Ambient light control for privacy displays

Abstract

A control system for a switchable privacy display device includes an ambient light sensor, a controllable light source, and a display controller arranged to control the light source and display brightness. High image visibility is provided for a public mode of operation, while in a privacy mode, a level of visual safety above a perceived privacy threshold can be achieved by controlling image brightness and display illumination in response to the output of the ambient light sensor.

Description

Technical Field

[0001] The present disclosure relates generally to illumination from light modulating devices, and more particularly to the control of privacy displays. Background Art

[0002] Privacy displays provide image visibility to the primary user, typically in an on-axis position, and reduced visibility of the image content to peepers, typically in an off-axis position. Privacy functionality can be provided by micro-louver optical films, which transmit some light from the display in an on-axis direction while providing low brightness in off-axis positions. However, such films have high loss for front lighting, and micro-louvers can introduce moiré artifacts due to their tolerance for the pixels of the spatial light modulator. The spacing of the micro-louvers may need to be selected based on the panel resolution, increasing inventory and cost.

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

[0004] Control can be provided by means of brightness reduction, for example by means of a switchable backlight for a liquid crystal display (LCD) spatial light modulator. Display backlights typically employ waveguides and edge emitting sources. Certain imaging directional backlights have the additional capability of directing illumination through the display panel into the viewing window. An imaging system can be formed between multiple sources and corresponding window images. An example of an imaging directional backlight is an optical valve that can employ a folded optical system and, therefore, can also be an example of a folded imaging directional backlight. Light can propagate through the optical valve in one direction with essentially no loss, while light propagating in the opposite direction can be extracted by reflection off of an inclined facet, as described in U.S. Patent No. 9,519,153, which is incorporated herein by reference in its entirety.

[0005] In known privacy displays, privacy mode is provided by adding a removable blind film (such as that marketed by 3M Corporation), which may not be reliably fitted or removed by the user and is therefore not diligently attached every time the user is outside the office. In another known privacy display, control of the privacy mode is electronically activated, but control is vested in the user who must perform a keystroke to enter the privacy mode. Summary of the Invention

[0006] According to a first aspect of the present disclosure, there is provided a privacy display device, the privacy display device comprising: a display device, the display device being arranged to display an image, the display device being capable of providing a privacy function, in which visibility of the image to an off-axis viewer is reduced compared to visibility of the image to an on-axis viewer; at least one privacy light source, the at least one privacy light source being arranged to provide illumination from an illuminated area, the illumination being arranged to illuminate the display device along an incident direction for reflection to a predetermined viewer position that makes a polar angle greater than 0° with respect to a normal to the display device; and a control system, the control system being arranged to control the display device, wherein the control system is arranged to control a luminous flux of the at least one privacy light source when the privacy function is provided.

[0007] Therefore, when privacy is provided, illumination from the illuminated area is provided along the incident direction onto the display device and is reflected in the direction of an off-axis viewer at a predetermined viewer position. Consequently, the illuminated area suppresses the visibility of the displayed image to off-axis viewers by providing external illumination. Generally, this suppression occurs because the external illumination reduces the contrast of the displayed image, thereby improving the level of visual safety. By contrast, illumination from the illuminated area is not reflected toward an on-axis viewer, who therefore does not perceive the reduction in image visibility.

[0008] The privacy display device can be applied in a range of situations. As a non-limiting example, the privacy display device can be applied in a motor vehicle. In this case, conveniently, the illuminated area can be part of a door of the motor vehicle.

[0009] The display device may further comprise an ambient light sensor arranged to detect an illumination level of ambient light.

[0010] The control system may use the illuminance level of ambient light detected by the ambient light sensor to derive a measure of the illuminance of light along an incident direction on the display, and selectively control the luminous flux of the at least one privacy light source based on the derived measure.

[0011] In case the ambient light sensor is a directional sensor arranged to detect the illuminance level of ambient light incident on the display device along an incidence direction, the measure of illuminance of light along the incidence direction on the display Iθ is the illuminance level of ambient light detected by the ambient light sensor.

[0012] Where the ambient light sensor is arranged to detect illuminance levels of ambient light from a range of directions, the control system may use both the illuminance levels of ambient light detected by the ambient light sensor and the luminous flux of the at least one privacy light source to derive a measure of the illuminance of light along an incident direction on the display.

[0013] Advantageously, the control system is arranged to control the luminous flux of the at least one privacy light source to maintain the relationship Iθ ≥ Iθ 最小 , where Iθ is a derived measure of the illuminance of light on the display along the incident direction, and Iθ 最小 is given by the following equation:

[0014]

[0015] where Y 最大 is the maximum output brightness of the display device, Y 最大 The unit is Iθ 最小 divided by the unit of solid angle in steradian, ρ(θ) is the reflectivity of the display device for light along the incident direction, P(θ) is the ratio of the luminance of the display device along the incident direction to the maximum output luminance of the display device, and S 最小 Has a value of 1.0 or greater.

[0016] By maintaining the relationship Iθ ≥ Iθ when the privacy function is provided 最小 The luminous flux of the at least one privacy light source is controlled in such a manner based on a derived measure of the illuminance of light along an incident direction on the display that the perceived safety level of the operation of the display device can be maintained at or above a limit value S at a predetermined viewer position having a polar angle greater than 0° with respect to the normal to the display device. 最小 , even when the ambient light illumination level and the brightness of the display device vary. In this way, the perceived safety level is maintained at or above the limit value S 最小 , off-axis viewers cannot perceive the displayed image.

[0017] Advantageously, S 最小 Can have a value of 1.5 or greater. S 最小 Such an increased limit value of achieves a higher level of visual safety in which, for most images and most observers, the image is not visible to an off-axis viewer, ie the off-axis viewer cannot even perceive that the image is being displayed.

[0018] Advantageously, S 最小 Can have a value of 1.8 or greater. S 最小 Such an increased limit value of achieves a higher level of visual safety, in which the image is invisible to all observers, regardless of the image content.

[0019] At least one privacy light source may be positioned in various locations to provide illumination from the illuminated area, such as as follows.

[0020] One possibility is that the illuminated area is a surface, and at least one privacy light source is arranged to illuminate the illuminated area. In this case, the illumination from the illuminated area is provided by reflection from the surface.

[0021] Another possibility is that at least one privacy light source is arranged in the illuminated area so as to provide the illumination as light output therefrom.

[0022] In either of these possibilities, advantageously, the privacy light source is not visible to an on-axis viewer.

[0023] The display device may be capable of operating in at least a public mode and a private mode, wherein in the private mode, a privacy function is provided and the visibility of the image to off-axis viewers is reduced compared to the public mode, and the control system can selectively operate the display device in the public mode or the private mode for at least one area of ​​the display device. This provides selective operation in the public mode or the private mode depending on the use of the display device. For example, the private mode can be used in a public place (such as a cafe or a train) to enable the primary user to keep working but prevent bystanders or snoopers from seeing or photographing data from the screen, and the public mode can be used when discussing the content on the screen with colleagues (for example, in a company office).

[0024] The control system may be arranged to selectively operate the display device in a public mode or a private mode in response to a detected level of ambient light.

[0025] According to a second aspect of the present disclosure, there is provided a method for controlling a display device, wherein the display device is arranged to display an image and is capable of providing a privacy function, in which the visibility of the image to an off-axis viewer is reduced compared to the visibility of the image to an on-axis viewer, the method comprising: providing at least one privacy light source, the at least one privacy light source being arranged to provide illumination from an illuminated area, the illumination being arranged to illuminate the display device along an incident direction for reflection to a predetermined viewer position that makes a polar angle greater than 0° with respect to a normal to the display device; and controlling the luminous flux of the at least one privacy light source when the privacy function is provided.

[0026] The second aspect of the present invention corresponds to the operating method of the display device according to the first aspect of the present disclosure, and offers similar advantages.

[0027] Embodiments of the present disclosure can be used in various optical systems. Embodiments can comprise various projectors, projection systems, optical components, displays, microdisplays, computer systems, processors, self-contained projector systems, vision and / or audio-visual systems and electrical devices and / or optical devices, or work together with various projectors, projection systems, optical components, displays, microdisplays, computer systems, processors, self-contained projector systems, vision and / or audio-visual systems and electrical devices and / or optical devices. Aspects of the present disclosure can be used together with any in fact any equipment relevant to optical devices and electrical devices, optical systems, demonstration systems or any equipment that can comprise any type of optical system. Therefore, embodiments of the present disclosure can be used in optical systems, visual presentation and / or the device used in optical presentation, visual peripherals etc. and in several computing environments.

[0028] Before continuing to discuss the disclosed embodiments in detail, it should be understood that the present disclosure is not limited to the details of the specific arrangements shown in its application or creation, as the present disclosure is capable of other embodiments. Moreover, various aspects of the present disclosure can be set forth in different combinations and arrangements to define unique embodiments. Furthermore, the terminology used herein is for the purpose of description, not limitation.

[0029] These and other advantages and features of the present disclosure will become apparent to those of ordinary skill in the art after reading this disclosure in its entirety. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The embodiments are illustrated by way of example in the accompanying drawings, in which like reference numerals indicate similar parts, and in which:

[0031] Figure 1 is a schematic diagram illustrating a front view of a privacy display including a privacy control system operating in a privacy mode at a first visual security level;

[0032] Figure 2 is a schematic diagram illustrating a top perspective view of a privacy display including a privacy control system operating in a privacy mode at a first visual security level;

[0033] Figure 3 is a schematic graph illustrating the variation of output brightness with viewing angle for a typical collimated backlight arranged in conjunction with a switchable retarder to provide a high level of visual security to a wide range of snooper locations;

[0034] Figure 4 is a schematic graph illustrating a variation in visual safety level with off-axis relative luminance for a switchable privacy display operating in a privacy mode;

[0035] Figure 5 is a schematic graph illustrating the variation of reflectivity with extreme viewing angles for two types of privacy displays;

[0036] Figure 6 Is a diagram illustrating Figure 5 Schematic graph showing the change of visual safety level with extreme viewing angles for two types of privacy displays;

[0037] Figure 7A is a schematic graph illustrating a transfer function between front display luminance and ambient illuminance;

[0038] Figure 7B is a schematic graph illustrating a transfer function between a ratio of measured ambient illuminance to front display luminance and ambient illuminance;

[0039] Figure 8A is a schematic graph illustrating a variation in safety factor with polar angle for a lux / nit ratio of 3.0 for an illustrative privacy display operating in privacy mode;

[0040] Figure 8B is a schematic graph illustrating the variation of safety factor with polar angle for a lux / nit ratio of 0.5 for an illustrative privacy display operating in public mode;

[0041] Figure 8C is a schematic graph illustrating the variation of safety factor with polar angle for a lux / nit ratio of 0.5 for an illustrative privacy display operating in public mode;

[0042] Figure 8D is a schematic graph illustrating a variation in safety factor with polar angle for a lux / nit ratio of 3.0 for an illustrative privacy display operating in privacy mode;

[0043] Figure 9A is a schematic graph illustrating a user-selectable transfer function between front display brightness and ambient illumination;

[0044] Figure 9B is a schematic flow chart illustrating a method for operating a user-selectable transfer function;

[0045] Figure 9C is a schematic graph illustrating changes in perceived privacy with visual security levels;

[0046] Figure 9D It is a graphic illustration Figure 1-2 and Figure 3-4 Flowchart of the privacy control system;

[0047] Figure 10A is a schematic diagram illustrating a top view of a motor vehicle being operated in a dark environment and a control system including interior vehicle lighting arranged to provide desired visual safety;

[0048] Figure 10B Illustration Figure 10A Flowchart of the privacy control system;

[0049] Figure 10C is a schematic diagram illustrating a top view of a tabletop privacy display and light sources arranged to increase image security from off-axis snoopers in response to ambient lighting conditions;

[0050] Figure 10D is a schematic diagram illustrating a front view of a privacy display and a light source;

[0051] Figure 10E It is a graphic illustration Figure 10D A schematic diagram of a top view of a privacy display;

[0052] Figure 11A is a schematic diagram illustrating a top view of a privacy display and an off-axis ambient light sensor;

[0053] Figure 11B is a schematic graph illustrating polar regions for measuring ambient illumination for a privacy display;

[0054] Figure 12A 、 12B and 12C are diagrams used in Figure 11B Schematic diagram of a top view of an off-axis ambient light sensor for measuring ambient illumination in a polar region of FIG.

[0055] Figure 13 is a schematic diagram illustrating, in front perspective, a switchable directional display device including a directional backlight and a switchable liquid crystal retarder;

[0056] Figure 14 is a schematic diagram illustrating, in a perspective side view, an arrangement of a switchable liquid crystal retarder including a passive negative C-plate compensating retarder in a privacy mode of operation;

[0057] Figure 15A is a schematic graph illustrating polar and azimuthal variations in output brightness of a collimated backlight and a spatial light modulator;

[0058] Figure 15Bis a schematic graph illustrating polar and azimuthal variations in transmission of a switchable retarder arranged between parallel polarizers;

[0059] Figure 15C is a schematic graph illustrating polar and azimuthal variations in relative reflection of a switchable retarder disposed between a reflective polarizer and an absorbing polarizer;

[0060] Figure 15D Is a diagram illustrating Figure 13 Schematic graphs of polar and azimuthal variations in total display reflectivity when arranged in a privacy mode of operation;

[0061] Figure 15E Is a diagram illustrating Figure 13 A schematic graph showing polar and azimuthal variations in output brightness when the device is in a privacy operation mode;

[0062] Figure 15F Is a diagram illustrating Figure 13 The arrangement in the privacy mode of operation, for the value Y measured in nits 最大 Schematic diagram of the polar and azimuthal angle variation of the visual safety level S for the frontal luminance of a display, with the value Y measured in nits 最大 The display front brightness is half the illuminance of value I measured in lux;

[0063] Figure 15G Is a diagram illustrating Figure 13 The arrangement in the privacy mode of operation, for the value Y measured in nits 最大 Schematic diagram of the polar angle variation of the visual safety level S for the front luminance of the display for zero elevation angle, the value Y measured in nits 最大 The display front brightness is half the illuminance of value I measured in lux;

[0064] Figure 16 is a schematic diagram illustrating, in perspective side view, an arrangement of a switchable retarder in a common mode of operation, wherein the switchable retarder comprises a switchable liquid crystal layer having a homeotropic orientation and a passive C-plate compensating retarder;

[0065] Figure 17A Is a diagram illustrating Figure 13 A schematic graph showing polar and azimuthal variations in output brightness when the arrangement is in a common operating mode;

[0066] Figure 17B Is a diagram illustrating Figure 13 The arrangement in the common operating mode, for the value Y measured in nits 最大Schematic diagram of the polar angle variation of the visual safety level S for the front luminance of the display for zero elevation angle, the value Y measured in nits 最大 The display front brightness is half the illuminance of value I measured in lux;

[0067] Figure 17C It is a diagram illustrating the maximum brightness Y 最大 A schematic curve diagram of the polar angle and azimuth angle changes of the backlight source in the direction of 最大 The direction is not perpendicular to the display;

[0068] Figure 18A is a side view illustrating the output light of the spatial light modulator passing through the Figure 13 Schematic diagram of the propagation of the optical stack;

[0069] Figure 18B is a top view illustrating the privacy mode of operation with ambient lighting passing through Figure 13 Schematic diagram of the propagation of the optical stack;

[0070] Figure 19A is a side view illustrating the output light of the spatial light modulator in the common operating mode through Figure 13 Schematic diagram of the propagation of the optical stack;

[0071] Figure 19B Is a graphic illustration of Figure 19A A schematic graph showing the variation of the output brightness of the transmitted light with the polar direction;

[0072] Figure 19C This diagram illustrates the ambient lighting in public operation mode. Figure 13 Schematic diagram of the propagation of the optical stack;

[0073] Figure 19D Is a graphic illustration of Figure 19C A schematic graph showing a change in reflectivity of reflected light with polar direction;

[0074] Figure 20 is a schematic diagram illustrating, in front perspective, a switchable directional display device comprising a directional backlight and two switchable liquid crystal retarders, each switchable liquid crystal retarder being disposed between a pair of polarizers;

[0075] Figure 21A is a schematic graph illustrating polar and azimuthal variations in output brightness of a transmitting spatial light modulator;

[0076] Figure 21Bis a schematic graph illustrating polar and azimuthal variations in transmission of a first switchable retarder disposed between a first pair of parallel polarizers;

[0077] Figure 21C is a schematic graph illustrating polar and azimuthal variations in relative reflection of a first switchable retarder disposed between a reflective polarizer and an absorbing polarizer;

[0078] Figure 21D Is a diagram illustrating Figure 20 Schematic graphs of polar and azimuthal variations in total display reflectivity when arranged in a privacy mode of operation;

[0079] Figure 21E is a schematic graph illustrating polar and azimuthal variations in transmission of a second switchable retarder disposed between a second pair of parallel polarizers;

[0080] Figure 21F Is a diagram illustrating Figure 20 A schematic graph showing polar and azimuthal variations in output brightness when the device is in a privacy operation mode;

[0081] Figure 21G Is a diagram illustrating Figure 20 The arrangement in the privacy mode of operation, for the value Y measured in nits 最大 Schematic diagram of the polar and azimuthal angle variation of the visual safety level S for the frontal luminance of a display, with the value Y measured in nits 最大 The display front luminance is half the illuminance of value I measured in lux; and

[0082] Figure 21H Is a diagram illustrating Figure 20 The arrangement in the privacy mode of operation, for the value Y measured in nits 最大 Schematic diagram of the polar angle variation of the visual safety level S for the front luminance of the display for zero elevation angle, the value Y measured in nits 最大 The display front brightness is half the illuminance of value I measured in lux. DETAILED DESCRIPTION

[0083] Terms related to the privacy display appearance will now be described.

[0084] The private mode of operation of a display is an operating mode in which the viewer sees low contrast sensitivity, making the image unclear. Contrast sensitivity is a measure of the ability to distinguish different levels of brightness in a static image. Reverse contrast sensitivity can be used as a measure of visual safety, as a high visual safety level (VSL) corresponds to low image visibility.

[0085] For a privacy display that provides an image to an observer, visual security can be given as:

[0086] V = (Y + R) / (Y – K) Equation 1

[0087] Where V is the visual safety level (VSL), Y is the luminance of the display's white state at the peeper's viewing angle, K is the luminance of the display's black state at the peeper's viewing angle, and R is the luminance of the reflected light from the display.

[0088] The panel contrast ratio is given by:

[0089] C=Y / K Equation 2

[0090] So the visual safety level can be further given as:

[0091] V=(PY 最大 +I.ρ / π) / (P.(Y 最大 -Y 最大 / C)) Equation 3

[0092] Where: Y 最大 is the maximum brightness of the display; P is the off-axis relative brightness, which is usually defined as the brightness at the viewer's angle divided by the maximum brightness Y 最大 C is the image contrast ratio; ρ is the surface reflectance; and I is the illuminance. 最大 The units of are I divided by the units of the solid angle in steradians.

[0093] The brightness of the display changes with the angle, so the maximum brightness of the display is Y 最大 Occurs at a specific angle depending on the configuration of the display.

[0094] In many displays, the maximum brightness Y 最大 Any display device disclosed herein can be arranged to have the maximum brightness Y occurring in the front direction. 最大 In this case, the maximum brightness Y of the display device 最大 References to may be replaced with references to brightness perpendicular to the display device.

[0095] Alternatively, any display described herein may be arranged to have a maximum luminance Y occurring at a polar angle greater than 0° from the normal to the display device. 最大 For example, the maximum brightness Y 最大 This can occur at a non-zero polar angle and below an azimuth angle, such as a zero sideways angle, so that maximum brightness is provided to an on-axis user looking down at the display device. The polar angle can be, for example, 10 degrees, and the azimuth angle can be in the direction of north (90° counterclockwise from the east). The viewer can therefore desirably see high brightness at typical non-normal viewing angles.

[0096] The off-axis relative luminance P is sometimes referred to as the privacy level. However, such a privacy level P describes the relative luminance of the display at a given polar angle compared to the frontal luminance, rather than being a measure of the appearance of privacy.

[0097] Illuminance, I, is the luminous flux per unit area incident on the display and reflected from the display toward the observer's location. For Lambertian illumination, and for displays illuminated by a Lambertian front diffuser, illuminance, I, is invariant with polar and azimuthal angles. For arrangements with displays in which a non-Lambertian front diffuser is arranged in an environment with directional (non-Lambertian) ambient light, illuminance, I, varies with polar and azimuthal angles of observation.

[0098] Therefore, in a completely dark environment, a high contrast display has a VSL of approximately 1.0. As the ambient illumination increases, the perceived image contrast decreases, the VSL increases, and the privacy image is perceived.

[0099] For typical LCDs, the panel contrast C is higher than 100:1 for almost all viewing angles, so that the visual safety level is approximated as:

[0100] V=1+I.ρ / (π.PY 最大 ) Equation 4.

[0101] In the present embodiment, in addition to the exemplary definition of Equation 4, other measures of the visual security level V may also be provided, for example, to include the effects of image visibility to a snooper at the snooper's location, image contrast, image color and white point, and subtended image feature size. Thus, the visual security level may be a measure of the degree of privacy of a display, but may not be limited to the parameter V.

[0102] Perceived image safety can be determined by the logarithmic response of the eye so that

[0103] S=log 10 (V) Equation 5.

[0104] The desired limit for S is determined in the following manner. In a first step, a privacy display is provided. Measurements of the display's privacy level P(θ) as a function of polar viewing angle and of the display's reflectivity ρ(θ) as a function of polar viewing angle are performed using photopic measurement equipment. A light source, such as a light box of substantially uniform brightness, is arranged to provide illumination from the illuminated area, the illumination being arranged to illuminate the privacy display along an incident direction for reflection to a viewer position at a polar angle greater than 0° to the normal to the display. The variation of the illuminance I(θ) of the substantially Lambertian emitting light box as a function of polar viewing angle is determined by measuring the recorded reflected brightness as a function of polar viewing angle, taking into account the variation of the reflectivity ρ(θ). The measurements of P(θ), r(θ) and I(θ) are used to determine the safety factor S(θ) as a function of polar viewing angle along the zero elevation axis.

[0105] In the second step, a series of high contrast images are provided on the privacy display, including (i) a small text image with a maximum font height of 3 mm, (ii) a large text image with a maximum font height of 30 mm, and (iii) a moving image.

[0106] In the third step, each observer (with vision correction for viewing at 1000 m, where appropriate) views each of the images from a distance of 1000 m and adjusts their polar viewing angle at zero elevation until image invisibility is achieved for one eye from a position on the display at or near the centerline of the display. The polar coordinate locations of the observer's eyes are recorded. From the relationship S(θ), the safety factor at the polar coordinate location is determined. For different images, for various display brightnesses Y 最大 The measurements were repeated for different light box illuminances I(q=0), for different background lighting conditions, and for different observers.

[0107] From the above measurements, S < 1.0 provides low or no visual safety, 1.0 ≤ S < 1.5 provides visual safety that depends on the contrast, spatial frequency, and temporal frequency of the image content, 1.5 ≤ S < 1.8 provides acceptable image invisibility for most images and most observers (that is, no image contrast is observed), and S ≥ 1.8 provides complete image invisibility, regardless of image content for all observers.

[0108] Compared to privacy displays, wide-angle displays are expected to be easily viewed under standard ambient illumination conditions. One measure of image visibility is given by contrast sensitivity, such as Michelson contrast, which is given by the following equation:

[0109] M=(I 最大 –I 最小) / (I 最大 +I 最小 ) Equation 6

[0110] And therefore:

[0111] M=((Y+R)–(K+R)) / ((Y+R)+(K+R))=(YK) / (Y+K+2.R) Equation 7.

[0112] Therefore, the visual safety level (VSL) V is equal to 1 / M (but not the same as 1 / M). In this discussion, for a given off-axis relative luminance P, the wide-angle image visibility W is approximated as:

[0113] W=1 / V=1 / (1+I.ρ / (π.PL)) Equation 8.

[0114] It would be desirable to provide controls for switchable privacy displays.

[0115] Figure 1 is a schematic diagram illustrating a front view of a privacy display device 200 including a privacy display apparatus 100 controlled by a privacy control system 500 operating in a privacy mode at a first visual security level. The display apparatus 100 displays an image.

[0116] The display device 200 includes a display apparatus 100 capable of implementing a privacy mode and a control system 500. The display apparatus 100 is arranged to display an image and is capable of operating in at least a public mode and a private mode, wherein in the private mode, a privacy function is provided and the visibility of the image to off-axis viewers is reduced compared to the public mode, while the visibility of the image to a primary user in an on-axis position remains visible in both the private mode and the public mode. The control system 500 selectively operates the display apparatus 100 in either the public mode or the private mode for at least one region of the displayed image, typically the entire displayed image.

[0117] Examples of suitable types of display devices are described further below.

[0118] The means by which privacy mode operation is determined will now be described.

[0119] For front-facing users in typical ambient lighting environments, the display device 100 desirably provides a displayed image 101 having a brightness that enables high image visibility W in both the private and public modes of operation.

[0120] The display device 200 may also include inputs related to situations in which it is desirable to provide a private image, or conversely, situations in which it is undesirable to provide a public image. Such desirable and undesirable situations may be determined by a policy 240, such as provided by a company policy, a government policy, a medical ethics policy, or provided by a user preference setting.

[0121] The display device 200 has a primary ambient light sensor 232 that detects the illumination level of ambient light. The control system 500 can be arranged to selectively operate the display apparatus 100 in a public mode or a private mode in response to the detected level of ambient light. The primary ambient light sensor 232 can be of any suitable type, such as a photodiode that can have a photopic filter or a photopic light response current, voltage, or digital value.

[0122] Some types of displays have multiple optical effects that improve privacy performance, exemplary optical effects are described below. If more than one privacy optical effect is available, the mode that gives the primary user the widest viewing freedom while still maintaining a sufficient level of visual safety at the ambient light level experienced can be selected by the control system 500. This advantageously protects privacy and maintains user productivity.

[0123] Airplane mode 270 may be selected, indicating that a low light level ambient environment may exist, and visual safety level controls may be adapted accordingly.

[0124] Advantageously, in public mode, the display apparatus 100 may have greater image uniformity and greater viewing freedom for primary users, as well as be visible from multiple viewing locations.

[0125] A visual security level indicator 280 may be provided on the display, which is a measure of the level of privacy achieved. Figure 1 In the illustrative example of , indicator 282 may be an amber privacy alert indicating that there may be some residual image visibility to off-axis snoopers. When switched to privacy mode, control system 500 may be arranged to control display device 100 to display image 101 with information such as indicator 280 indicating the visibility of the image to off-axis viewers, for example, to provide a visual security level V. Advantageously, users or their supervisors can have confidence in the level of privacy achieved in the particular environment in which they are operating.

[0126] The appearance of the display in privacy mode as seen by a snoop will now be described, along with further inputs for controlling the level of visual security.

[0127] Figure 2is a schematic diagram illustrating a top perspective view of a privacy display including a privacy control system 500 operating in a privacy mode at a first visual security level. Figure 2 Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0128] As will be described below, off-axis privacy may be provided through control of the off-axis brightness, reflectivity, and image contrast of the image 103 provided by the switchable privacy display device 100 to unwanted snoopers.

[0129] In one example, the display device may include an emissive spatial light modulator. In this case, the privacy control system 500 may control the brightness of the displayed image by controlling the emission of light by the spatial light modulator.

[0130] In another example, a display device may include a backlight and a transmissive spatial light modulator arranged to receive light from the backlight. In this case, the control system may be arranged to control the brightness of the displayed image by controlling the brightness of the backlight and / or by controlling the transmission of light by the spatial light modulator.

[0131] In operation in privacy mode, a limited output cone angle 402C is provided, generally centered on the optical axis 199, which is generally the surface normal 199 of the display device 100. Off-axis brightness is reduced. In other embodiments (not shown), the cone 402C can be tilted relative to the surface normal 199, such as for use in off-axis displays (such as center console mounted displays).

[0132] Ambient light source 604 illuminates the display surface with light 605. Reflected ambient light 610A from display 100 provides reflected area 606 that provides light 606 that contributes to enhancing visual safety level V as described elsewhere herein.

[0133] Some light 608 may be incident on primary ambient light sensor 232. Primary ambient light sensor 232 may be a separate component or may be incorporated into the camera 230 detection system.

[0134] Ambient illumination detection 234 provides a calculation of ambient illumination and is input into control system 500. VSL calculation 250 is used to determine desired display setting characteristics and is output to display control 710. Display control 710 can control display brightness setting 278 and can further be used to provide visual safety level indicator 280 level 282. Display control 710 is further described below with respect to the example of a privacy display.

[0135] Figure 2 Further illustrated is a privacy light source 600 arranged to provide light 601 which, after reflecting from the privacy display 100, is reflected as light 603. In the present embodiment, as will be further described herein, the privacy light source 600 is controlled by the display control 710 to adjust the brightness of the reflected light 603, 606 corresponding to the brightness of the display in the cone 402C.

[0136] In public display mode, the larger solid angle output light cone 402D provided from the switchable privacy display device 100, as shown in FIG11 , can be adjusted to be greater than in privacy mode to increase off-axis display brightness. In addition, the illumination from the privacy light source 600 can be reduced or removed to advantageously achieve improved image visibility.

[0137] Switching between an exemplary privacy brightness curve and a public brightness curve will now be described.

[0138] Figure 3 is a schematic graph illustrating the variation in output brightness with viewing angle for a typical collimated backlight arranged in conjunction with a plurality of retarders 300 to provide a high level of visual security to a wide range of snooper locations. Figure 3 Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0139] Figure 3 The diagram illustrates a desired brightness curve 486 for a backlight 20 operated in privacy mode for use in privacy mode with the switchable liquid crystal retarder 300 of FIG. 11. Curve 486 is modified by the switchable liquid crystal retarder 300 to provide an illustrative curve 490 that advantageously achieves an off-axis relative brightness of less than 0.5% at a 45-degree lateral angle and zero-degree elevation angle, which may be, for example, Figure 12C The target snooper is shown viewing polar coordinate locations 27L, 27R.

[0140] The control of the visual safety level will now be described in more detail.

[0141] Figure 4 is a diagram illustrating an exemplary embodiment of a switchable privacy display 100 in a privacy mode and with reference to Figure 13 Schematic graph of the visual safety level of the switchable privacy display as a function of the off-axis relative luminance. Figure 4Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0142] Figure 4 The graph illustrates a visual security level V (calculated from Equation 4 above in each illustrative embodiment) versus an illustrative embodiment for varying the privacy level achieved at the target snooper viewing location 26L, 26R, as shown in Table 1. A display reflectivity of 30% or greater can be achieved for a display that includes a reflective polarizer 302, while a display reflectivity of approximately 5% can be achieved for a display that does not include a reflective polarizer 302.

[0143]

[0144] Table 1

[0145] At the 0.5% privacy level, various visual safety level points 458 may be provided depending on the display structure, ambient illumination and display brightness.The present embodiment further provides an indicator 280 for displaying the visual safety level, which may be provided, for example, by means of a traffic light indicator.

[0146] The variation of display reflectivity with viewing angle will now be described.

[0147] Figure 5 is a schematic graph illustrating the variation of reflectivity of two types of privacy displays with extreme viewing angles (which may be sideways angles for zero degree elevation). Figure 13 The change in reflectivity of the illustrative embodiment of FIG, and curve 822 illustrates the change in reflectivity of the illustrative embodiment of FIG. Figure 13 302. Both curves include Fresnel reflectivity at the external polarizer 318 and are therefore enhanced at high polar angles.

[0148] The variation of the visual safety level V with the viewing angle will now be described.

[0149] Figure 6 Is a diagram illustrating Figure 5 11 with the reflective polarizer 302, and VSL curve 826 illustrates the output of the display of FIG. 11 with the reflective polarizer 302 omitted. The VSL curves are illustrated for the same ambient illuminance I. The limit value V is further described below. 限值 , higher than the limit V 限值, image visibility is non-existent. The angular range 825 of snooper locations for curve 824 is therefore greater than the angular range 827 for curve 826. Reflective polarizer 302 achieves above-threshold visual security levels over a wider range of polar coordinates, advantageously providing improved protection against snoopers. Furthermore, frontal brightness can be increased for a given ambient illumination, thereby improving image visibility for the display user.

[0150] Selective control of the relationship between desired display brightness and ambient light illumination will now be described.

[0151] The control system 500 controls the brightness of the displayed image based on the detected level of ambient light according to the transfer function. The transfer function can be selected to optimize the visibility of the displayed image for an on-axis viewer. Similar techniques for optimizing the visibility of a displayed image based on the detected level of ambient light are commonly used for display devices of portable devices (such as mobile phones) and can be applied here. However, as described below, when a privacy function is provided, the transfer function can be adapted for use with the privacy display device 100.

[0152] Typically, the transfer function provides a higher brightness of the displayed image in the public mode than in the private mode.Some illustrative examples will now be described.

[0153] Figure 7A is a diagram illustrating transfer function curves 802, 804, 852, 854 between display front luminance measured in nits and detected ambient illuminance measured in lux; Figure 7B are schematic graphs illustrating transfer function curves 803, 805, 853, 857 between the measured ambient illuminance and the ratio of the front display luminance measured in lux / nit and the ambient illuminance measured in lux.

[0154] Variations 802 and 803 are illustrative curves for a public operating mode, and variations 804 and 805 are illustrative curves for a private operating mode.

[0155] The control system 500 is arranged to selectively control the brightness of the displayed image in response to the detected level of ambient light in public mode and in private mode according to different transfer functions 802, 804 that relate the brightness level to the detected level of ambient light in public mode and in private mode, respectively. The transfer function 802 in public mode relates a higher brightness level to the detected level of ambient light than the transfer function 805 in private mode.

[0156] consider Figure 7A Curve 856 and Figure 7BThe corresponding curve 857 provides a linear variation of the display brightness Y0 compared to the measured ambient illuminance with a constant ratio of 0.5 lux / nit for all illuminance levels. In operation, such a display has a high brightness compared to the background illuminance over all illuminance ranges.

[0157] Curves 802 and 803 differ from curves 856 and 857 in that the lux / nit ratio increases with increasing brightness. Advantageously, such curves achieve visually comfortable images with high image visibility and low perceived glare over a wide illuminance range.

[0158] In switchable privacy displays (as discussed below with respect to Figure 13 In a switchable privacy display as described herein, such curves 856, 857 and 802, 803 may be desirable for a public mode of operation. As will be further described below, curves 856, 857, 802, 803 advantageously achieve high image visibility (desirably, W ≥ 0.8) and a low image safety factor (desirably, S ≤ 0.1) over a wide polar region, for both on-axis and off-axis viewing locations.

[0159] consider Figure 7A Curve 854 and Figure 7B The corresponding curve 855 provides a linear variation of the display brightness Y0 compared to the measured ambient illuminance with a constant ratio of 3.0 lux / nit for all illuminance levels. In operation, such a display has a reduced brightness compared to the background illuminance over all illuminance ranges compared to the display with curves 802, 803.

[0160] Curves 804, 805 differ from curves 854, 855 in that for brightness levels below 150 nit, the lux / nit ratio increases with increasing brightness.

[0161] In switchable privacy displays (as discussed below with respect to Figure 13 802, 803 advantageously achieves high image visibility (desirably, W ≥ 0.8) and a low image safety factor (desirably, S ≤ 0.1) over a wide polar region, for both on-axis and off-axis viewing locations. Advantageously, such curves 854, 855 and 804, 805 can achieve desired brightness and image visibility for a display user at lower illumination levels. Furthermore, such curves 856, 857, 802, 803 achieve improved image safety at higher illumination levels.

[0162] When operating in privacy mode, a privacy transfer function 804 is selected and the control system uses the measured ambient light level to control the display brightness so that the desired visual security level V at at least one off-axis snooper viewing angle is provided for different ambient lighting levels. Advantageously, display security can remain constant under different lighting conditions.

[0163] When operating in the common operating mode, the common transfer function 802 may be selected to provide a desired image visibility W for different ambient lighting levels. Advantageously, for off-axis observers, display visibility may remain constant under different lighting conditions.

[0164] As will be discussed below Figure 10A As further described, the illumination level can be changed by controlling the ambient light source, the display brightness can be changed, or the display brightness and the ambient illumination can be changed to achieve the desired image visibility for the primary user 45 and the desired visual security for the snoopers 47 in the privacy operating mode.

[0165] The variation of the safety factor with display control and ambient illumination level will now be further described.

[0166] Figure 8A is a schematic graph illustrating a variation in safety factor with polar angle for a lux / nit ratio of 3.0 for an illustrative privacy display operating in privacy mode; Figure 8B is a schematic graph illustrating a variation in safety factor with polar angle for a lux / nit ratio of 0.5 for an illustrative privacy display operating in privacy mode; Figure 8C is a schematic graph illustrating the variation of safety factor with polar angle for a lux / nit ratio of 0.5 for an illustrative privacy display operating in public mode; and Figure 8D is a schematic graph illustrating the variation of safety factor with polar angle for a lux / nit ratio of 3.0 for an illustrative privacy display operating in public mode.

[0167] Figure 8A The -D curve is given by Figure 13 Illustrative embodiments are provided for different ratios of illuminance to frontal luminance Y0 as will now be described. The primary display user 45 is located in a polar region near a lateral angle of 0° and an elevation angle of 0°. The snooper is typically located at a polar location with a lateral angle > 25°, and more typically at a polar location with a lateral angle > 35°.

[0168] exist Figure 8AIn the embodiment, the display 100 is arranged to provide low off-axis brightness (e.g. Figure 3 Curve 490 is shown in the lateral direction) and high off-axis reflectivity (as Figure 5 Curve 820 is shown in the lateral direction. Display front luminance Y0 is controlled by controlling light source 15 of backlight 20 so that luminance Y0, measured in nits, is one-third the illuminance measured in lux (which is assumed to be the same for all polar angles). Around the coaxial direction, S ≤ 0.1, and the image is seen with high image visibility W ≥ 0.8. Advantageously, arrangement 8A represents a desired polar curve for security factor S for privacy operation.

[0169] Through with Figure 8A For comparison, Figure 8B The diagram illustrates the variation of the safety factor S with extreme viewing angles for luminance Y0 measured in nits, where luminance Y0 measured in nits is twice the illuminance measured in lux (i.e., an arrangement suitable for public mode viewing). Undesirably, the extreme region within which the safety factor S ≥ 1.5 is greatly reduced. Off-axis display users can see more than Figure 8A The arrangement of multiple image data.

[0170] exist Figure 8C In the embodiment, the display 100 is arranged to provide enhanced off-axis brightness (e.g., Figure 3 486 in the lateral direction) and reduced off-axis reflectivity (as shown in FIG. Figure 5 (Curve 822 is shown in the lateral direction). The display front luminance Y0, measured in nits, is controlled to be three times the illuminance measured in lux. Around the on-axis direction, S ≤ 0.1, and the image is visible with high image visibility W ≥ 0.8. Arrangement 8A is a desirable polar curve for the security factor S for privacy operation. Advantageously, the polar region where S ≤ 0.1 is significantly increased, allowing off-axis observers to see the image on display 100 with high image visibility.

[0171] Through with Figure 8C For comparison, Figure 8D The diagram illustrates the variation of the safety factor S with extreme viewing angles for luminance Y0 measured in nits, which is 1 / 3 the illuminance measured in lux (i.e., an arrangement suitable for private viewing). Undesirably, the extreme region within which the safety factor S > 1.5 is greatly reduced. Off-axis display users may undesirably see a larger image than the image above. Figure 8C The arrangement of less image data.

[0172] Advantageously, the control system of the present embodiment achieves desired performance in both the private mode of operation and the public mode of operation for different illumination levels.

[0173] It may be desirable for a user to select a transfer function that achieves a desired brightness level.

[0174] Figure 9A is a schematic graph illustrating a user-selectable transfer function between front display brightness and ambient illuminance. Figure 7A Compared to the arrangement of , optional curves 830, 832, 834, 836, 838, 840, 842, 844, 846, 848 may be provided, each shaped as a step function of the brightness of the displayed image with increasing detected levels of ambient light.

[0175] Advantageously, Figure 9A Curve control of φ(t) can be provided at low cost and complexity due to the step function shape of the transfer function. The control system 500 can similarly provide a single curve from among multiple curves to achieve the same benefits.

[0176] An illustrative operating example will now be described. The display can be operated in a bright environment (e.g., 450 lux). In such an environment, the display can default to its maximum peak brightness of 250 nit provided to the front user. If necessary, the user can further reduce the display brightness. Advantageously, high visual safety can be provided for a wide range of ambient illuminations. The curve can be selected using a step function as shown in the figure to reduce the number of settings and reduce driving costs by selecting different curves. Alternatively, a smooth curve that changes continuously with the ambient illumination can be provided.

[0177] In the default setting 838, when the ambient illuminance drops (e.g., between 250 lux and 175 lux), the display switches between 150 nit and 100 nit. The visual safety level for snoopers remains above the threshold. A time constant can be applied to the switching of the curves so that the change is not visible as display flicker. The time constant can be, for example, a few seconds.

[0178] At high illumination levels, a single display maximum brightness Y can be provided for all curves as shown in the figure. 最大 , or the step function can continue to vary with brightness.

[0179] In some environments, the user may prefer a brighter front image with some limited reduction in visual safety and therefore may select curve 832 instead of the default setting. In other environments for which a high level of visual safety is desired, curve 848 may be selected with a lower front brightness and an increased level of visual safety.

[0180] In other words, the user can cause the default display brightness setting to change from the curve shown in the figure as default curve 838. If the ambient illumination changes, the display can follow a selected brightness step curve, such as curve 830 as shown.

[0181] During periods of time where ambient illumination is changing, or the user selection of a curve is altered, switching between curves may be provided over extended periods of time (such as seconds) to achieve a seamless change in the appearance of the display.

[0182] Figure 9B is a schematic flow chart illustrating a method for operating a user-selectable transfer function.

[0183] A display device (e.g., a notebook computer) may have a system-level PWM (pulse width modulation) generator 860. Inputs to the system-level PWM generator 860 may include a setting for global brightness 868 set by the operating system, and the output of a separate ambient light sensor (not shown) may be used as input.

[0184] The inputs to the global brightness setting 868 may also include user inputs that can bias or adjust the default display brightness. A PWM input 878 is received by a timing controller (TCON) board 862, which may include a microcontroller that performs processing functions. The TCON board 862 also includes an input from a privacy enable 876 signal that determines whether the display is in privacy mode. If the display is not in privacy mode, a PWM output 880 may follow the PWM input 878. The TCON board 862 further includes an input from an ambient light sensor 872, which may be different from the ambient light sensor provided by the system. Specifically, as shown, the ambient light sensor 872 may be provided with a direct connection to the TCON 862. This connection may be independent of operating system control. The PWM output 880 sent to the LED controller 864 can be modified by the TCON 862. The time response function 874 takes input from the ambient light sensor 872 and enables the TCON 862 to provide a PWM output 880 so that changes in ambient illumination result in changes in the signal to the LED controller 864 that are gradual over time so that the user does not experience flickering or jumping in the brightness of the display. The time response function 874 can also suppress the effects of frequency components of the ambient illumination (e.g., 50 or 60 Hz) that may be caused by fluorescent tubes, etc.

[0185] The LED controller 864 is connected to the LED light bar 15 of the privacy display 100, which may be a PCB or flexible PCB incorporated within the backlight 20 of the privacy display 100, as shown, for example, in FIG11 below. In other arrangements (not shown), the LED controller 864 may be provided by a display controller arranged to control the brightness of the emissive spatial light modulator 48, such as an emissive OLED display or an emissive micro-LED display.

[0186] Advantageously, Figure 9A Curve control can be provided at low cost and complexity.

[0187] Desirable limits on front brightness for a display operating in privacy mode will now be described.

[0188] Figure 9C is a schematic graph illustrating the variation 806 of perceived visual safety at an observation angle θ as a function of the visual safety level V. The visual safety level V is a measured quantity for any given display and varies with extreme viewing angles.

[0189] The perceived visual safety is a subjective judgment of the visibility of the displayed privacy image caused by the response of the human visual system at the viewing angle, as compared with the visual safety level V.

[0190] In operation, it has been found that the threshold limit value V above the visual safety level V 限值 , then no image information is perceived. This transition of perceived visibility with changes in the visual safety level V is very fast, e.g. Figure 7B The threshold value V of the curve in 限值 That is, as the visual safety level V increases, initially the perceived visibility decreases only gradually and the image is essentially viewable. However, upon reaching the threshold limit V 限值 When the perceived image ceases to be visible, it does so very quickly, in a way that is surprising to watch in practice.

[0191] In an observation of surprising results, for text document images of interest for privacy applications, it was found that the perceived image seen by the snooper quickly ceased to be visible for a V of 10. In region 810 where V values ​​were above 10, all displayed text had zero visibility. In other words, for V of 10 or greater, the perceived text quickly ceased to be visible in a manner that was surprising to see in practice.

[0192] Below V 限值 In the region 812 below V', the text is visible with low contrast, and in the region 814 below V', the text is clearly visible.

[0193] It would be desirable to maximize the front display brightness to achieve high image visibility for the primary display user. It would further be desirable to achieve a high level of image security against snoopers at viewing angles. Selective control of front brightness will now be described in more detail.

[0194] For an observation angle θ, the maximum display output brightness Y 最大 (usually front brightness) is prevented from exceeding the brightness limit Y 限值 , at the brightness limit Y 限值 At the point where the visual safety level V is higher than the threshold limit V 限值 , so that the image is not perceived at the observation angle θ, the brightness limit Y 限值 is given by the following equation:

[0195]

[0196] where Rθ is the reflected ambient illuminance at viewing angle θ, Kθ is the display black state luminance at viewing angle θ, and Pθ is the maximum display output luminance Y at viewing angle θ. 最大 The relative luminance of the display compared to the front luminance (usually measured in nits). For a display reflectivity ρθ and a Lambertian light source with illuminance Iθ reflected by the display at the viewing angle measured in lux, the luminance limit Y 限值 It is also given by the following equation:

[0197]

[0198] Because the illuminance Iθ depends on the amount of ambient light, the brightness of the display device can be controlled by the control system 500 according to these relationships. Specifically, the privacy transfer function 804 used by the control system 500 as described above can be selected to maintain the relationship Y 最大 ≤Y 限值 , so that the image is not perceived as visible at a desired viewing angle θ (e.g., a viewing angle θ of 45 degrees sideways and zero degrees elevation from the normal to the display device).

[0199] Subject to this limit, the brightness is preferably as high as possible to optimize the performance of front viewing. Therefore, the privacy transfer function 804 used by the control system 500 as described above can also be selected to maintain the following Figure 7B The relationship Y shown by the curve 805 in 最大 / Iθ≥1lux / nit. The illuminance Iθ may be the perceived ambient illuminance averaged from the illuminated scene.

[0200] Advantageously, a display may be provided that has high image security against off-axis snoopers while achieving high image visibility for front-facing users for varying illumination levels.

[0201] A further description of the control of the privacy display will now be described.

[0202] Figure 9D It is a graphic illustration Figure 1-2 and Figure 3-4 Flowchart of the privacy control system.

[0203] The display operating environment 261 may include, but is not limited to, network ID 201, date / time 262, GPS 206 data, primary ambient light sensor 232 detection, and airplane mode 270 settings.

[0204] The corporate privacy policy 240 may include definitions under which the display should be operated in a privacy mode (including when and where); documents and applications; and visual security level specifications.

[0205] Other inputs may include display design parameters 272 and information about the document and application being viewed 274 .

[0206] Data processor 290 is used to analyze the display operating environment 261, display design parameters 272, viewed documents and applications 274, and compare to the company privacy policy 240. The output determines whether to operate the display in a private mode or a public mode, so that switch 292 is set for the private operating mode or the public operating mode based on the data processor 290 output.

[0207] For the privacy mode of operation, settings are provided that are applied to the display device 100 using the display control system 710 and the image 101 using the image control system 296 to achieve a desired level of visual security. Further indication of the level of visual security using the indicator 280 may be provided.

[0208] For common operating modes, appropriate lighting control (including cone angle changes by the display control system 710 and brightness using the LED driver 715 ) is provided to the display device 100 .

[0209] The controller 500 may continue to monitor the status of the display operating environment 261 and appropriate changes to the policies 240 and adjust the display device 100 and image 101 appropriately to maintain a target visual safety level.

[0210] Advantageously, the control system 500 can enable a visual safety level to be set for the current environment of the device (which can be a visual safety level that will be reliably calculated and compared to the level of the company policy 240). The visual safety level can be adjusted to the level required by the display device 100 environment so that the primary user maintains optimal viewing freedom and comfort consistent with achieving the specified company privacy policy privacy level.

[0211] Not discussed in more detail Figure 9D Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0212] It may be desirable to provide improved visual safety at low ambient illumination levels (eg, in a motor vehicle at night).

[0213] Figure 10A is a schematic diagram illustrating a top view of an automobile cabin and a control system, including vehicle lighting arranged to provide desired visual safety, operated in a low exterior light (dark) environment.

[0214] Vehicle 650 includes a privacy display device 100, which is part of a privacy display apparatus 200 as described herein. In this example, the privacy display device 100 is located in front of a non-driver passenger 45 (in this example, the vehicle is left-hand drive), and the driver is the voyeur in the privacy operating mode. Passenger 45 is therefore an on-axis viewer. Driver 47 is therefore an off-axis viewer. Privacy mode is selected when it is desired that the displayed image not be visible to driver 47, typically including when the vehicle is being driven so that driver 47 is not distracted by the displayed image.

[0215] Display device 200 also includes dashboard privacy light 600 and / or door privacy light 602, each of which provides illumination from an illuminated area 652 as follows. In this example, illuminated area 652 is the surface of a panel of a door of the vehicle.

[0216] Dashboard privacy light source 600 is mounted in the dashboard of a vehicle, so as to be remote from an illuminated area 652, but is arranged to illuminate the illuminated area 652. Illumination from dashboard privacy light source 600 is reflected as light 610, so that illumination is provided from the illuminated area 652 by reflection. Dashboard privacy light source 600 may include one or more light-emitting elements and may be directional so as to direct light primarily toward the illuminated area 652. Dashboard privacy light source 600 may be controlled to vary its luminous flux.

[0217] Door privacy light source 602 is installed in illuminated area 652 (i.e., a panel of a portion of the vehicle's door). Therefore, illumination is provided from illuminated area 652 by light directly output by second privacy light source 602. Door privacy light source 602 can be a diffuse source. Door privacy light source 602 can include one or more light-emitting elements. Door privacy light source 602 can be controlled to vary its luminous flux.

[0218] The display device 200 can include either or both of the dashboard privacy light source 600 and the door privacy light source 602, as convenient in the vehicle configuration. Regardless of which is provided, the dashboard privacy light source 600 and the door privacy light source 602 are positioned so that they are not visible to the on-axis viewer 47.

[0219] Illumination area 652 is positioned so that it provides illumination of display device 100 along the incident direction for reflection to a predetermined viewer position of driver 47, who in this example is an off-axis viewer. The predetermined driver 47 position may be at a 45-degree angle to the normal of display 100. Alternatively, it may be desirable to provide a high safety factor, such as an angle of less than 30 degrees or less than 25 degrees, for a driver leaning toward passenger 45 with the intent to view the display.

[0220] Therefore, both the incident direction and the reflection from the display device 100 are at a polar angle greater than 0° with respect to the normal to the display device. In this example, the direction is set by the configuration of the vehicle, but in other examples, the incident direction and reflection can be designed with reference to the location of an off-axis viewer who does not wish to see the displayed image. For example, the polar angle can be 45°.

[0221] When the privacy function of display device 100 is enabled in privacy mode, control system 500 controls the luminous flux of instrument panel privacy light source 600 and door privacy light source 602 so that illumination is provided from illuminated area 652 along the incident direction onto display device 100. This light is reflected in the direction of driver 47. As a result, illuminated area 652 suppresses the visibility of the image displayed by display device 100 by providing external illumination. Generally speaking, suppression occurs because external illumination reduces the contrast of the displayed image. This effect can be similarly seen in the fact that the visual safety level V shown in Equation 4 is improved by increasing the illuminance I. By comparison, illumination from illuminated area 652 is not reflected toward passenger 45, who therefore does not perceive the reduced visibility of the image.

[0222] As described above, the display device includes a primary ambient light sensor 232, and the control system 500 uses the illumination level of ambient light detected by the primary ambient light sensor 232 to control the brightness of the displayed image. Thus, in operation, the primary ambient light sensor 232 is used to determine the desired image brightness for the passenger 45. The primary ambient light sensor 232 is of a type that detects the illumination level of ambient light incident on the display device in a non-directional manner. Thus, the primary ambient light sensor 232 measures the overall cabin illumination, such as through the ambient lighting 604.

[0223] Optionally, the display device 200 further includes a directional ambient light sensor 231 of a type that detects the illuminance level of ambient light incident on the display device 100 along an incident direction for reflection to a predetermined viewer position of the driver 47 at a polar angle greater than 0° relative to the normal of the display device 100. Thus, the directional ambient light sensor 231 detects the illuminance level of light from an illuminated area 652 of the interior of the vehicle on the display device 100. For example, by providing a capture lens 233 in the directional ambient light sensor 231, the directional ambient light sensor 231 can be configured to collect light from the illuminated area 652 only over a cone angle θa. With the aid of calibration, the illuminance of the display device 100 caused by the illuminated area 652 can be determined from the output of the directional ambient light sensor 231.

[0224] At low light levels, natural ambient lighting may decrease, and the brightness of the reflected light seen by driver 47 may be insufficient to achieve the desired level of visual safety. As described elsewhere herein, the brightness of display device 100 can be reduced to achieve reduced image visibility for driver 47. However, image brightness can be reduced to a level that is undesirable for passenger 45. Therefore, to selectively control the luminous flux of instrument panel privacy light source 600 and door privacy light source 602, for example as follows, control system 500 can use either the illuminance level of ambient light detected by primary ambient light sensor 232 or the illuminance level of ambient light detected by directional ambient light sensor 231 (if provided). In each case, a measure of the illuminance of light along the incident direction on display device 100, Iθ, is determined using the detected illuminance level of ambient light. The luminous flux of instrument panel privacy light source 600 and door privacy light source 602 is then controlled based on the derived measure.

[0225] Where a directional ambient light sensor 231 is used, as this provides a measure of the illuminance level of ambient light incident on the display device 100 along the incident direction from the illumination area 652, the measure Iθ of the illuminance of light along the incident direction on the display device 100 derived by the control system 500 is the illuminance level of the ambient light detected by the directional ambient light sensor 231 due to the directional nature of the directional ambient light sensor 231.

[0226] In the case of using the primary ambient light sensor 232, because this detects the illuminance of light on the display device 100 in a non-directional manner, but the illuminance along the incident direction from the illuminated area 652 is increased, the output of the primary ambient light sensor 232 does not directly provide a reliable measure of the illuminance Iθ of the light along the incident direction on the display device 100. However, the increased illuminance along the incident direction from the illuminated area 652 depends on the luminous flux output by the dashboard privacy light source 600 and the door privacy light source 602. Therefore, the control system uses both the illuminance level of ambient light detected by the primary ambient light sensor 232 and the luminous flux of the dashboard privacy light source 600 and the door privacy light source 602 to derive a measure of the illuminance Iθ of the light along the incident direction on the display device.

[0227] The dependence of the measure of illuminance along the incident direction Iθ on the illuminance level of the ambient light detected by the primary ambient light sensor 232 and the luminous flux of the dashboard privacy light source 600 and the door privacy light source 602 can be derived for a given display device 200 in a given situation by taking a calibrated measurement of the directional illuminance along the incident direction and the output of the primary ambient light sensor 232 while varying the amount of ambient light and the luminous flux of the dashboard privacy light source 600 and the door privacy light source 602.

[0228] The level of visual safety for the driver 47 is then determined from the measure Iθ of the illuminance of the light along the direction of incidence on the display device 100 .

[0229] When a measure Iθ of the illuminance of light along an incident direction on the display device 100 falls below a threshold, the control system 500 is arranged to provide at least one of the following operations: (i) reducing the brightness of the display 100 by means of control of the transmission or emission of the display backlight and / or spatial light modulator, and (ii) increasing the illuminance of the display 100 from the lighting area 652 by means of control of the instrument panel privacy light source 600 and the door privacy light source 602.

[0230] In operation, as naturally occurring ambient illumination decreases, such as at night, the ambient light sensor 231 detects reduced brightness from the illuminated area 652 .

[0231] As the light level is adjusted, the display brightness for the passenger 45 is adjusted to provide a comfortable image. However, such a comfortable image may provide undesirable perceived image visibility for the driver 47. In order to achieve the desired visual safety level, the luminous flux of the instrument panel privacy light source 600 and the door privacy light source 602 is controlled to provide an illuminance Iθ of light along the incident direction on the display device 100 to provide a perceived image safety level S of 1.0 or greater, preferably 1.5 or greater, and most preferably 1.8 or greater to the driver 47. Specifically, this can be achieved by maintaining the relationship Iθ≥Iθ by the control system 500. 最小 is achieved where Iθ is a derived measure of the illuminance of light on the display device along the direction of incidence, and Iθ 最小 is given by the following equation:

[0232]

[0233] where Y 最大 is the maximum output brightness of the display device 100, Y 最大 The unit is Iθ 最小 divided by the unit of solid angle in steradian, ρ(θ) is the reflectivity of the display device for light along the incident direction, P(θ) is the ratio of the luminance of the display device along the incident direction to the maximum output luminance of the display device 100, and S 最小 has a desired value. Equation 11 is derived from Equation 4 in consideration of both the reflectivity ρ and the ratio (relative brightness) P relative to the driver 47 .

[0234] Among them S 最小 With a value of 1.0 or greater, the driver 47 cannot perceive the displayed image.

[0235] Among them S 最小 With a value of 1.5 or greater, for most images and most observers, the driver 47 may not even perceive that an image is being displayed.

[0236] Among them S 最小 With a value of 1.8 or greater, the image is invisible to the driver 47 , regardless of the image content for all observers.

[0237] Advantageously, therefore, an image having a desired brightness can be provided to the passenger 45 , and a desired level of perceived image safety can be provided to the driver 47 .

[0238] Furthermore, the position of the driver 47 can be monitored by means of a head detection sensor 644. The portion 654 of the illuminated area 652 that provides illumination to the driver can be calculated from the position of the driver's 47 head, and only this portion 654 can be illuminated, for example, by means of the light source 602 in the portion 654. Advantageously, the size of the illuminated area 652 that is illuminated can be reduced, and the overall ambient light provided within the vehicle can be reduced.

[0239] Figure 10B Illustration by Figure 10A Flowchart of a method for controlling privacy light sources 600, 602 executed by the privacy control system 500.

[0240] In step S1 , the illumination level of ambient light is detected by the primary ambient light sensor 232 and optionally also by the directional ambient light sensor 231 (if provided).

[0241] In step S2, the brightness level of the display device 100 is set using the illumination level of the ambient light detected by the primary ambient light sensor 232. This step is performed as described above to optimize the displayed image for the passenger 45.

[0242] In step S3, a measure of the illuminance Iθ of the light along the incident direction on the display device is derived. This is performed as described above, for example, from the illuminance level of the ambient light detected by the directional ambient light sensor 231 (if provided), or from both the illuminance level of the ambient light detected by the main ambient light sensor 232 and the luminous flux of the privacy light sources 600, 602.

[0243] In optional step S4, the location of driver 47 is measured by sensor 644, and a portion 654 of illuminated area 652 that provides illumination to driver 47 is determined. If step S4 is performed, step S5, described below, is performed on the incident direction from this portion 654 of illuminated area 652. Otherwise, driver 47 is assumed to be at a predetermined location known from the configuration of the vehicle, so step S5, described below, is performed on the incident direction corresponding to this location.

[0244] In step S5, the visual safety level V with respect to the driver 47 and the corresponding incident direction is calculated using Equation 4. This calculation uses the maximum output brightness Y of the display device 100 最大, and a measure of the illuminance Iθ of the light along the incident direction on the display device 100, which is derived as illuminance I in step S3, since the driver 47 is being considered. This calculation also uses the reflectivity ρ(θ) of the display device 100 for light along the incident direction, and the luminance drop-off P(θ) of the display device 100 along the incident direction for reflections toward the driver 47 (i.e., the ratio of the luminance of the display device along the incident direction to the maximum output luminance of the display device 100). The perceived safety level S is then derived from the visual safety level V according to Equation 5.

[0245] In step S6, it is determined whether the privacy light source 600, 602 is currently on. In the case of a positive determination in step S6, the light remains on, so the method proceeds to step S9. In the case of a negative determination in step S6, then the method proceeds to step S7.

[0246] In step S7, based on the perceived safety level S derived in step S5 and the minimum level S 最小 The comparison of the perceived security level S in step S5 determines whether to turn on the privacy light source 600, 602. 最小 , the method proceeds to step S8, in which the privacy light sources 600, 602 are turned on to improve visual safety, after which the method proceeds to step S9. Otherwise, the method returns to step S1, so that the privacy light sources 600, 602 remain off.

[0247] As described above, the minimum level S of the perceived image safety level S of the driver 47 is 最小 It may be chosen to have a value of 1.0 or greater, so as to achieve the effect that the driver 47 cannot perceive the displayed image; a value of 1.5 or greater, so as to achieve the effect that for most images and most observers the driver 47 cannot even perceive that the image is being displayed; or a value of 1.8 or greater, so as to achieve the effect that the image is invisible to the driver 47, regardless of the image content for all observers.

[0248] In step S9, the luminous flux of the privacy light sources 600, 602 is adjusted based on the perceived security level S derived in step S5. If the privacy light sources 600, 602 are currently off, they are turned on. If the privacy light sources 600, 602 are currently on, their luminous flux is adjusted based on the perceived security level S and the minimum level S. 最小 If the perceived image safety S is less than the minimum level S 最小 , the luminous flux of the privacy light sources 600, 602 is increased. On the contrary, if the perceived image security S is greater than the minimum security level S 最小, the luminous flux of the privacy light sources 600 and 602 is reduced.

[0249] This control can be achieved by a feedback loop using the measure of illuminance along the incident direction Iθ as a feedback parameter. Specifically, the control system 500 can adjust the luminous flux of the privacy light sources 600, 602 so as to satisfy the relationship Iθ≥Iθ 最小 , where Iθ is given by Equation 11 above. In this way, the perceived safety level S can be maintained at or above the minimum level S 最小 .

[0250] Advantageously, passengers 45 can view comfortable images under a wide range of ambient lighting conditions, and driver 47 can view the images with little or no distraction.

[0251] Although Figure 10A The example of the display device 200 in a vehicle 650 is described, but similar techniques can be applied to a display device 200 to be used in any environment where it is desirable to limit the visibility of an image to off-axis viewers. By way of example, an example will now be described where the display device 200 is arranged to improve image security against snoopers in an office environment.

[0252] Figure 10C is a schematic diagram illustrating a top view of a privacy display device 200 intended for tabletop use and including a privacy light source 660 arranged to respond to ambient lighting conditions to improve image security from off-axis snoopers. Figure 10C Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0253] The privacy display device 200 is provided in a laptop computer 98, which is shown placed on the surface of a table 670 for a seated user 47, who is an on-axis viewer. The off-axis viewer is a standing peeper 47 who sees a reflection from at least a portion of the display device 100. The display device 200 includes a privacy light source 660, which is arranged in (or near) the laptop computer 98 to illuminate an illuminated area 662 of the table 670. This corresponds to Figure 10A The illuminated area 652 in the example is seen by the snooper 47 as reflected in the display device 100. The display device 200 includes a main ambient light sensor 232 and a directional light sensor 231 and operates in the same manner to provide the same Figure 10A The same effect as in the example above.

[0254] Figure 10Dis a schematic diagram illustrating a front view of the privacy display 100 and the light source 660; and Figure 10E It is a graphic illustration Figure 10D Schematic diagram of a top view of the privacy display 100. Not discussed in detail Figure 10D Features of the embodiments of -E may be assumed to correspond to features having equivalent reference numbers as discussed above, including any possible variations in features.

[0255] and Figure 10C Compared with the implementation plan, Figure 10D Alternative embodiment -E illustrates that light source 660 may be arranged to illuminate the front surface of display 100 to provide reflected light 668 to snooper 47 which may be controlled as described elsewhere herein. Advantageously, the safety factor of display 100 may be increased in environments where other sources of ambient light are limited or absent.

[0256] Some possible arrangements for directional ambient light sensors 231 that measure ambient illuminance in directions corresponding to locations where ambient light reflectivity contributes to visual safety for off-axis viewers will now be described.

[0257] Figure 11A is a schematic diagram illustrating a top view of a privacy display and an off-axis ambient light sensor. Ambient light source 604 is reflected by privacy display 100 toward peeper 47. Ambient light sensor 231 is arranged to measure the ambient illuminance in light cone 605R. In operation, the output of ambient light sensor 231 is arranged to adjust the brightness for user 47 to achieve a desired level of visual safety for the ambient illuminance. In operation, for a typical display without a diffuser or with a limited diffuser (e.g., with a front surface diffuser having a diffusion of AG50 or less), the peeper sees only reflected ambient light reflected from an area surrounding the direction of light cone 605R.

[0258] Figure 11B is a schematic graph illustrating polar regions for measuring ambient illumination for a privacy display. Figure 11B Thus, polar coordinate locations 605R, 605L are indicated within which ambient light sources can be arranged to contribute to the visual safety level observed by an off-axis snoop. Ambient light sources located elsewhere do not contribute to the visual safety factor. Providing a reduction in frontal brightness to compensate for ambient illumination that does not provide an increased level of visual safety (that is, light sources outside of regions 605L, 605R) is undesirable.

[0259] An ambient light sensor that preferentially measures illumination in the polar regions 605L, 605R will now be described.

[0260] Figure 12A -C is a graphic illustration used in Figure 11B Schematic diagram of a top view of an off-axis ambient light sensor for measuring ambient illumination in a polar region.

[0261] Figure 12A The diagram illustrates an ambient light sensor 231 comprising a mask 237 having apertures 241R, 241L separated from the mask 237 by a spacer 239. Sensor 235L measures ambient illumination from an off-axis ambient light source 604L, while sensor 235R measures ambient illumination from an off-axis ambient light source 604R. Advantageously, in a privacy mode of operation, the level of visual security provided to a snooper can be increased in response to appropriately placed ambient light sources 604R, 605L.

[0262] Figure 12B Similar to Figure 12A , except that the two sensors 235L, 235R are replaced by a single sensor 235C. Advantageously, costs are reduced.

[0263] Figure 12C The following embodiment is illustrated in which the sensors 235L, 235R and the masks 237L, 237R are tilted relative to the normal direction of the display device 100, wherein the optical axis 299L, 299R is oriented toward the center of the region 605L, 605R. Figure 12A Compared with the arrangement of FIG, stray light can be reduced and measurement accuracy is improved.

[0264] exist Figure 12A -C embodiment, the hole 241 and the sensor 231 can be shaped to achieve the measurement direction and Figure 11B The polar coordinate locations 605L and 605R match.

[0265] An illustrative example of a display capable of switching between a private mode and a public mode will now be described.

[0266] Figure 13 is a schematic diagram illustrating a front perspective switchable directional display device 100 , which includes a backlight 20 , a switchable liquid crystal retarder 300 , and a spatial light modulator 48 .

[0267] Display device 100 includes a directional backlight 20, such as a collimated backlight arranged to output light, the backlight 20 including a directional waveguide 1; and a plurality of light sources 15 arranged to input light into the waveguide 1, the waveguide 1, a back reflector, and a light control film 5 arranged to direct light from the light sources 15 into a solid angle range 402A. The light control film 5 may include, for example, a turning film and a diffuser.

[0268] In this disclosure, the solid angle range is the solid angle of the light cone within which the brightness is greater than a given relative brightness relative to the peak brightness. For example, the brightness rolloff can be 50% relative brightness so that the solid angle range has the same angular width as the full width at half maximum (FWHM) in a given direction (e.g., the lateral direction).

[0269] Backlight 20 may be arranged to provide a solid angle range 402A of angular light having reduced brightness compared to frontal brightness for off-axis viewing positions.

[0270] The display control system 710 is arranged to provide control of the light source driver 715. The brightness of the LEDs 15 can be controlled by the control system so that the absolute off-axis brightness to a peeper can be controlled.

[0271] The spatial light modulator 48 may include a liquid crystal display including substrates 212, 216 and a liquid crystal layer 214 having red pixels 220, green pixels 222, and blue pixels 224. The spatial light modulator 48 has an input display polarizer 210 and an output display polarizer 218 on opposite sides thereof. The output display polarizer 218 is arranged to provide a high extinction ratio for light from the pixels 220, 222, 224 of the spatial light modulator 48. Typical polarizers 210, 218 may be absorbing polarizers, such as dichroic polarizers.

[0272] Optionally, a reflective polarizer 208 may be provided between the dichroic input display polarizer 210 and the backlight 210 to provide recycled light and improve display efficiency. Advantageously, efficiency may be improved.

[0273] An optical stack that provides control of off-axis brightness will now be described.

[0274] Reflective polarizer 302 , plurality of retarders 300 , and additional polarizer 318 are arranged to receive the output light of spatial light modulator 48 .

[0275] The plurality of retarders 300 are disposed between the reflective polarizer 302 and the additional polarizer 318. The polarizers 210, 218, 318 may be absorbing type polarizers, such as iodine polarizers, while the reflective polarizer 302 may be a stretched birefringent film stack, such as APF from 3M Corporation, or a wire grid polarizer.

[0276] The plurality of retarders 300 comprises a switchable liquid crystal retarder 301 comprising a layer 314 of liquid crystal material and substrates 312, 316 arranged between a reflective polarizer 302 and an additional polarizer 318. The retarder 300 further comprises a passive retarder 330 as will be described further below.

[0277] As described below, the plurality of retarders 300 do not affect the brightness of light that passes through the reflective polarizer 302, the retarder 300, and the additional polarizer 318 along axes that are normal to the plane of the retarder 300, but the retarder 300 reduces the brightness of light that passes through the reflective polarizer 302, the retarder 300, and the additional polarizer 318 along axes that are tilted toward the normal to the plane of the retarder 300, at least in one of the switchable states of the switchable retarder 301. This is caused by the presence or absence of a phase shift introduced by the retarder 300 to light along axes that are at different angles relative to the liquid crystal material of the retarder 300.

[0278] The transparent substrates 312, 316 of the switchable liquid crystal retarder 301 comprise electrodes arranged to provide a voltage across a layer 314 of liquid crystal material 414 therebetween. The control system 752 is arranged to control the voltage applied by the voltage driver 350 across the electrodes of the switchable liquid crystal retarder 301 .

[0279] Not discussed in more detail Figure 13 Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0280] As will be described further below, the additional polarizer 318, the plurality of retarders 300, and the reflective polarizer 302 may be arranged to provide polar control of the output brightness from the ambient lighting 604 and the reflectivity of the front surface.

[0281] Examples of optical stacks that provide control of off-axis brightness will now be described.

[0282] Figure 14 is a schematic diagram illustrating an arrangement of a plurality of retarders 300 in a privacy mode of operation in a perspective side view, the plurality of retarders 300 comprising a negative C-plate passive retarder 330 and a homeotropically oriented switchable liquid crystal retarder 301 in a privacy mode of operation. Figure 14 In the figures, some layers of the optical stack are omitted for clarity. For example, the switchable liquid crystal retarder 301 is shown with the substrates 312, 316 omitted.

[0283] The switchable liquid crystal retarder 301 includes two surface alignment layers, which are disposed on electrodes 413, 415 and adjacent to and on opposite sides of a liquid crystal material layer 414, and each surface alignment layer is arranged to provide a homeotropic orientation in the adjacent liquid crystal material 414. The liquid crystal material layer 414 of the switchable liquid crystal retarder 301 includes a liquid crystal material having negative dielectric anisotropy. The liquid crystal molecules 414 may be provided with a pre-tilt angle (e.g., 88 degrees) relative to the horizontal direction to remove degeneracy in switching.

[0284] The electric vector transmission direction of the reflective polarizer 302 is parallel to the electric vector transmission direction of the output polarizer 218. Furthermore, the electric vector transmission direction 303 of the reflective polarizer 302 is parallel to the electric vector transmission direction 319 of the additional polarizer 318.

[0285] The switchable liquid crystal retarder 301 comprises a layer 314 of liquid crystal material 414 having negative dielectric anisotropy. The passive retarder 330 comprises a negative C-plate having an optical axis perpendicular to the plane of the retarder 330, schematically illustrated by the orientation of the disc-shaped material 430.

[0286] The liquid crystal retarder 301 further includes transmissive electrodes 413, 415 arranged to control the liquid crystal material, and the liquid crystal material layer can be switched by adjusting the voltage applied to the electrodes. The electrodes 413, 415 can be spread across the layer 314 and arranged to apply the voltage for controlling the liquid crystal retarder 301. The transmissive electrodes are on opposite sides of the liquid crystal material layer 414 and can be, for example, ITO electrodes.

[0287] Alignment layers may be formed between electrodes 413, 415 and the liquid crystal material 414 of layer 314. The orientation of the liquid crystal molecules in the xy plane is determined by the pretilt directions of the alignment layers, such that each alignment layer has a pretilt angle, wherein the pretilt angle of each alignment layer has a pretilt direction having a component 417a, 417b in the plane of layer 314, the components 417a, 417b being parallel, antiparallel, or orthogonal to the electric vector transmission direction 303 of the reflective polarizer 302.

[0288] Driver 350 applies voltage V to electrodes 413, 415 on layer 314 of switchable liquid crystal material 414 to tilt the liquid crystal molecules relative to vertical. The plane of tilt is determined by the pre-tilt angle direction of the alignment layer formed on the inner surface of substrates 312, 316.

[0289] In a typical use case for switching between public and private modes, the liquid crystal material layer can switch between two states: a first state, a public mode that allows the display to be used by multiple users, and a second state, a private mode for a primary user with minimal visibility to snoopers. Switching can be accomplished by applying voltages to electrodes. Generally speaking, such a display can be considered to have a first, wide-angle state and a second, reduced off-axis brightness state.

[0290] Not discussed in more detail Figure 14 Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0291] We will now describe Figure 13 Polar plots of various elements of an illustrative embodiment of a stack.

[0292] Figure 15A is a schematic graph illustrating the polar and azimuthal variations in output brightness of a collimated backlight and a spatial light modulator.

[0293] Figure 15B is a schematic graph illustrating the polar and azimuthal variation of transmission of a switchable retarder arranged between parallel polarizers for the illustrative embodiment of Table 2.

[0294]

[0295] Table 2

[0296] Figure 15C is a schematic graph illustrating the polar and azimuthal variations in relative reflection of a switchable retarder disposed between a reflective polarizer and an absorbing polarizer for the illustrative embodiment of Table 2.

[0297] Figure 15D Is a graphic illustration of Figure 13 Schematic graph of the polar and azimuthal variations in total display reflectivity when arranged in a privacy mode of operation (that is, a polar coordinate graph ρ(θ, φ) of reflectivity, where θ is the polar angle and φ is the azimuthal angle).

[0298] Figure 15E Is a graphic illustration of Figure 13 Schematic graph of polar and azimuthal variations in output brightness when the arrangement is in a privacy operation mode (that is, a polar coordinate curve P(θ, φ) with respect to the privacy level).

[0299] Figure 15F Is a graphic illustration of Figure 13 The arrangement in the privacy mode of operation, for the value Y measured in nits最大 Schematic graph of the polar and azimuthal angle variations of the visual safety level S(θ,φ) of the front luminance of the display, with the value Y measured in nits 最大 The display front brightness is half the illuminance of the value I measured in lux. Contour lines for S = 1.0, S = 1.5, and S = 1.8 are illustrated to show extreme regions of image privacy and image visibility. Contour lines for S = 0.1 are illustrated to show extreme regions of high image visibility.

[0300] Figure 15G Is a diagram illustrating Figure 13 The arrangement in the privacy mode of operation, for the value Y measured in nits 最大 Schematic diagram of the polar angle variation of the visual safety level S for the front luminance of the display for zero elevation angle, the value Y measured in nits 最大 The display front brightness is half the illuminance of the value I measured in lux. At 45 degrees, the display is controlled so that the display's I / Y 最大 The ratio (lux / nit) is set to 2.0, and the image is not visible at extreme angles of + / - 45 degrees.

[0301] We will now describe Figure 13 The display is operated in public mode.

[0302] Figure 16 is a schematic diagram illustrating the arrangement of the retarder 300 in a common mode of operation in a perspective side view. In this embodiment, as in Table 2, zero volts is provided across the liquid crystal retarder 301.

[0303] and Figure 14 Compared to the arrangement of , no voltage is applied and the molecules of the liquid crystal material 414 are arranged substantially perpendicular to the alignment layer and electrodes 413 , 415 .

[0304] Not discussed in more detail Figure 16 Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0305] Figure 17A Is a diagram illustrating Figure 13 A schematic graph showing polar and azimuthal variations in output brightness when the arrangement is in a common operating mode; Figure 17B Is a diagram illustrating Figure 13 The arrangement in the common operating mode, for the value Y measured in nits 最大Schematic diagram of the polar angle variation of the visual safety level S for the front luminance of the display for zero elevation angle, the value Y measured in nits 最大 The front brightness of the display is half the illuminance of the value I measured in lux. Figure 15F Compared to the arrangement of FIG, the display remains visible to the user over a wide polar area near the axis with highest visibility.

[0306] Figure 17C It is a diagram illustrating the maximum brightness Y 最大 A schematic graph showing the polar angle and azimuth angle changes of the output brightness of the backlight source in the direction of 最大 The direction of is not perpendicular to the display. With Y at location 890 (which is the display normal) 最大 of Figure 17A compared to, Figure 17C Illustration Description 最大 At a location 892 above the axis. Advantageously, the display brightness can be increased for a user looking down at the display. The propagation of polarized light from the output polarizer 218 will now be considered for both on-axis and off-axis directions of a display operating in privacy mode.

[0307] Figure 18A is a side view illustrating the output light of the spatial light modulator passing through the Figure 13 Schematic diagram of the propagation of the optical stack.

[0308] When the layer 314 of liquid crystal material 414 is driven to operate in the privacy mode, the retarder 300 does not provide a total transformation of the polarization component 360 to output light 400 that passes through the retarder 300 along an axis perpendicular to the plane of the switchable retarder, but provides a total transformation of the polarization component 361 to light 402 that passes through the retarder 300 for some polar angles that are acute angles to the perpendicular to the plane of the retarder.

[0309] Polarization component 360 from output polarizer 218 is transmitted by reflective polarizer 302 and is incident on retarder 300. On-axis light has polarization component 362 that does not modify component 360, while off-axis light has polarization component 364 that is transmitted by retarder 300. Minimally, polarization component 361 is converted to linear polarization component 364 and is absorbed by additional polarizer 318. More generally, polarization component 361 is converted to an elliptically polarized component, that is, is partially absorbed by additional polarizer 318.

[0310] Figure 15BThe polar distribution of light transmission shown in modifies the polar distribution of the luminance output of the underlying spatial light modulator 48. Where the spatial light modulator 48 comprises a directional backlight 20, the off-axis luminance may be further reduced as described above.

[0311] Not discussed in more detail Figure 18A Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0312] Advantageously, a privacy display is provided that has low brightness for off-axis snoopers while maintaining high brightness for on-axis observers.

[0313] The operation of reflective polarizer 302 on light from ambient light source 604 will now be described for a display operating in a privacy mode.

[0314] Figure 18B is a top view illustrating the privacy mode of operation with ambient lighting passing through Figure 13 Schematic diagram of the propagation of the optical stack.

[0315] Ambient light source 604 illuminates display device 100 with unpolarized light. Additional polarizer 318 transmits light 410 perpendicular to display device 100 together with a first polarization component 372 , which is a linear polarization component parallel to electric vector transmission direction 319 of additional polarizer 318 .

[0316] In both operating states, polarization component 372 remains unmodified by retarder 300, so transmitted polarization component 382 is parallel to the transmission axis of reflective polarizer 302 and output polarizer 218, so ambient light is directed through spatial light modulator 48 and lost.

[0317] By comparison, for ray 412, off-axis light is directed through retarder 300 so that polarization component 374 incident on reflective polarizer 302 can be reflected. Such polarization component is converted again into component 376 after passing through retarder 300 and is transmitted through additional polarizer 318.

[0318] Thus, when the layer of liquid crystal material 314 is in the second of the two states, the reflective polarizer 302 provides no reflected light for ambient light 410 that passes through the additional polarizer 318 and then through the retarder 300 along an axis perpendicular to the plane of the retarder 300, but provides reflected light 412 for ambient light that passes through the additional polarizer 318 and then through the retarder 300 at some polar angle that is acute to the perpendicular to the plane of the retarder 300; wherein the reflected light 412 passes back through the retarder 300 and is then transmitted by the additional polarizer 318.

[0319] The retarder 300 therefore does not provide a total transformation of the polarization component 380 into ambient light 410, which passes through the additional polarizer 318 and then passes through the retarder 300 along an axis perpendicular to the plane of the switchable retarder, but does provide a total transformation of the polarization component 372 into ambient light 412, which passes through the absorbing polarizer 318 and then passes through the retarder 300 at some polar angles, some of which are acute angles with respect to the perpendicular to the plane of the retarder 300.

[0320] Figure 15C The polar distribution of light reflection shown in thus illustrates that high reflectivity can be provided at a typical snooper location by means of the privacy state of the retarder 300. Thus, in the privacy mode of operation, the reflectivity for off-axis viewing positions is as follows: Figure 15C , and for off-axis light from the spatial light modulator the brightness is as follows Figure 15B The reduction is shown in .

[0321] In a common operating mode, the control system 710, 752, 350 is arranged to switch the switchable liquid crystal retarder 301 to a second retarder state in which a phase shift is introduced to a polarization component passing through the switchable liquid crystal retarder 301 along an axis tilted relative to the normal to the plane of the switchable liquid crystal retarder 301.

[0322] By comparison, the solid angle range 402D may be substantially the same as the solid angle range 402B in the common operating mode.Such control of the output solid angle ranges 402C, 402D may be achieved by synchronous control of the multiple groups 15, 17 of light sources and at least one switchable liquid crystal retarder 300.

[0323] Advantageously, a privacy mode with low image visibility for off-axis viewing can be achieved, and for a public mode of operation, a large solid angle range can be provided with high efficiency for sharing display images among multiple users and improving image spatial uniformity.

[0324] An additional polarizer 318 is arranged on the same output side of the spatial light modulator 48 as the display output polarizer 218, which may be an absorbing dichroic polarizer. The display polarizer 218 and the additional polarizer 318 have parallel electric vector transmission directions 219, 319. As will be described below, such a parallel orientation provides high transmission for central viewing locations.

[0325] The transmissive spatial light modulator 48 is arranged to receive output light from the backlight source; the input polarizer 210 is arranged on the input side of the spatial light modulator, between the backlight source 20 and the spatial light modulator 48; the output polarizer 218 is arranged on the output side of the spatial light modulator 48; the additional polarizer 318 is arranged on the output side of the output polarizer 218; and the switchable liquid crystal retarder 300 includes a liquid crystal material layer 314 arranged in this case between at least one additional polarizer 318 and the output polarizer 318, wherein the additional polarizer 318 is arranged on the output side of the output polarizer 218; and the control system 710 is arranged to synchronously control the light sources 15, 17 and at least one switchable liquid crystal retarder 300.

[0326] The control system 710 further comprises control of a voltage controller 752 arranged to provide control of the voltage driver 350 in order to enable control of the switchable liquid crystal retarder 301 .

[0327] Not discussed in more detail Figure 18B Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0328] Advantageously, a privacy display is provided that has high reflectivity to off-axis snoopers while maintaining low reflectivity to on-axis observers. As described above, such increased reflectivity provides enhanced privacy performance for displays in ambient-lit environments.

[0329] The operation in the public mode will now be described.

[0330] Figure 19A is a side view illustrating the output light from the spatial light modulator in a common mode of operation through Figure 1 a schematic diagram of the propagation of an optical stack; and Figure 19B Is a diagram illustrating Figure 19A A schematic graph showing the variation of the output brightness of the transmitted light with the polar direction.

[0331] Not discussed in more detail Figure 19A and Figure 19B Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0332] When the liquid crystal retarder 301 is in the first of the two states, the retarder 300 does not provide a total transformation of the polarization components 360, 361 to output light that passes through the switchable retarder 301 perpendicular to the plane of the switchable retarder 301 or at an acute angle to the perpendicular to the plane of the switchable retarder 301. That is, the polarization component 362 is substantially the same as the polarization component 360, and the polarization component 364 is substantially the same as the polarization component 361. Therefore, Figure 19B The angular transmission curve of φ(R) is substantially uniform over a wide polar area. Advantageously, the display can be switched to a wide field of view.

[0333] Figure 19C The top view illustrates the common operating mode with ambient lighting through Figure 1 a schematic diagram of the propagation of an optical stack; and Figure 19D Is a diagram illustrating Figure 19C A schematic graph showing the change in reflectivity of reflected light with polar direction.

[0334] Thus, when the liquid crystal retarder 301 is in the first of the two states, the retarder 300 does not provide an overall transformation of the polarization component 372 into ambient light 412, which passes through the additional polarizer 318 and then through the retarder 300, that is, perpendicular to the plane of the retarder 300 or at an acute angle to the perpendicular to the plane of the retarder 300.

[0335] In the common mode of operation, input light 412 has polarization state 372 after being transmitted through additional polarizer 318. For both the head-on and off-axis directions, polarization conversion does not occur, and therefore the reflectivity for light 402 from reflective polarizer 302 is low. Light 412 is transmitted by reflective polarizer 302 and is not reflected by display polarizers 218, 210, or Figure 1 Backlight, or Figure 2 The transmitted light is lost in the optical isolators 218 , 518 in the spatial light modulator 38 .

[0336] Not discussed in more detail Figure 19C and Figure 19D Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0337] Advantageously, in a public mode of operation, high brightness and low reflectivity are provided over a wide field of view. Such a display can be conveniently viewed by multiple observers with high contrast.

[0338] A display device including an emissive display will now be described.

[0339] Figure 20 is a schematic diagram illustrating a switchable directional display device in a front perspective view, the switchable directional display device including a directional backlight and two switchable liquid crystal retarders, each switchable liquid crystal retarder being arranged between a pair of polarizers. Figure 13 Compared to the arrangement of CMOS, an emissive display such as an OLED display or a micro-LED display includes a further quarter wave plate 202 between the pixel layer 214 and the output polarizer 218. Advantageously, undesirable reflectivity from the backplate 214 is reduced.

[0340] Not discussed in more detail Figure 20 Features of the embodiments may be assumed to correspond to features having equivalent reference numerals as discussed above, including any possible variations in features.

[0341] Figure 21A is a schematic graph illustrating polar and azimuthal variations in output brightness of an emitting spatial light modulator.

[0342] Figure 21B is a schematic graph illustrating the polar and azimuthal variations in transmission of a first switchable retarder disposed between a first pair of parallel polarizers for the illustrative embodiment of Table 3.

[0343]

[0344] Table 3

[0345] Figure 21C is a schematic graph illustrating the polar and azimuthal variations in relative reflection of a first switchable retarder 300A disposed between a reflective polarizer 302 and an absorptive polarizer 318A for the illustrative embodiment of Table 3.

[0346] Figure 21D Is a graphic illustration of Figure 20 Schematic graphs of polar and azimuthal variations of the total display reflectivity ρ(θ, φ) for an arrangement in a privacy mode of operation.

[0347] Figure 21E is a schematic graph illustrating the polar and azimuthal variations in transmission of a second switchable retarder 300B disposed between a second pair of parallel polarizers for the illustrative embodiment of Table 3.

[0348] Figure 21F Is a graphic illustration of Figure 20 Schematic graph of polar and azimuthal angle variations of output brightness P(θ, φ) when arranged in privacy operation mode.

[0349] Figure 21G Is a graphic illustration of Figure 20 The arrangement in the privacy mode of operation, for the value Y measured in nits 最大 Schematic diagram of the polar and azimuthal angle variation of the visual safety level S for the frontal luminance of a display, with the value Y measured in nits 最大 The display front brightness is half the illuminance of value I measured in lux.

[0350] Figure 21H Is a diagram illustrating Figure 20 The arrangement in the privacy mode of operation, for the value Y measured in nits 最大 Schematic diagram of the polar angle variation of the visual safety level S for the front luminance of the display for zero elevation angle, the value Y measured in nits 最大 The display front brightness is half the illuminance of the value I measured in lux. Desirably, at + / - 45°, the safety level S is greater than 1.8.

[0351] Other types of switchable privacy displays will now be described.

[0352] The display device 100, which can be switched between a private mode of operation and a public mode of operation, includes an imaging waveguide and a light source array as described in U.S. Patent No. 9,519,153, which is incorporated herein by reference in its entirety. The imaging waveguide images the light source array onto an optical window that can be controlled to provide high brightness on-axis and low brightness off-axis in the private mode, and high brightness with a large cube angle for public operation.

[0353] Liquid crystal displays with switchable angular contrast curves are described in Japanese Patent Publication No. JPH130783 and U.S. Patent Publication No. 2017-0123241, both of which are incorporated herein by reference in their entirety. Such displays can provide out-of-plane tilting of liquid crystal molecules in the liquid crystal layer 214 of the liquid crystal display and can achieve reduced off-axis image contrast in a privacy mode of operation. The control system 500 for the display device 100 can further include control of the out-of-plane tilting of the liquid crystal molecules.

[0354] As may be used herein, the terms "substantially" and "approximately" provide industry-accepted tolerances for their corresponding terms and / or correlations between terms. Such industry-accepted tolerances range from zero percent to ten percent and correspond to, but are not limited to, component values, angles, etc. Such correlations between terms range from approximately zero percent to ten percent.

[0355] 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, not limitation. Therefore, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined solely in accordance with any claims issuing from this disclosure and their equivalents. In addition, the above advantages and features are provided in the described embodiments, but the application of such issued claims should not be limited to processes and structures that achieve any or all of the above advantages.

[0356] In addition, the section headings herein are provided for suggested consistency under 37 CFR 1.77 or to provide organizational clues. These headings should not limit or characterize the one or more embodiments that may be set forth in any claims that may be published from this disclosure. Specifically and by way of example, although a heading refers to a "Technical Field," the claims should not be limited by the language selected under that heading to describe the so-called technical field. Further, the description of the technology in the "Background" section is not to be interpreted as an admission that a certain technology is prior art to any one or more embodiments in this disclosure. "Summary of the Invention" is also not to be considered as a description of the features of one or more embodiments set forth in the published claims. In addition, any reference to the singular "invention" in this disclosure should not be used to prove that there is only one novel point in this disclosure. According to the limitations of the multiple claims published from this disclosure, multiple embodiments may be set forth, and these claims accordingly define the one or more embodiments protected by them, and their equivalents. In all cases, the scope of these claims should be understood in accordance with the essence of these claims themselves in light of this disclosure, and should not be limited by the headings listed herein.

Claims

1. A privacy display device, comprising: a display device arranged to display an image, the display device being capable of providing a privacy function in which visibility of the image to an off-axis viewer is reduced compared to visibility of the image to an on-axis viewer; at least one privacy light source arranged to provide illumination from an illuminated area, the illumination arranged to illuminate the display device along an incident direction for reflection to a predetermined viewer position at a polar angle greater than 0° to a normal to the display device; as well as A control system is arranged to control the display device and, when the privacy function is provided, to control the luminous flux of the at least one privacy light source. 2 . The privacy display device of claim 1 , further comprising an ambient light sensor arranged to detect an illumination level of ambient light.

3. A privacy display device according to claim 2, wherein the control system is arranged to use the illuminance level of ambient light detected by the ambient light sensor to derive a measure of the illuminance of light along the incident direction on the display device, and selectively control the luminous flux of the at least one privacy light source based on the derived measure.

4. The privacy display device according to claim 3, wherein the ambient light sensor is a directional sensor arranged to detect the illuminance level of ambient light incident on the display device along the incident direction, whereby the measure Iθ of the illuminance of light along the incident direction on the display device is the illuminance level of the ambient light detected by the ambient light sensor.

5. A privacy display device according to claim 3, wherein the ambient light sensor is arranged to detect illuminance levels of ambient light from a range of directions, and the control system is arranged to use both the illuminance levels of ambient light detected by the ambient light sensor and the luminous flux of the at least one privacy light source to derive a measure of the illuminance of light along the incident direction on the display device.

6. The privacy display device according to any one of claims 3 to 5, wherein the control system is arranged to control the luminous flux of the at least one privacy light source to maintain the relationship Iθ≥Iθ 最小 , where Iθ is a derived measure of the illuminance of light on the display along the incident direction, and Iθ 最小 is given by the following equation: in Y 最大 is the maximum output brightness of the display device, Y 最大 The unit is Iθ 最小 Divide by the units of solid angle in steradians, ρ(θ) is the reflectivity of the display device for light along the incident direction, P(θ) is the ratio of the luminance of the display device along the incident direction to the maximum output luminance of the display device, and S 最小 Has a value of 1.0 or greater.

7. The privacy display device according to claim 6, wherein S 最小 Has a value of 1.5 or greater.

8. The privacy display device according to claim 6, wherein S 最小 Having a value of 1.8 or greater.

9. A privacy display device according to any one of claims 2 to 5, wherein the control system is further arranged to control the brightness of the displayed image based on the detected level of ambient light.

10. The privacy display device according to any one of claims 1 to 5, wherein the illuminated area is a surface, and the at least one privacy light source is arranged to illuminate the illuminated area so as to provide the illumination by reflection from the surface. 11 . The privacy display device according to claim 1 , wherein the at least one privacy light source is arranged in the illuminated area so as to provide the illumination as light outputted thereby.

12. The privacy display device according to any one of claims 1 to 5, wherein the privacy light source is invisible to the on-axis viewer.

13. The privacy display device according to any one of claims 1 to 5, wherein the illuminated area is a portion of a door of a motor vehicle.

14. A privacy display device according to any one of claims 2 to 5, wherein the display device is capable of operating in at least a public mode and a private mode, wherein in the private mode, the privacy function is provided and the visibility of the image to off-axis viewers is reduced compared to the public mode, and the control system is capable of selectively operating the display device in the public mode or the private mode.

15. A privacy display apparatus according to claim 14, wherein the control system is arranged to selectively operate the display device in the public mode or the privacy mode in response to the detected level of ambient light. 16 . The privacy display device of claim 14 , wherein the transfer function provides a higher brightness of the displayed image in the public mode than in the privacy mode. 17 . The privacy display apparatus according to claim 1 , wherein a maximum output brightness of the display device is along a normal line of the display device.

18. A method of controlling a display device, the display device being arranged to display an image and being capable of providing a privacy function in which visibility of the image to an off-axis viewer is reduced compared to visibility of the image to an on-axis viewer, the method comprising: providing at least one privacy light source arranged to provide illumination from an illuminated area, the illumination being arranged to illuminate the display device along an incident direction for reflection to a predetermined viewer position at a polar angle greater than 0° to a normal to the display device; and When the privacy function is provided, the luminous flux of the at least one privacy light source is controlled. The method of claim 18 , further comprising detecting an illumination level of ambient light.

20. The method according to claim 19, wherein The method further comprises using the detected illuminance level of the ambient light sensor to derive a measure of the illuminance of light on the display along the incident direction; and The step of controlling the luminous flux of the at least one privacy light source is performed based on the derived measure.

21. A method according to claim 20, wherein the detected illuminance level is the illuminance level of ambient light incident on the display device along the incident direction, whereby the measure Iθ of the illuminance of light along the incident direction on the display is the illuminance level of ambient light detected by the ambient light sensor.

22. The method according to claim 20, wherein The detected illumination levels are the illumination levels of ambient light from a range of directions, and A measure of the illuminance of light along the direction of incidence on the display is derived using both the illuminance level of ambient light detected by the ambient light sensor and the luminous flux of the at least one privacy light source.

23. The method of claim 20, further comprising controlling the luminous flux of the at least one privacy light source to maintain the relationship Iθ≥Iθ 最小 , where Iθ is a derived measure of the illuminance of light on the display along the incident direction, and Iθ 最小 is given by the following equation: in Y 最大 is the maximum output brightness of the display device, Y 最大 The unit is Iθ 最小 Divide by the units of solid angle in steradians, ρ(θ) is the reflectivity of the display device for light along the incident direction, P(θ) is the ratio of the luminance of the display device along the incident direction to the maximum output luminance of the display device, and S 最小 Has a value of 1.0 or greater.

24. The method according to claim 23, wherein S 最小 Has a value of 1.5 or greater.

25. The method according to claim 24, wherein S 最小 Having a value of 1.8 or greater.

26. A method according to any one of claims 19 to 25, further comprising controlling the brightness of a displayed image based on the detected level of ambient light.

27. A method according to any one of claims 18 to 25, wherein the illuminated area is a surface and the at least one privacy light source is arranged to illuminate the illuminated area so as to provide the illumination by reflection from the surface.

28. The method of any one of claims 18 to 25, wherein the at least one privacy light source is arranged in the illuminated area so as to provide the illumination as light output therefrom.

29. The method of any one of claims 18 to 25, wherein the privacy light source is invisible to the on-axis viewer.

30. A method according to any one of claims 18 to 25, wherein the illuminated area is part of a door of a motor vehicle.

31. A method according to any one of claims 18 to 25, wherein the display device is capable of operating in at least a public mode and a private mode, wherein in the private mode the privacy function is provided and the visibility of the image to an off-axis viewer is reduced compared to the public mode.

32. The method of any one of claims 18 to 25, wherein the maximum output brightness of the display device is along a normal to the display device.

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