detail may be assumed to correspond to features having similar reference numbers as described above, including any potential variations of the features.

[0391] The viewing angle control optical element 260 is applied to the output side of a display device and includes a spatial light modulator 48 arranged to output light, the spatial light modulator 48 including an output polarizer 218 arranged on the output side of the spatial light modulator 48, and the viewing angle control optical element 260 includes an additional polarizer 318, a retarder 300 and a further plurality of retarders 1300 arranged between the output polarizer 218 and the additional polarizer 318.

[0392] In use, the viewing angle control optics 260 may be user or factory attached to the polarized output spatial light modulator 48. The viewing angle control optics 260 may be provided as a flexible film for curved and bent displays. The viewing angle control optics 260 may be provided on a rigid substrate, such as a glass substrate.

[0393] Advantageously, retrofit privacy and / or stray light control elements may be provided that combine the effect of the retarders 300 as described above with further retarders 1300 as described below with reference to Figure 23 A. Viewing angle control optics 260 may further be provided as factory-fitted to the spatial light modulator 48.

[0394] By attaching the viewing angle control optical element 260 of FIG. 22 to an existing display device, a display device as shown in FIG. 20B can be formed.

[0395] Figure 23A is a schematic diagram showing an example arrangement of a further plurality of retarders 1300 in a side perspective view, including a passively controlled retarder 1308B, a passive compensation retarder 1308A, and a switchable liquid crystal retarder 1301. The arrangement shown in Figure 23A may be applied as an additional component in a display device, as described in any of the above embodiments.

[0396] FIG. 23B is a schematic graph showing the variation in transmittance versus polarity direction of the transmitted light of the arrangement of FIG. 23A in privacy mode, and FIG. 23C is a schematic graph showing the variation in transmittance versus polarity direction of the transmitted light of the arrangement of FIG. 23A in public mode.

[0397] The further plurality of retarders 1300 will now be described in more detail. The switchable liquid crystal retarder 1301 may include two surface alignment layers 1419a, 1419b adjacent to either side of the liquid crystal material 1421, each arranged to provide homogeneous alignment in the adjacent liquid crystal material layer 1421. The liquid crystal material layer 1421 of the switchable liquid crystal retarder 1301 may include a liquid crystal material 1421 having positive dielectric anisotropy. The liquid crystal material layer 1421 may have a retardance in the range of 500nm to 900nm, preferably in the range of 600nm to 850nm, and most preferably in the range of 700nm to 800nm ​​for light of wavelength 550nm. The further plurality of retarders 1300 includes a pair of passive retarders 1308A, 1308B having optical axes in the planes of the intersecting retarders, each passive retarder of the pair having a retardance in the range of 300 nm to 800 nm, preferably in the range of 350 nm to 650 nm, and most preferably in the range of 450 nm to 550 nm, for light of a wavelength of 550 nm.

[0398] The substrates 1312, 1316 of the switchable liquid crystal retarder 1301 may include electrodes arranged to provide a voltage across the layer of liquid crystal material 1421. A voltage driver 1350 may apply a voltage across the electrodes of the switchable liquid crystal retarder 1301.

[0399] The passive retarder 1330 may be provided by a pair of A-plates 1308A, 1308B with crossed optical axes 1309A and 1309B. In this embodiment, "crossed" refers to an angle of substantially 90° between the optical axes of the two retarders in the plane of the retarder. To reduce the cost of the retarder material, it may be desirable to provide a material with some variation in retarder orientation, for example due to stretching errors during film manufacturing. Variations in retarder orientation away from the preferred direction may reduce the front brightness and increase the minimum transmittance. Preferably, the angle 1310A is at least 35° and at most 55°, more preferably at least 40° and at most 50°, and most preferably at least 42.5° and at most 47.5°. Preferably, the angle 1310B is at least 125° and at most 145°, more preferably at least 130° and at most 135°, and most preferably at least 132.5° and at most 137.5°.

[0400] Passive retarders can be implemented using stretched films, advantageously providing low cost and high uniformity. The further field of view of the homogeneously aligned liquid crystal retarder is increased while restoring visibility of the flow of liquid crystal material under pressure.

[0401] The schematic graphs shown in Figures 24A and 24B were obtained using a further number of retarders 1300 with parameters listed in the first and second rows of Table 5. The desired ranges of retardation and other properties of the layers have been established by simulation of the retarder stack and experimentation with display optics stacks. [Table 8]

[0402] Figure 23D is a schematic graph showing the variation of reflectance versus polarity orientation of reflected light in privacy mode for an arrangement similar to Figure 23A where the output polarizer 210 is provided by an additional polarizer 318 and the input polarizer 1318 is provided by a reflective polarizer 302, as further shown in Figures 27A, 27G, and 27M, and using the parameters in Table 5. Control of off-axis reflectance may be provided to provide a switchable privacy display with high off-axis visual security levels.

[0403] It may be desirable to provide a spatial light modulator that has a broader brightness profile than can be achieved by the collimated backlight described with reference to Figures 29A-31.

[0404] Figure 24A is a schematic diagram illustrating in side view a switched privacy display for use in ambient lighting that includes a transmissive spatial light modulator 48 that includes a recirculating backlight 20. Features of the embodiment of Figure 24A that are not discussed in further detail can be assumed to correspond to features having like reference numbers described above, including any potential variations of the features.

[0405] A further additional polarizer 1318 is arranged at the output side of the first mentioned additional polarizer 318. The further additional polarizer 1318 is arranged to pass a linearly polarized polarization state. At least one further retarder 1300 is arranged between the first mentioned additional polarizer 318, 302 and the further additional polarizer 1318. The further retarder in the embodiment of Fig. 24A is of the type shown in Fig. 23A, but may be any other retarder 1300 or 300 exemplified elsewhere herein.

[0406] In Figure 24A the additional polarizer 318 is a reflective polarizer 302 and the at least one further retarder 1300 comprises a switchable liquid crystal retarder 1301 including a layer of liquid crystal material 1421 and electrodes 1413, 1415 arranged to apply a voltage to switch the layer of liquid crystal material. The at least one retarder 330 arranged between the pair of quarter wave plates 296A, 296B comprises at least one passive retarder having a retardance in the range of -100nm to -400nm, or +200nm to +600nm, preferably in the range of -200nm to -300nm, or preferably in the range of +300nm to +500nm, for light of wavelength 550nm.

[0407] The recycling backlight 20 includes an optical stack 5 that includes a diffuser 804 and prismatic films 806, 808. In operation, light from an input light source 15 is guided into the waveguide 1 and output to the diffuser 804. The light is recycled by refraction and total internal reflection at the prismatic films 806, 808 and reflected by the reflective recycling polarizer 208 to provide the profile as shown by contour 850. Advantageously, by recycling the backlight, high efficiency can be achieved in a thin package with high uniformity.

[0408] The switchable retarder 1300 is disposed between the reflective polarizer 302 and the additional polarizer 318. A plurality of passive retarders 300, including a passive retarder 330 disposed between the quarter wave plate 296A and the quarter wave plate 296B, are disposed between the reflective polarizer 302 and the output polarizer 218 of the spatial light modulator 48. The passive retarders have the arrangement of FIG. 17C and the parameters in the second column of Table 4A.

[0409] Figure 24B is a schematic graph showing the variation of output luminance with outer viewing angle for various components of the stack of Figure 24A. Profile 852 is provided by the multiple retarders 300 having retarder values ​​as shown in the second column of Table 4A, and profile 854 for the switchable retarder 1300 operating in privacy mode, with retarder parameters shown in Table 5. The polar angle θ1 of the switchable retarder 1300 is shown as 45 degrees, and the polar angle θ2 of the multiple retarders 1300 is greater than 90 degrees, i.e., the null is not reached at the widest viewing angle.

[0410] Thus, retarder 300 has a different polar angle θ for minimum transmission than that provided by retarder 1300 .

[0411] FIG. 24C is a schematic graph showing the variation of output luminance with outer viewing angle for the arrangement of FIG. 24A.

[0412] In the public mode of operation, a profile 856 is provided by multiple retarders 300 disposed between the display polarizer 218 and the reflective polarizer 302, which modify the profile 850 of the recycling backlight 20. Advantageously, the frontal brightness is substantially unchanged and the display has some visibility at outside angles greater than 50 degrees.

[0413] In a private mode of operation, profile 858 is provided by the additional effect of multiple retarders 1300 switched between the reflective polarizer 302 and the additional polarizer 318. Advantageously, the front brightness is substantially unchanged, and furthermore the brightness in the off-axis viewing regions 862R and 862L is reduced. Advantageously, a highly efficient and thin recycling backlight can provide a high level of visual security at high viewing angles.

[0414] In comparison, Figure 24D is a schematic graph showing the variation of output brightness with outer viewing angle for an arrangement similar to Figure 24A, but omitting the passive retarder disposed between the quarter wave plates, resulting in an unnecessary increase in brightness in regions 862R, 862L and a reduced level of visual security against off-axis observers, as compared to the arrangement of Figure 24C.

[0415] The arrangement of FIG. 24C can provide some color variation with viewing angle, especially at higher viewing angles, due to the variation of retardance with wavelength of each retarder 330, 296A, 296B.

[0416] It is desirable to provide reduced visibility of color change with viewing angle.

[0417] 25A is a schematic graph showing the variation of output intensity with wavelength when the additional polarizer 318 is broadband absorbing and when the additional polarizer 318 is leaky absorbing. Profile 870 shows the variation of transmitted intensity with parallel broadband polarizers. Profile 872 shows the variation of transmitted intensity with parallel leaky polarizers. Profile 874 shows the variation of transmitted intensity with crossed broadband polarizers and profile 876 shows the variation of transmitted intensity with crossed leaky polarizers. The leaky polarizer 318 has increased leakage and increased transmission in the blue spectral band.

[0418] The additional polarizer when crossed with a second conceptual polarizer of the same material has a transmittance at wavelengths from 520 nm to 560 nm that is less than the transmittance at wavelengths from 450 nm to 490 nm. The transmittance at wavelengths from 450 nm to 490 nm is greater than 1%, preferably greater than 2%, and most preferably greater than 3%. The transmittance at wavelengths from 520 nm to 560 nm is less than 3%, preferably less than 2%, and most preferably less than 1%.

[0419] Figure 25B is a schematic graph showing the variation of output color Δε with polarity direction of transmitted light for the arrangement of Figure 23A in the public mode for the broadband absorbing polarizer 318 of Figure 25A and the multiple retarders 300 of Figures 18A-18B. Figure 25C is a schematic graph showing the variation of output color Δε with polarity direction of transmitted light for the arrangement of Figure 23A in the public mode for the leaky absorbing polarizer 318 of Figure 25A and the multiple retarders 300 of Figures 18A-18B.

[0420] Advantageously, the color variation of a display including a leaky absorbing polarizer 318 is reduced compared to the color variation of a broadband polarizer 318. Additionally, the transmission efficiency is improved.

[0421] FIG. 25D is a schematic graph showing the variation of output color Δε versus polarity direction of transmitted light for the arrangement of FIG. 23A in the public mode of the broadband absorbing polarizer of FIG. 25A and the multiple retarders 300 of FIGS. 19A-19B, and FIG. 25E is a schematic graph showing the variation of output color Δε versus polarity direction of transmitted light for the arrangement of FIG. 23A for the leaky absorbing polarizer of FIG. 25A and the multiple retarders 300 of FIGS. 19A-19B.

[0422] Compared to the arrangement of Figures 25B-25C, color variation with polar angle is advantageously reduced while decreasing brightness at off-axis positions.

[0423] It is desirable to further reduce the color variation with viewing angle.

[0424] FIG. 26A is a schematic diagram showing a side view of a switchable privacy display for use in ambient lighting, including an emissive spatial light modulator 48 and a switchable retarder 1300 arranged between the reflective polarizer 302 (i.e., the additional polarizer 318) and a further additional polarizer 1318, where the multiple retarders 300 include a passive retarder 330 between the quarter-wave plates 296A, 296B arranged between the reflective polarizer 302 and the output polarizer 218 of the spatial light modulator 48, and an air gap 619 and a diffuser 805 arranged between the output polarizer 218 and the reflective polarizer 302.

[0425] The display device further comprises a reflective polarizer 302 arranged between the output polarizer 218 and the at least one further retarder 1300, the output polarizer 218 and the reflective polarizer 302 arranged to pass the same linearly polarised polarization state. The at least one further retarder comprises a switchable liquid crystal retarder comprising a layer of liquid crystal material and electrodes arranged to apply a voltage to switch the layer of liquid crystal material 1314. The at least one retarder 330 arranged between the pair of quarter wave plates 296A, 296B comprises at least one passive retarder 330 having a retardance in the range of 100nm to -400nm, or +200nm to +600nm, preferably in the range of -200nm to -300nm, or preferably +300nm to +500nm, for light with a wavelength in the range of 550nm.

[0426] The diffuser 805 in the air gap 619 provides mixing of the output angles from the passive retarder 330 and advantageously reduces the visibility of color shifts with polar viewing angles. The diffuser 805 and air gap 619 may be provided by, for example, a bulk diffuser layer that includes scattering elements.

[0427] It may be desirable to provide a diffuser for color mixing at the input of the spatial light modulator 48 .

[0428] FIG. 26B is a schematic diagram showing a side view of a switchable privacy display for use in ambient lighting, including a transmissive spatial light modulator 48, a recirculating backlight 20, and a switchable retarder 1300 arranged between a reflective polarizer 302 and a further additional polarizer 1318, the multiple retarders 300 including a passive retarder 330 between quarter-wave plates 296A, 296B arranged between the reflective recirculating polarizer 208 (i.e. the additional polarizer 318) and the input polarizer 210 of the spatial light modulator 48, an air gap 619 and a diffuser 805.

[0429] The reflective recirculating polarizer 208 operates differently than the reflective polarizer 302, as described elsewhere herein. The angular variation provided by the multiple retarders 300 may be similar to that shown in FIG. 25C, for example. The surface relief diffuser 805 can provide angular mixing of the light from the display and reduce the variation in color shift. Advantageously, privacy performance can be enhanced while using a thin and efficient recirculating backlight. Additionally, the retardance of the passive retarder 330 can be increased, providing a reduction in brightness at smaller polar angles, advantageously improving privacy performance while minimizing color difference.

[0430] In other embodiments, a collimated backlight may be provided, such as those described below with respect to Figures 29A-31. Off-axis brightness can be reduced and advantageous thickness and off-axis color variation can be achieved.

[0431] Features of the embodiments of Figures 26A-26B that are not discussed in further detail can be assumed to correspond to features having like reference numbers as described above, including any potential variations of the features.

[0432] Various stacking arrangements for the multiple retarders 300, 1300 and the multiple retarders 300A, 300B are illustrated herein.

[0433] Figures 27A-27F are schematic diagrams illustrating, in side view, various arrangements of passive and active retarders in series with an emissive spatial light modulator 48. Features of the embodiments of Figures 27A-27F that are not discussed in further detail can be assumed to correspond to features having like reference numbers as described above, including any potential variations of the features.

[0434] 27G-X and 27AA-AF are schematic diagrams showing various arrangements of passive and active retarders in series with a transmissive spatial light modulator 48 in side view. For clarity, some components such as the backlight are omitted. Features of the embodiments of FIGS. 27G-X and 27AA-AF that are not discussed in further detail can be assumed to correspond to features with like reference numbers as described above, including any potential variations of the features. The multiple retarders 300, 300A, 300B may include passive or active retarder components as described elsewhere herein.

[0435] The positions of the multiple retarders 300, 1300 or 300A, 300B can be optimized to provide improved performance, including visual security level, image visibility, transmission efficiency, screen thickness, total thickness, and cost.

[0436] 26B, 27G, 27H, 27K, and 27L show embodiments in which at least one retarder between crossed quarter-wave plates is disposed between the reflective recirculating polarizer 208 and the display input polarizer 210.

[0437] 27M, 27N, 27Q, and 27R show embodiments in which at least one retarder between crossed quarter-wave plates is disposed between the absorbing additional polarizer 318 and the display input polarizer 210.

[0438] Thus, an output polarizer 218 is positioned at the output side of the spatial light modulator 48, and a further polarizer 1318, 318B is positioned at the output side of the output polarizer 210, the output polarizer 210 and the further polarizer are positioned to pass their respective linearly polarized polarization states, and at least one further retarder 1300, 300B is positioned between the output polarizer 218 and the further polarizer 1318, 318B.

[0439] FIG. 28A is a schematic diagram showing a side perspective view of a viewing angle control element 260 including a passive compensation retarder stack 700, a reflective polarizer 302, and a switchable liquid crystal retarder 301 between a quarter wave plate 296A and a quarter wave plate 296B arranged between the reflective polarizer 302 and an additional polarizer 318.

[0440] Figure 28A shows an embodiment of the viewing angle control element 260 different from that shown in Figure 8B. In addition to the embodiment of Figure 8B, the viewing angle control element 260 may include a passive retarder stack 700 including passive retarders 702A, 702B, 702C and 702D. Features not discussed in further detail may be assumed to correspond to features with like reference numbers as described above, including any potential variations of the features.

[0441] In use, the viewing angle control element 260 may be attached by a user to a polarizing output spatial light modulator. The viewing angle control element 6260 may be provided as a flexible film for curved and bent displays. The viewing angle control element 601 may be provided on a rigid substrate, such as a glass substrate.

[0442] Advantageously, a post-add privacy and / or stray light control element may be provided that combines the effect of multiple retarders 300 as described above with the passive retarder stack 700 described below. The viewing angle control element may be cut to the size and orientation of the output polarizer 218.

[0443] FIG. 28B is a schematic diagram illustrating a side perspective view of an example passive retarder stack 700 that includes a passive controlled retarder that includes a series of four aligned A-plates.

[0444] The passive retarder stack 700 is discussed in more detail herein. The in-plane angle refers to the in-plane rotation of the slow axis orientation of the retarder material compared to the electric vector transmission directions 211, 1319 of the input polarizer 210 and the further additional polarizer 1318, respectively.

[0445] Thus, the passive retarder stack 700 may include a pair of passive retarders 702A, 702D with slow axes in the plane of the crossed retarders. The pair of retarders each includes a plurality of A-plates with their respective slow axes oriented at different angles to each other. The passive retarder stack 700 may further include an additional pair of passive retarders 702B, 702C disposed between the first mentioned pair of passive retarders 702A, 702D. The pair of passive retarders 702B, 702C may have their slow axes extending at 90° and 0°, respectively, with respect to an electric vector transfer direction that is parallel to the electric vector transfer 211 of the display polarizer 210. Thus, the pair of passive retarders 702B, 702C is an example of a passive retarder 330 including a plurality of passive retarders with their optical axes parallel to the plane of the crossed passive retarders.

[0446] Each passive retarder 702A, 702B, 702C, 702D may include birefringent molecules 704A, 704B, 704C, 704D, which may be fixed liquid crystals, such as UV cured reactive mesogens, stretched polymers, or other known birefringent materials.

[0447] The pair of passive retarders 702A, 702D may have their slow axes extending at 45° and 135°, respectively, with respect to an electric vector propagation direction 211 that is parallel to the electric vector propagation of the output polarizer 210.

[0448] FIG. 28C is a schematic graph showing the variation in transmittance with respect to the polar direction of a transmitted light beam in the arrangement of FIG. 28B.

[0449] The graph represents the variation in polarity of the relative transmission of the retarder 700 stack between the parallel input polarizer 210 and the additional polarizer 1318. Thus, on-axis light rays do not vary substantially in brightness except for losses from polarized front transmission and Fresnel reflection losses. Off-axis light rays experience a decrease in brightness as shown.

[0450] The schematic graph shown in Figure 28C was obtained using a number of additional retarders 1300 with the parameters listed in Table 6. The desired ranges of retardation and other properties of the layers have been established by simulation of the retarder stack and experimentation with display optics stacks. [Table 9]

[0451] Table 7 is an example of another passive retarder stack 700 that is the same as that shown in Table 6, except for the different parameters set out in the table. The quarter wave plates 702A, 702D in Table 6 are zero order wave plates, and the quarter wave plates 702A, 702D in Table 7 are second order quarter wave plates (q=2 in Equation 13).

[0452] Optionally, the present disclosure may exclude either or both of the cases shown in Tables 6 and 7.

[0453] Optionally, the present disclosure may exclude cases where the pair of quarter-wave plates have slow axes that intersect at 45° and 135°, respectively, and the retarder further includes an additional pair of passive retarders disposed between the quarter-wave plates and having slow axes in the plane of the intersecting passive retarders.

[0454] Optionally, the present disclosure may exclude cases where one of the pair of quarter-wave plates has a slow axis that extends at least 40° and at most 50° relative to an electric vector transmission direction parallel to the electric vector transmission of the display polarizer, and the other of the pair of quarter-wave plates has a slow axis that extends at least 130° and at most 140° relative to an electric vector transmission direction parallel to the electric vector transmission of the display polarizer, and the retarder further includes an additional pair of passive retarders disposed between the quarter-wave plates, one of the additional pair of passive retarders having a slow axis in the plane of the passive retarder that extends at least -10° and at most 10° relative to an electric vector transmission direction parallel to the electric vector transmission direction of the display polarizer, and the other of the additional pair of passive retarders having a slow axis in the plane of the passive retarder that extends at least -10° and at most 10° relative to an electric vector transmission direction parallel to the electric vector transmission of the display polarizer. [Table 10]

[0455] Figure 29A is a schematic diagram illustrating the operation of an imaging waveguide 1 in a narrow angle mode of operation from a rear perspective view. The example waveguide 1 shown in Figure 29A can be combined with any of the embodiments of the display device 100, as described above.

[0456] Light rays 400A, 400B provided by an input set of light sources 15 are directed to the reflecting end 4 and are directed within the waveguide 1 to the extraction features 12 by total internal reflection within the waveguide 1. The waveguide 1 includes a light input face 2 extending along a first end of the directional waveguide 1 and a reflecting end 4 extending along a second end of the directional waveguide 1 opposite the first end 2 for reflecting the input light back along the waveguide 1, the reflecting end 4 having a positive refractive power in the lateral direction. The second guide face 8 includes light extraction features 12 and intermediate regions 10 between the light extraction features 12, the light extraction features 12 being oriented to polarize the input light 400A, 400B reflected through the first guide face 6 as output light, and the intermediate region 10 being arranged to direct the light through the waveguide 1 without extracting light.

[0457] Imaging waveguides and optical systems incorporating imaging waveguides are described in US Pat. Nos. 9,519,153 and 10,054,732, both of which are incorporated herein by reference in their entireties.

[0458] As shown in FIG. 29A, the light extraction features may have positive lateral optical power that cooperates with the optical power of the curved reflective end 4 to provide imaging of the set of light sources 15.

[0459] The reflected light rays 400A, 400B are directed in the z-direction towards the optical window 26 in the window surface 106. The optical window 26 is an image of the set of light sources 15 in the lateral (y-axis).

[0460] Thus, the second guide surface 8 is arranged to polarise the reflected input light 400A, 400B through the first guide 6 surface as output light, and the waveguide 1 is arranged to image the set of light sources 15 laterally such that the output light from the light sources is directed to the respective optical windows 26 in output directions that are dispersed depending on the input positions of the light sources of the set of light sources 15.

[0461] Advantageously, the imaging waveguide demonstrates an off-axis relative luminance p at 45 degree outer angle of less than 1.5%. In this embodiment, such a waveguide 1 can achieve an off-axis luminance in privacy mode of less than 0.1% when the liquid crystal retarder is appropriately driven. Very high levels of visual image security, VSL, can be achieved, including VSL>4. Such displays may have improved performance in low light environments when display settings of >1 nit / lux are desired.

[0462] FIG. 29B is a schematic graph showing the viewing luminance plot of the output of FIG. 29A when used in a display device that does not have a switchable liquid crystal retarder.

[0463] Advantageously, waveguide imaging can achieve relatively low levels of off-axis brightness for off-axis viewing systems, for example providing less than 5% front-on brightness at an outside angle of 45 degrees and an elevation of 0 degrees.

[0464] Such a waveguide 1 has a thickness limited by its efficiency, which is determined by the height of the light source and the relative heights of the reflecting end 4 and the input end 2. It would be desirable to reduce the thickness.

[0465] Figure 30 is a schematic diagram showing a side view of a backlight 20 including a collimating waveguide 901. The example backlight 20 shown in Figure 30 can be combined with any of the embodiments of display device 100, as described above.

[0466] The directional waveguide 901 includes first and second light input faces 902A, 902B extending laterally along opposing ends of the waveguide 901, a set of light sources 915, 917 disposed along each respective light input face 902A, 902B, and opposing first and second light guiding faces 906, 908 extending from the light input face 902A across the waveguide 901 for guiding input light 400 along the waveguide 901 by total internal reflection, the waveguide being arranged to polarize the input light guided through the waveguide for output through the first guiding face 906.

[0467] The at least one optical input surface includes a first input surface 902A extending along a first end of the waveguide 901 and a second input surface 902B extending along a second optical input end of the waveguide 901, the second optical input end facing the first optical input end.

[0468] Reflector 903 may be positioned to polarize light scattered to the rear of backlight 20 to increase efficiency. Output light beam 400 is output from the waveguide and may be further polarized by turning film 927 and diffused by surface 950 located on or near the output of turning film 927.

[0469] FIG. 31 is a schematic graph showing the viewing luminance plot of the output of FIG. 30 when used in a display device that does not have a switchable liquid crystal retarder.

[0470] Advantageously, such an illumination profile can provide highly efficient illumination to a front user. Moreover, such a profile is substantially uniform along the length of the light guide 901, achieving high brightness uniformity and reduced power consumption. Such a profile can also be used to provide a privacy mode operation of the backlight 20 for a privacy display 100.

[0471] The operation of the retarder layer between parallel polarizers for off-axis illumination will now be further described. In the various devices described above, the at least one retarder is arranged between the reflective polarizer 318 and the additional polarizer 218 in various different configurations. In each case, the at least one retarder is configured to reduce the intensity of light passing through the reflective polarizer 318, the at least one retarder, and the additional polarizer 218 along an axis normal to the plane of the retarder, but not affect the intensity of light passing through the reflective polarizer 318, the at least one retarder, and the additional polarizer 218 along an axis tilted with respect to the normal to the plane of the retarder, in at least one of the switchable states of the compensated switchable retarder 300. This effect will now be described in more detail. This principle can be generally applied to all devices described above.

[0472] 32A is a schematic diagram illustrating illumination of a retarder layer by off-axis light in a perspective view. The compensation retarder 630 may include a birefringent material represented by an index ellipsoid 632 with an optical axis direction 634 of 0 degrees relative to the x-axis and having a thickness 631. Ordinary ray 636 propagates such that its path length in the material is the same as the thickness 631. Ray 637 has an increased path length in the yz-plane, but the birefringence of the material is substantially the same as ray 636. In comparison, ray 638, which is in the xz-plane, has an increased path length in the birefringent material and also has a different birefringence than ordinary ray 636.

[0473] The retardance of retarder 630 thus depends on the angle of incidence of each ray, and the plane of incidence for ray 638 in xz has a different retardance than the normal 636 and ray 637 in the yz plane.

[0474] We now describe the interaction of polarized light with retarder 630. To distinguish between the first and second polarization components during operation in a directional backlight 101, the following description refers to the third and fourth polarization components.

[0475] FIG. 32B is a schematic diagram showing, in a perspective view, illumination of the retarder layer by off-axis light in a third linear polarization state at 90 degrees relative to the x-axis, and FIG. 32C is a schematic diagram showing, in a perspective view, illumination of the retarder layer by off-axis light in a fourth linear polarization state at 0 degrees relative to the x-axis. In such a configuration, the input linear polarization state is aligned with the optical axis of the birefringent material represented by the ellipse 632. Thus, no phase difference is provided between the third and fourth orthogonal polarization components, resulting in no change in the polarization state of the linearly polarized input for each of the light rays 636, 637, 638. Thus, the retarder 630 does not introduce a phase shift in the polarization components of the light that passes through the polarizer on the input side of the retarder 630 along an axis normal to the plane of the retarder 630. Thus, the retarder 630 does not affect the brightness of the light passing through the retarder 630, and the polarizers (not shown) on either side of the retarder 630. Although Figures 29A-29B are specifically directed to a passive retarder 630, a similar effect can be achieved with a switched liquid crystal retarder, and with multiple retarders in the devices described above.

[0476] FIG. 32D is a schematic diagram illustrating, in a perspective view, illumination of a layer of retarder 630 with off-axis light of a 45 degree linearly polarized polarization state. The linear polarization state can be decomposed into third and fourth polarization components that are orthogonal and parallel, respectively, to the direction of the optical axis 634. The retarder thickness 631 and material retardance, as indicated by the index ellipsoid 632, can have the net effect of relatively shifting the phase of the normally incident third and fourth polarization components, as indicated by light ray 636, by half a wavelength for the design wavelength. The design wavelength can be, for example, in the range of 500-550 nm.

[0477] For the design wavelength and light propagating vertically along ray 636, the output polarization may be rotated 90 degrees at -45 degrees to a linear polarization state 640. Light propagating along ray 637 may experience a phase difference similar to, but not identical to, the phase difference along ray 637 due to thickness changes, and thus an elliptical polarization state 639 may be output that may have a principal axis similar to the linear polarization axis of the output light in ray 636.

[0478] In contrast, the phase difference of the incident linear polarization state along the light ray 638 may be significantly different, and in particular, a lower phase difference may be provided. Such a phase difference may result in an output polarization state 644 that is substantially circular at a given tilt angle 642. Thus, the retarder 630 introduces a phase shift to the polarization components of the light passing through the polarizer on the input side of the retarder 630 along an axis corresponding to the light ray 638 that is tilted with respect to the normal to the plane of the retarder 630. Although FIG. 29B relates to a passive retarder 630, a similar effect is achieved with a switchable liquid crystal retarder, and in the multiple retarders described above, in the switched states of the switchable liquid crystal retarder corresponding to the privacy mode.

[0479] To illustrate the off-axis behavior of the retarder stack, we now describe the angular brightness control of the C-plates 308A, 308B between the additional polarizer 318 and the output display polarizer 218 for various off-axis illumination configurations with reference to the operation of the C-plate between parallel polarizers 500, 210.

[0480] FIG. 33A is a schematic diagram showing, in a perspective view, the illumination of a C-plate layer with off-axis polarized light with a positive elevation angle. An incident linearly polarized component 704 enters the birefringent material 632 of the retarder 560, which is a C-plate with an optic axis direction 507 perpendicular to the plane of the retarder 560. The output polarization component is the same as component 704, since the polarization component 704 experiences no net phase difference in transmission of the liquid crystal molecules. Thus, maximum transmission occurs at the polarizer 210. Thus, the retarder comprises a retarder 560 with an optic axis 561 perpendicular to the plane of the retarder 560, which is the xy plane. A retarder with an optic axis perpendicular to the plane of the retarder 560 comprises a C-plate.

[0481] FIG. 33B is a schematic diagram showing, in a perspective view, illumination of a C-plate layer with off-axis polarized light having a negative outer angle. As with the arrangement of FIG. 33A, polarization state 704 results in no net phase difference and is transmitted with maximum brightness. Thus, retarder 560 introduces no phase shift to the polarization components of light passing through the polarizer on the input side of retarder 560 along an axis normal to the plane of retarder 560. Thus, retarder 560 has no effect on the brightness of light passing through retarder 560, and on the polarizers (not shown) on either side of retarder 560. Although FIGS. 29A-B are specifically directed to a passive retarder 560, similar effects are achieved with switchable liquid crystal retarders, and with multiple retarders in the devices described above.

[0482] Figure 33C is a schematic diagram showing illumination of a C-plate layer with off-axis polarized light with positive elevation angle and negative outer angle in a perspective view. Compared to the arrangement of Figures 33A-B, the polarization state 704 decomposes into eigenstates 703, 705 with respect to the birefringent material 632, resulting in a net phase difference in transmission of the retarder 560. The resulting elliptically polarized component 656 is transmitted through the polarizer 210 and has reduced brightness compared to the light beam shown in Figures 33A-B.

[0483] FIG. 33D is a schematic diagram showing in a perspective view the illumination of a C-plate layer with off-axis polarized light with positive elevation and positive outward angles. Similar to FIG. 33C, the polarized light components 704 are decomposed into eigenstates 703, 705 that experience a net phase difference, resulting in an elliptically polarized light component 660 that reduces the brightness of each off-axis ray after transmission through the polarizer. Thus, the retarder 560 introduces a phase shift to the polarized components of light that pass through the polarizer on the input side of the retarder 560 along an axis that is tilted with respect to the normal to the plane of the retarder 560. Although FIG. 29B relates to a passive retarder 560, a similar effect is achieved with a switchable liquid crystal retarder and in the multiple retarders described above, in which the switchable states of the switchable liquid crystal retarder correspond to the privacy modes.

[0484] Figure 34 is a schematic graph showing the variation of output transmittance versus polarity direction for the transmitted light beam of Figures 33A-33D. Thus, the C-plate can provide brightness reduction in the polar quadrants. In combination with a switched liquid crystal retarder 314 described elsewhere herein, (i) the C-plate brightness reduction can be eliminated in a first, wide angle operating state, and (ii) an extended polar region for brightness reduction can be achieved in a second, privacy operating state.

[0485] To illustrate the off-axis behavior of the retarder stack, we now describe the angular brightness control of crossed A-plates 308A, 308B between the additional polarizer 318 and the output display polarizer 218 for various off-axis illumination configurations.

[0486] FIG. 35A is a schematic diagram showing the illumination of a crossed A-plate retarder layer with off-axis polarized light having a positive elevation angle in a perspective view. A linear polarizer 218 with an electric vector transfer direction 219 is used to produce a linear polarization state 704 on the first A-plate 308A of the crossed A-plates 308A, 308B that is parallel to the horizontal direction. The optical axis direction 309A is tilted +45 degrees to the horizontal direction. The retardance of the retarder 308A for the off-axis angle θ1 in the positive elevation direction provides a resultant polarization component 650 at the output side that is approximately elliptical. The polarization component 650 is incident on the second A-plate 308B of the crossed A-plates 308A, 308B with an optical axis direction 309B perpendicular to the optical axis direction 309A of the first A-plate 308A. 35A, the retardance of the second A-plate 308B for off-axis angle θ1 is equal and opposite to the retardance of the first A-plate 308A. Thus, the input polarization component 704 has a net zero retardation and the output polarization component is identical to the input polarization component 704.

[0487] The output polarization components match the electric vector transmission direction of the additional polarizer 318 and are therefore efficiently transmitted. Advantageously, light rays with zero transverse angular components experience substantially zero loss, resulting in perfect transmission efficiency.

[0488] 35B is a schematic diagram showing in a perspective view the illumination of a crossed A-plate retarder layer with off-axis polarized light having a negative outer angle. Thus, the input polarization component is converted by the first A-plate 308A into an intermediate polarization component 652, which is an approximately elliptical polarization state. The second A-plate 308B again introduces an equal and opposite retardation to the first A-plate, so that the output polarization component is identical to the input polarization component 704, and the light is efficiently transmitted through the polarizer 318.

[0489] The retarder thus comprises a pair of retarders 308A, 308B which are crossed and have their optic axes in the plane of the retarders 308A, 308B, i.e. the xy plane in this embodiment. The pair of retarders 308A, 308B have optic axes 309A, 309B each extending at 45° with respect to an electric vector propagation direction which is parallel to the electric vector propagation direction of the polarizer 318.

[0490] Advantageously, light rays having a zero elevation angular component experience substantially zero loss, resulting in perfect transmission efficiency.

[0491] 35C is a schematic diagram illustrating illumination of a crossed A-plate retarder layer with off-axis polarized light having a positive elevation angle and a negative outer angle in a perspective view. Polarized light component 704 is converted to an elliptically polarized light component 654 by the first A-plate 308A. The resulting elliptically polarized light component 656 is output from the second A-plate 308B. The elliptically polarized light component 656 is analyzed by the input polarizer 318 and has a reduced intensity compared to the input intensity of the first polarized light component 704.

[0492] 35D is a schematic diagram illustrating illumination of a crossed A-plate retarder layer with off-axis polarized light having a positive elevation angle and a positive outward angle in a perspective view. Polarization components 658 and 660 are introduced by the first A-plate 308A and the second A-plate 308B because the net retardance of the first and second retarders does not compensate.

[0493] Thus, brightness is reduced for rays with non-zero lateral and non-zero elevation components. Advantageously, visibility for the primary display user is not substantially reduced, but the privacy of the display can be improved for peepers located in the viewing quadrant.

[0494] Figure 36 is a schematic graph showing the variation of output transmittance with polar direction for transmitted light beams of Figures 35A-35D. Compared to the arrangement of Figure 34, the area of ​​brightness drop increases for off-axis viewing. However, with the switchable liquid crystal retarder layer 301, the uniformity may decrease compared to the C-plate arrangement for off-axis viewing in the first wide angle operating mode state.

[0495] As may be used herein, the terms "substantially" and "approximately" provide an industry-accepted tolerance for corresponding terms and / or relative items. Such industry-accepted tolerances may range from 0 percent to 10 percent, including, but not limited to, component values, angles, and the like. Such relative items may range from about 0 percent to 10 percent.

[0496] Although various embodiments according to the principles disclosed herein have been described above, it should be understood that they are presented by way of example only and not by way of limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the exemplary embodiments described above, but should be defined only in accordance with any of the claims and their equivalents derived from this disclosure. Furthermore, while the above-mentioned advantages and features are provided in the described embodiments, it is not intended to limit the application of such published claims to methods and structures that achieve any or all of the above-mentioned advantages.

[0497] In addition, section headings herein are provided as organizational markers, as required by 37 CFR 1.77, and are not intended to limit or characterize the embodiment(s) set forth in the claims that may arise from this disclosure. Specifically, and by way of example only, there is a heading entitled "Technical Field," but the language selected under this heading to describe the so-called field is not intended to limit the scope of the claims. Furthermore, any description of technology set forth in the "Background" should not be construed as an admission that a particular technology is prior art to any embodiment(s) in this disclosure. Nor should the "Summary" be considered as characterizing the embodiment(s) set forth in the published claims. Furthermore, any reference to "invention" in the singular form in this disclosure should not be used to assert that there is only one novel aspect of this disclosure. Multiple embodiments may be set forth by this disclosure, subject to the limitations of the published claims. Accordingly, these claims are intended to define and protect this embodiment(s) and their equivalents. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, and should not be constrained by the headings set forth herein.

Claims

1. 1. A display device, comprising: a spatial light modulator arranged to output light along an output direction; a display polarizer disposed on a side of the spatial light modulator; an additional polarizer disposed on the same side of the spatial light modulator as the display polarizer, the display polarizer and the additional polarizer being positioned to pass their respective linearly polarized polarization components; a first quarter-wave plate and a second quarter-wave plate arranged between the additional polarizer and the display polarizer, the first quarter-wave plate being arranged on the input side of the second quarter-wave plate and arranged to convert a linearly polarized polarization state passing through one of the display polarizer and the additional polarizer at the input side of the first quarter-wave plate into a circularly polarized polarization state, and the second quarter-wave plate on the output side being arranged to convert a circularly polarized polarization state incident thereon into a linearly polarized polarization state passing through the other of the display polarizer and the additional polarizer at the output side of the second quarter-wave plate; and at least one retarder disposed between the pair of quarter wave plates.

2. 10. The display device of claim 1, wherein the pair of quarter wave plates each comprises a passive quarter wave plate including a layer of oriented uniaxially birefringent material.

3. 3. The display device of claim 2, wherein the pair of quarter wave plates have crossed optical axes.

4. 4. A display device according to claim 2 or 3, wherein each of the pair of quarter wave plates has an optical axis arranged at 45 degrees to an electric vector propagation direction of the adjacent display polariser or the additional polariser.

5. A display device according to any one of claims 2 to 4, wherein the pair of quarter wave plates each have a retardance in the range of 110nm to 175nm, preferably 130nm to 140nm, for light with a wavelength of 550nm.

6. A display device according to any preceding claim, wherein the retarder comprises a liquid crystal retarder comprising a layer of liquid crystal material.

7. 7. The display device of claim 6, wherein the liquid crystal retarder is a switched liquid crystal retarder further comprising electrodes arranged to apply a voltage to switch the layer of liquid crystal material.

8. 8. A display device as claimed in claim 6 or 7, wherein the liquid crystal retarder comprises two surface alignment layers adjacent to and disposed on either side of the layer of liquid crystal material, each arranged to provide a homogeneous alignment within the adjacent liquid crystal material.

9. 9. The display device of claim 8, wherein each of the two surface alignment layers has an alignment direction disposed at 45 degrees to an electric vector propagation direction of at least one of the display polarizer and the additional polarizer.

10. A display device according to any one of claims 6 to 9, wherein the liquid crystal material layer has positive dielectric anisotropy.

11. A display device according to any one of claims 6 to 10, wherein the layer of liquid crystal material has a twist.

12. 12. The display device of claim 11, wherein the twist is (90+m*180) degrees, where m is zero or a positive integer.

13. A display device according to any one of claims 6 to 12, wherein the layer of liquid crystal material has a twist of 90 degrees and a retardance for light of 550 nm wavelength in the range 420 nm to 550 nm, most preferably in the range 460 nm to 480 nm.

14. A display device according to any one of claims 6 to 12, wherein the layer of liquid crystal material has a twist of 270 degrees and a retardance for light of 550 nm wavelength in the range 650 nm to 800 nm, most preferably in the range 700 nm to 720 nm.

15. A display device according to any one of claims 6 to 12, wherein the layer of liquid crystal material has a twist of 360 degrees and a retardance for light of 550 nm wavelength in the range 1100 nm to 1400 nm, most preferably in the range 1150 nm to 1300 nm.

16. A display device according to any one of claims 6 to 12, wherein the layer of liquid crystal material has a twist of 450 degrees and a retardance for light of 550 nm wavelength in the range 820 nm to 1000 nm, most preferably in the range 880 nm to 920 nm.

17. A display device according to any one of claims 6 to 11, wherein the layer of liquid crystal material has a twist of 225 degrees and a retardance for light of 550 nm wavelength in the range 750 nm to 1200 nm, most preferably in the range 900 nm to 1100 nm.

18. the first and second quarter wave plates and the at least one retarder are arranged to introduce a phase shift into the polarization components of light passing through one of the display polarizer and the additional polarizer that is on an input side of the at least one retarder, and the at least one retarder reduces the brightness of light passing through the other of the display polarizer and the additional polarizer along an axis tilted with respect to the optical axis; The liquid crystal retarder has a refractive index of 1.0 to 1.5 for light having a wavelength of 550 nm, the refractive index being expressed by the formula: |R|=-0.02947*θ^3+5.81385*θ^2-397.950*θ+10090 and has a total retardance that is within 10% of the value R in nanometers given by A display device according to any one of claims 6 to 17, wherein θ is the outer angle of the tilt axis at which the maximum reduction in luminance occurs for a tilt axis with an elevation angle of 0 degrees.

19. 20. The display device of claim 18, wherein the liquid crystal retarder has a retardance for light with a wavelength of 550 nm that is within 4% of the value R.

20. The display device of any one of claims 6 to 19, wherein the at least one retarder further comprises at least one passive retarder.

21. 21. The display apparatus of claim 20, wherein the at least one passive retarder comprises a passive retarder having an optic axis perpendicular to a plane of the passive retarder.

22. A display device according to any preceding claim, wherein said at least one retarder comprises at least one passive retarder.

23. 23. The display apparatus of claim 22, wherein the at least one passive retarder comprises a passive retarder having an optic axis perpendicular to a plane of the passive retarder.

24. 24. A display device as claimed in claim 22 or 23, wherein the at least one passive retarder has a retardance for light of a wavelength of 550 nm in the range of -400 nm to -1000 nm, or +500 nm to +1200 nm, preferably in the range of -750 nm to -850 nm, or preferably in the range of +950 nm to +1050 nm.

25. 24. A display device as claimed in claim 22 or 23, wherein the at least one passive retarder comprises a pair of passive retarders having optical axes parallel to a plane of intersection of the passive retarders.

26. 26. The display apparatus of claim 25, wherein the pair of passive retarders have slow axes extending at 45[deg.] and 135[deg.], respectively, with respect to an electric vector propagation direction that is parallel to an electric vector propagation direction of the display polarizer.

27. 27. A display device as claimed in claim 25 or 26, further comprising an additional pair of passive retarders disposed between the first mentioned pair of passive retarders and having slow axes in the planes of the intersecting passive retarders.

28. 28. The display apparatus of claim 27, wherein the additional pair of passive retarders have slow axes each extending at 0° and 90°, respectively, with respect to an electric vector propagation direction that is parallel to an electric vector propagation direction of the display polarizer.

29. the first and second quarter wave plates and the at least one retarder are arranged to introduce a phase shift into the polarization components of light passing through one of the display polarizer and the additional polarizer that is on an input side of the at least one retarder, and the at least one retarder reduces the brightness of light passing through the other of the display polarizer and the additional polarizer along an axis tilted with respect to the optical axis; The at least one passive retarder has a retardation coefficient (Rc) of 1.0 for light having a wavelength of 550 nm, the retardation coefficient (Rc) being expressed by the formula: |R|=-0.02947*θ^3+5.81385*θ^2-397.950*θ+10090 and has a total retardance that is within 10% of the value R in nanometers given by A display device according to any one of claims 22 to 28, wherein θ is the outer angle of the tilt axis at which the maximum reduction in luminance occurs for a tilt axis with an elevation angle of 0 degrees.

30. 30. The display apparatus of claim 29, wherein said at least one passive retarder has a retardance for light with a wavelength of 550 nm that is within 4% of said value R.

31. A display device according to any preceding claim, wherein the display polariser and the additional polariser have electric vector propagation directions that are parallel.

32. The display device of any preceding claim, wherein the at least one retarder comprises a retarder comprising an oriented uniaxial birefringent material.

33. 33. A display device according to any one of claims 1 to 32, wherein the at least one retarder is arranged to introduce a phase shift along an axis tilted relative to the optical axis of the retarder to the polarised components of light passing through one of the display polariser and the additional polariser and the first quarter wave plate on the input side of the at least one retarder.

34. 34. The display device of claim 33, wherein the at least one retarder comprises at least one switchable liquid crystal retarder arranged to, in a switched mode of the switchable liquid crystal retarder, introduce a phase shift to polarization components of light passing through one of the display polarizer and the additional polarizer and the first quarter wave plate on the input side of the at least one retarder along an axis tilted relative to the optical axis of the retarder.

35. A display device as claimed in any one of claims 1 to 34, wherein the at least one retarder is arranged so as not to introduce a phase shift along an axis along the optical axis of the retarder to the polarised components of light passing through one of the display polariser and the additional polariser and the first quarter wave plate on the input side of the at least one retarder.

36. 36. A display device according to any one of claims 1 to 35, wherein the display polarizer is an output polarizer arranged on the output side of the spatial light modulator, and the additional polarizer is arranged on the output side of the display polarizer, whereby the one of the display polarizer and the additional polarizer on their input sides becomes the display polarizer.

37. 37. The display device of claim 36, further comprising a reflective polarizer disposed between the display polarizer and the first quarter-wave plate, the display polarizer and the reflective polarizer being arranged to pass the same linearly polarized polarization state.

38. a further additional polarizer arranged at the output side of the first-mentioned additional polarizer, the further additional polarizer being arranged to pass a linearly polarized polarization state; 38. A display device as claimed in claim 36 or 37, further comprising at least one further retarder arranged between the first mentioned further polariser and the further further polariser.

39. the additional polarizer is a reflective polarizer; 39. The display device of claim 38, wherein the at least one further retarder comprises a switchable liquid crystal retarder comprising a layer of liquid crystal material and electrodes arranged to apply a voltage to switch the layer of liquid crystal material.

40. 40. A display device as claimed in claim 38 or 39, wherein the at least one retarder disposed between the pair of quarter wave plates comprises at least one passive retarder having a retardance for light of a wavelength of 550 nm in the range of -100 nm to -400 nm, or +200 nm to +600 nm, preferably in the range of -200 nm to -300 nm, or preferably in the range of +300 nm to +500 nm.

41. 36. A display device according to any one of claims 1 to 35, wherein the display polarizer is an input polarizer arranged on the input side of the spatial light modulator, and the additional polarizer is arranged on the input side of the display polarizer, whereby the one of the display polarizer and the additional polarizer on their input side becomes the additional polarizer.

42. an output polarizer disposed at the output side of the spatial light modulator; a further additional polarizer disposed on an output side of the output polarizer, the output polarizer and the further additional polarizer being arranged to pass their respective linearly polarized polarization states; 42. The display apparatus of claim 41, further comprising at least one additional retarder disposed between said output polarizer and said further additional polarizer.

43. the display device further comprising a reflective polarizer disposed between the output polarizer and the at least one further retarder, the output polarizer and the reflective polarizer being arranged to pass the same linearly polarized polarization state; 43. The display device of claim 42, wherein the at least one further retarder comprises a switchable liquid crystal retarder comprising a layer of liquid crystal material and electrodes arranged to apply a voltage to switch the layer of liquid crystal material.

44. 44. A display device as claimed in claim 42 or 43, wherein the at least one retarder disposed between the pair of quarter wave plates comprises at least one passive retarder having a retardance in the range of -100nm to -400nm, or +200nm to +600nm, preferably in the range of -200nm to -300nm, or preferably in the range of +300nm to +500nm, for light of a wavelength of 550nm.

45. 45. A display device according to any one of the preceding claims, wherein the additional polariser, when crossed with a second conceptual polariser of the same material, has a transmission for wavelengths between 520nm and 560nm which is less than its transmission for wavelengths between 450nm and 490nm.

46. said transmittance at wavelengths between 450 nm and 490 nm is greater than 1%, preferably greater than 2%, and most preferably greater than 3%, 46. ​​A display device according to claim 45, wherein said transmittance for wavelengths between 520nm and 560nm is less than 3%, preferably less than 2% and most preferably less than 1%.

47. the pair of quarter wave plates have crossed slow axes; A display device as claimed in any one of claims 1 to 46, wherein the retarder does not further comprise an additional pair of passive retarders disposed between the first and second quarter wave plates, the additional pair of passive retarders having slow axes in the plane of the intersecting additional pair of passive retarders.

48. one of the pair of quarter wave plates has a slow axis that extends at least 40° and at most 50° with respect to an electric vector transfer direction that is parallel to the electric vector transfer of the display polarizer, and the other of the pair of quarter wave plates has a slow axis that extends at least 130° and at most 140° with respect to an electric vector transfer direction that is parallel to the electric vector transfer of the display polarizer; 48. The display device of any one of claims 1 to 47, wherein the retarder further comprises an additional pair of passive retarders disposed between the first and second quarter wave plates, one of the additional pair of passive retarders having a slow axis in a plane of the additional pair of passive retarders extending at least -10° and at most 10° with respect to an electric vector transfer direction that is parallel to the electric vector transfer of the display polarizer, and the other of the additional pair of passive retarders having a slow axis in a plane of the additional pair of passive retarders extending at least -10° and at most 10° with respect to an electric vector transfer direction that is parallel to the electric vector transfer of the display polarizer.

49. 1. A viewing angle control optical element for application to an output side of a display device for use in ambient lighting, comprising a spatial light modulator arranged to output light, the spatial light modulator includes an output polarizer; The viewing angle control optical element is An additional polarizer; a first quarter wave plate and a second quarter wave plate; at least one retarder disposed between the first quarter-wave plate and the second quarter-wave plate.

50. 50. The viewing angle control optical element of claim 49, further comprising a reflective polarizer, wherein the first and second quarter wave plates and the at least one retarder are disposed between the reflective polarizer and the additional polarizer.

Images