Backlighting with an optical film, lighting device, screen, and optical film

Abstract

The invention relates to a backlighting (13) which extends in a flat manner, emits light, and has an optical film in order to monitor and limit the viewing angle range of a viewer. When viewed in the direction of a viewer, the optical film comprises a first polarization layer (1) with a first absorption axis, which forms an angle of 0° to 30° together with a surface normal of the optical film, at least one phase-shifting compensation layer for improving the limitation of the viewing angle range, and a second polarization layer (2) with a second absorption axis, which is oriented parallel to the surface of the optical film. According to the invention, different embodiments and combinations of compensation layers which are designed to be spatially homogenous and which are made of uniaxially or biaxially birefringent materials are provided, wherein the materials and thickness of the compensation layers are specified such that the luminance is minimal in a specified solid angle range which does not comprise the entire half space, apart from a recessed cone along the viewing direction.

Description

title

[0001] Backlight with an optical film and lighting device, screen and optical film Technical field of the invention

[0002] The invention relates to a backlight which is extensive, emits light and has an optical film for controlling and restricting a viewer's viewing angle range, as well as the optical film. Such a film comprises a first polarization layer with a first absorption axis which encloses an angle of 0° to 30° inclusive with a surface normal of the optical film, and a second polarization layer with a second absorption axis which is oriented parallel to the surface of the optical film. At least one phase-shifting compensation layer is arranged between them to improve the restriction of the viewing angle range. Viewed from the direction of a viewer, either the first or the second polarization layer can form the layer closest to the viewer.

[0003] In recent years, great strides have been made in widening the viewing angle range of LCDs. However, there are often situations in which the very wide viewing angle of a screen can be a disadvantage. Information such as banking details and other personal and sensitive data is also becoming increasingly available on mobile devices such as notebooks and tablet PCs. Accordingly, people need control over who can see this sensitive data. They must be able to choose between a wide viewing angle range - a public mode - in order to share information on their display with others, e.g. when looking at holiday photos or for advertising purposes. On the other hand, they need a narrow viewing angle range - in a private mode - if they want to keep the image information confidential.

[0004] A similar problem arises in vehicle construction: The driver must not be distracted by image content, such as digital entertainment programs, when the engine is running, while the passenger also wants to consume this content while driving. Therefore, a screen that can switch between the corresponding display modes is required.

[0005] Additional films based on micro-louvres have already been used for mobile displays to achieve visual privacy. However, these films were not switchable or reversible; they always had to be applied and removed manually. They also had to be transported separately from the display when not in use. A major disadvantage of using such louvre films is the associated light loss. State of the art

[0006] US Pat. No. 6,765,550 B2 describes such a privacy screen using micro-louvres. The biggest disadvantage here is the mechanical removal or installation of the filter, as well as the loss of light in protected mode.

[0007] US Pat. No. 5,993,940 A describes the use of a film with evenly spaced, small, strip-shaped prisms on its surface to achieve a private mode, i.e., a restricted viewing mode with a narrow viewing angle range. Development and production are technically quite complex.

[0008] In WO 2012 / 033583 A1, switching between unobstructed and restricted view is achieved by controlling liquid crystals between so-called "chromonic" layers. This results in light loss and requires considerable technical effort.

[0009] US 2012 / 0235891 A1 describes a very complex backlight in a display. According to Figs. 1 and 15, this system not only uses multiple light guides, but also other complex optical elements such as microlens elements 40 and prism structures 50, which reshape the light from the rear illumination on its way to the front illumination. This is expensive and technically complex to implement and also involves light loss. According to the variant shown in Fig. 17 in US 2012 / 0235891 A1, both light sources 4R and 18 produce light with a narrow illumination angle, with the light from the rear light source 18 first undergoing a complex conversion into light with a wide illumination angle. This complex conversion, as already mentioned above, significantly reduces brightness.

[0010] According to JP 2007-155783 A, special optical surfaces 19 are used, which are complex to calculate and manufacture, and which then deflect light into different narrow or wide areas depending on the angle of incidence. These structures resemble Fresnel lenses. Furthermore, there are interference edges that deflect light in undesirable directions. Thus, it remains unclear whether truly meaningful light distributions can be achieved.

[0011] US 2013 / 0308185 A1 describes a special light guide with steps that emits light over a large surface in different directions, depending on the direction from which it is illuminated from a narrow side. In conjunction with a transmissive image display device, e.g. an LC display, this makes it possible to create a screen that can be switched between free and restricted view mode. A disadvantage of this is that the restricted view effect can only be created for left / right or for top / bottom, but not for left / right / top / bottom simultaneously, as is necessary for certain payment transactions, for example. In addition, even in restricted view mode, residual light is still visible from blocked viewing angles.

[0012] The applicant's WO 2015 / 121398 A1 describes a screen with two operating modes, in which scattering particles are present in the volume of the corresponding light guide for switching between the operating modes. However, the polymer scattering particles selected there generally have the disadvantage that light is coupled out of both large surfaces, causing approximately half of the useful light to be emitted in the wrong direction, namely toward the backlight, where it cannot be adequately recycled due to their design. Furthermore, the polymer scattering particles distributed in the volume of the light guide can, under certain circumstances, particularly at higher concentrations, lead to scattering effects that reduce the privacy effect in the protected operating mode.

[0013] The approach of the “Electrical Birefringence (EDB)” technology is based on the idea of ​​using the switchable liquid crystals of an additionally applied LC- Panels to "filter" all light rays that do not exit the imaging layer at a specific beam angle. Disadvantages of this technology include high additional energy and cost requirements and the difficult-to-change + / -40 0 Sweet spot, i.e. the best possible viewing position. The absorption coefficient of the LC structures is also insufficient, since the attenuation of the light intensity increases again for viewing angles greater than the sweet spot, so that the light intensity for viewing angles greater than + / -40 0 up to 3% of the maximum light intensity.

[0014] US 2019 / 0094626 A1 describes an optical layer structure for controlling or restricting the viewing angle. It comprises two phase-difference plates arranged as compensation layers on a linear polarizer. Both phase-difference plates are λ / 4 plates with structured, optically anisotropic, lamellar or strip-shaped layers, between which a carrier material can be arranged. They can be constructed identically, but differ in their orientation in the finished arrangement. The topmost layer forms a polarization layer, in which the absorption axes of the transition dipole moments responsible for polarization are oriented in a direction perpendicular to the layer surface; this is also referred to as a "Z-polarizer."With a layer structured with lamellae in this way, there is always the possibility that visual artifacts such as moiré fringes may occur, so that such layers can only be used for screens whose characteristics such as resolution and dimensions, apertures, scattering properties, distance to the screen surface, etc. are known, but cannot be used universally and independently of the screen dimensions.

[0015] The aforementioned methods and arrangements generally have the disadvantage that they significantly reduce the brightness of the base screen and / or require a complex and expensive optical element for mode switching and / or reduce the resolution in the freely viewable, public mode and / or exhibit visual artifacts on very high-resolution displays. A further disadvantage is that the restriction of the viewing angle is not complete; sometimes, one can view the screen content with significantly reduced brightness, even though the viewing angle is intended to be limited. This can be disruptive, for example, when driving at night in a motor vehicle. Description of the invention

[0016] The object of the invention is to develop a backlight with an optical film for controlling and restricting the viewing angle of a viewer looking at the film - which is usually combined with a screen - with which the restriction of the viewing angle is improved, so that it is further difficult for unauthorized users to spy on protected image content, thus further improving the so-called privacy effect.

[0017] This object is achieved for an optical film and a backlight comprising such an optical film, with a layer structure as briefly described at the beginning, by special configurations of the at least one compensation layer, wherein the first and second polarization layers are first explained in more detail. The first polarization layer has an absorption axis which encloses an angle of 0° to 30° inclusive with a surface normal of the optical film. If the angle is 0°, i.e. the absorption axis is parallel to the surface normal or perpendicular to the film surface, this is a so-called "Z-polarizer". If angles deviate from this within the above-mentioned range up to 30°, the notation "Z*-polarizer" is also used below.The absorption of the first polarization layer is generally static, but can also be designed to be switchable, so that the angle-dependent absorption can be switched on and off. The second polarization layer has a second absorption axis, wherein the second absorption axis is oriented parallel to the surface of the optical film. The second polarization layer is therefore a conventional linear polarizer. At least one phase-shifting compensation layer is arranged between the first polarization layer and the second polarization layer to improve the limitation of the viewing angle range. Both the first and the second polarization layer can be arranged closest to a viewer. All layers are expediently firmly connected to one another, for example by means of a material bond, by welding, or by optical bonding.

[0018] For easier understanding, it is generally assumed in the following, without loss of generality, that the surface of the film is in a Cartesian coordinate system, which is defined by an x-direction, a y-direction and a z-direction lies in a plane parallel to an xy-plane spanned by the x-direction and the y-direction. Accordingly, the surface normal is then parallel to the z-direction.

[0019] To improve the privacy effect, i.e., to improve the limitation of the viewing angle range, the at least one phase-shifting compensation layer can fundamentally be designed in two ways. In a first alternative—which is also referred to below as alternative i—a first B* compensation layer is arranged between the first polarization layer and the second polarization layer, which consists of a first, biaxially birefringent material. A biaxially birefringent material has two optical axes and three principal refractive axes, and each of the three principal refractive axes has a unique, i.e., bijective, refractive index n. x , n y , n zDepending on the design of the first B* compensation layer and the position of its optical axes, either the principal refractive axis, to which the smallest refractive index corresponds, or the principal refractive axis, to which the largest refractive index corresponds, is parallel to the first absorption axis. The first B* compensation layer fulfills the condition

[0021] where d denotes the thickness of the first compensation layer, Aph a phase shift caused by the first B* compensation layer, and A a fundamentally arbitrarily specified wavelength at which the condition should be met. This defines an upper limit for the phase delay of the B* compensation layer, which indirectly also determines a maximum thickness.

[0022] In a second alternative - which is also referred to below as alternative ii. - at least two compensation layers made of uniaxially birefringent materials are arranged between the first and the second polarization layer, wherein a first, spatially homogeneous A* compensation layer consists of a first uniaxially birefringent material with a first optical axis and two first different principal refractive axes, wherein the first optical axis, which coincides with one of the first principal refractive axes, lies perpendicular or parallel to the first absorption axis of the first polarization layer. Behind it, as seen from the perspective of an observer, is a second, spatially homogeneous A* compensation layer consisting of a second uniaxially birefringent material with a second optical axis and two second principal refractive axes. The second optical axis of the second material, which coincides with one of the second principal refractive axes, is perpendicular to the first optical axis of the first material. The optical axes of uniaxial materials are also referred to as extraordinary axes.

[0023] "Spatially homogeneous" means that the respective compensation layer has no structuring in its interior or across the area parallel to the surface of the optical film and therefore exhibits the same behavior across the entire surface, unlike, for example, in US 2019 / 0094626 A1. To prevent moiré fringes from occurring in such lamellar structured layer bodies, the layer structure in a design according to US 2019 / 0094626 A1 must be specifically adapted to the resolution, spacing, apertures, surfaces, and scattering properties for each configuration, which makes production costly. In contrast, the optical films according to the invention with the spatially homogeneous compensation layers can be used universally for different screen sizes and screen resolutions without the need for special adaptation.

[0024] Each of the two A* compensation layers fulfills the condition

[0025] ph = — d ■ \n e — n01 < 2n (2), A

[0026] where d is the thickness of the respective A* compensation layer, n e is the extraordinary refractive index, and n0 is the ordinary refractive index. Aph denotes a phase shift caused by the first and second A* compensation layers, respectively, and A is a fundamentally arbitrarily specified wavelength at which the condition is to be fulfilled.

[0027] In both alternatives, the materials and the thicknesses d of the compensation layers are specified such that – measured in a spherical coordinate system originating on the surface of the film and in the plane of the film – the luminance is minimal only in a given solid angle range R. The solid angle range R covers only a part of the possible perceivable half-space, namely, on the one hand, the azimuthal angle cp, for which |q>| or 1180° - <p | kleiner as the magnitude of a given limit azimuthal angle (pi imis measured relative to a preferred direction in the plane of the film's surface. The choice of the preferred direction is essentially arbitrary, but depends on the application of the optical film. For example, if this optical film is used in a screen with a fixed orientation, such as in a vehicle, the preferred direction will be chosen so that it runs parallel to an imaginary line between the eyes of an upright driver, i.e., usually horizontally.

[0028] On the other hand, the solid angle range for which the luminance is minimal includes polar angles 0 whose magnitude is greater than a predetermined limiting polar angle 0üm, measured generally with respect to the first absorption axis and in a plane spanned by the surface normal and the first absorption axis - whose vectors have a common origin - i.e. all solid angles outside a cone with the limiting polar angle 0üm around the first absorption axis, which in this respect becomes the "zero axis". If the surface normal and the first absorption axis are parallel to one another, the polar angle is only measured with respect to the surface normal. If the optical film is used in a screen as described above as an example, the optical film effectively limits visibility because the luminance is minimal in the said solid angle range.A viewer positioned within the specified solid angle range with respect to the spherical coordinate system of the optical film will ideally perceive no content on the screen due to the minimum luminance in this range. The term "minimal luminance" is to be understood as meaning that the luminance is almost zero, with the luminance decreasing so sharply compared to the luminance outside the specified solid angle range that, ideally, a viewer cannot perceive any image content.

[0029] In order to define the solid angle range even more sharply, i.e. to achieve an even greater drop in luminance within compared to the luminance outside the said solid angle range, it is advantageous in the second alternative ii if a third, spatially homogeneous C* compensation layer is arranged between the first and the second A* compensation layer, which is constructed from a third uniaxially birefringent material with a third optical axis and two third principal refractive axes, wherein the third optical axis lies parallel to the first absorption axis of the first polarization layer.

[0030] For a symmetrical drop in luminance relative to the surface of the film, it is expedient if the first absorption axis is oriented perpendicular to the surface of the film. When using the optical film in a screen as described above, a viewer looking at the screen along the surface normal will experience a symmetrical drop in luminance on both sides depending on a change in viewing angle to the right or left or along the preferred direction, i.e., depending only on the magnitude of the polar angle. The limiting polar angle öüm is then measured with respect to the surface normal and is therefore identical for all azimuthal angles, unlike when measured with respect to the first absorption axis.

[0031] With such an orientation of the first absorption axis, the first polarization layer is also referred to as a Z-polarization layer. For positions of the first absorption axis that deviate from this in the range described above, the first polarization layer is referred to as a Z*-polarization layer. The use of the symbol , therefore, represents a generalization of the Z-polarization layer.

[0032] Likewise, the term A* compensation layer represents a generalization of the term A compensation layer and the term B* compensation layer represents a generalization of the term B compensation layer. The article “Optical anisotropy conversion of retarder film made of rod I ike and crosslike reactive molecules, and its dependence on the relative ratio and the orientation of the constituent molecules” by Ho-Jin Choi et al., published online in Optical Materials 99 (2020), with the article ID 109531, was used as a basis for the definition of the terms “Z polarization layer”, “A compensation layer” and “B compensation layer”, which are generally known from the state of the art.

[0033] If the first absorption axis is oriented perpendicular to the surface of the film, i.e., the first polarization layer is designed as a Z-polarization layer, several suitable configurations result: two configurations for the first alternative and a third configuration for the second alternative. In principle, the orientations of the optical axes of the compensation layers must be aligned with the orientation of the first absorption axis of the first polarization layer in order to achieve the desired restriction of the field of view. A different position of the first absorption axis may be advantageous, for example, if when the observer, for example the driver of a vehicle, does not look at the screen along the surface normal, but from an oblique angle.

[0034] In a first embodiment, according to the first alternative, in which the first absorption axis of the first polarization layer is oriented perpendicular to the surface of the optical film and the principal refractive axis, to which the smallest refractive index corresponds, is parallel to the first absorption axis, the first B* compensation layer is designed as a -B compensation layer. The optical axes lie in a plane spanned by the x-direction and the z-direction, and the three principal refractive axes correspond to the directions of the Cartesian coordinate system, where n x > n y > n z is and n xparallel to the second absorption axis of the second polarization layer. The latter condition also applies to small inclinations of the first absorption axis of up to about 10° with respect to the surface normal; otherwise, the principal refractive axes are also inclined and the position of the coordinate system in x, y, and z is oriented towards the position of the first absorption axis, which corresponds to the z-direction of the coordinate system, whereby it is essential that n x perpendicular to the first absorption axis.

[0035] In a second embodiment, according to the first alternative, in which the first absorption axis of the first polarization layer is oriented perpendicular to the surface of the optical film and the principal refractive axis, to which the highest refractive index corresponds, is parallel to the first absorption axis, the first B* compensation layer is designed as a +B compensation layer. The optical axes lie in a plane spanned by the y-direction and the z-direction, and the three principal refractive axes correspond to the directions of the Cartesian coordinate system, where n z > n x > n y is and n yparallel to the second absorption axis of the second polarization layer. The latter condition also applies to small inclinations of the first absorption axis of up to about 10° with respect to the surface normal; otherwise, the principal refractive axes are also inclined and the position of the coordinate system in x, y, and z is oriented towards the position of the first absorption axis, which corresponds to the z-direction of the coordinate system, whereby it is essential that n y perpendicular to the first absorption axis.

[0036] In a third embodiment, according to the second alternative, in which the first absorption axis of the first polarization layer (1 ) is also oriented perpendicular to the surface of the optical film, the first A* compensation layer is The first A* compensation layer is designed as a +A compensation layer, the second A* compensation layer as a -A compensation layer, or vice versa. If a third, C* compensation layer is present, this is designed as a -C or +C compensation layer.

[0037] In order to keep manufacturing costs low, it is advantageous if the first and second A* compensation layers are identical, ie both compensation layers are designed either as +A or -A compensation layers and have the same thickness.

[0038] In all of the aforementioned embodiments, in a particularly preferred further development, a liquid crystal layer that can be switched between at least two states can be arranged between the second polarization layer and the compensation layer arranged closest to the second polarization layer. This liquid crystal layer is designed to transmit light transmitted by the second polarization layer with unchanged polarization or with a linear polarization rotated by 90° in a first switching state, and to transmit light with circular, elliptical, or linear polarization in a second switching state. In the first switching state, the rotation by 90° is to be understood as meaning that the linearly polarized light, after passing through the switchable liquid crystal layer, becomes linearly or elliptically polarized light, with a large portion of the vectors of the electric field being rotated by 90°. Therefore, there is no exact rotation of the polarization by 90°.The addition of the switchable liquid crystal layer makes it possible to switch between a privacy mode and a public mode, which becomes noticeable when the optical film is integrated into a screen. The above-described drop in luminance within the specified solid angle range is permanent for the optical film, but can be eliminated by using the switchable liquid crystal layer. The first switching state corresponds to the privacy mode. The light remains linearly polarized. The second switching state corresponds to the public mode, in which the light is usually elliptically polarized, but can also be polarized differently depending on the choice of liquid crystal layer.In public mode, there is no or only a slight drop in luminance within the specified solid angle range, so that when used in a screen, image content is perceptible regardless of the viewer's position—i.e., unrestricted, within the limits of technical capabilities. In contrast, in privacy mode, image content is perceptible from the side—relative to the direction of the first absorption. onsachse - no longer perceptible to people standing. Alternatively, the liquid crystal layer can also be arranged between the first polarization layer and the compensation layer closest to it.

[0039] The switchable liquid crystal layer is typically used together with a static first polarization layer to create a switchable optical film. However, the switchable liquid crystal layer can also be omitted if its function can be performed by a switchable first polarization layer. In this case, the first polarization layer can be configured, for example, as a liquid crystal layer in which dyes are embedded, so-called "dye-LC cells." These are particularly suitable when the first absorption axis, i.e., the absorption axis of the first polarization layer, is parallel to the surface normal of the optical film.

[0040] The view-limiting effect in the given solid angle range, i.e. the drop in luminance to the minimum in this solid angle range, can preferably be achieved if the individual components of the optical film are matched to one another in such a way that in the given solid angle range R the loss function

[0042] is minimal, where T(cp, 0) is the angle-dependent transmission and Q denotes the solid angle. The natural logarithm of the angle-resolved transmission is calculated and integrated over the solid angle range in which the privacy screen is to be optimized. The logarithm represents a weighting, so that transmissions over different orders of magnitude are included in the optimization. Other weightings are also conceivable, for example a linear weighting. In this case, the application of the logarithm to the transmission is omitted. To coordinate the components while fulfilling this condition, commercially available optical design programs can be used as a basis and adapted for the inventive optical design, for example, LCD MASTER® from Uniglobe Kisco, Inc. or Tecwiz LCD 3D® from INCROPS.

[0043] For most applications, it has proven useful if the limit azimuthal angle q>ij mbetween 30° and 40° around a preferred direction and / or the limiting polar angle 0Hm is between 40° and 50° around the surface normal or the direction of the first absorption axis if it is inclined against the surface normal.

[0044] The backlight with the optical film described above can be integrated into a lighting device for a display screen, provided that the film is not equipped with a switchable liquid crystal layer as described above, and into a display screen if it is equipped with a switchable liquid crystal layer.

[0045] Specifically, the backlight with the non-switchable optical film, i.e., one not provided with a switchable liquid crystal layer, can be inserted into and used with a lighting device for a transmissive screen, in particular an LC display, wherein the lighting device is configured such that it can be operated in two operating modes: B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a solid angle range that is restricted compared to the free view mode. This lighting device comprises a planar backlight with backlight sources and a non-switchable optical film, as described above, and emits light.If the second polarization layer of the optical film is arranged in front of the first polarization layer in the viewing direction, the general rule for the backlight, even in other embodiments described below, is that the backlight sources of the backlight emit unpolarized light; with the reverse arrangement, the light emitted by the backlight sources of the backlight can also be (partially) polarized. Arranged in front of the backlight in one viewing direction—for a viewer looking in the direction of illumination—is a plate-shaped light guide with two large surfaces and narrow sides connecting them, which has output coupling elements on at least one of the large surfaces and / or within its volume. Illuminants are arranged laterally on at least one narrow side of the light guide.A linear polarization filter is arranged in the viewing direction in front of the backlight or in front of the light guide. This polarization filter can optionally correspond to the second polarization layer of the optical film, but can also be implemented separately. In this way, light emanating from the backlight and penetrating the linear polarization filter is restricted in its propagation direction. The polarization filters are arranged in the viewing direction. In operating mode B2 for restricted viewing mode, the backlight is switched on and the lamps are switched off. Only the backlight emits light into the restricted viewing angle. In operating mode B1 for free or public viewing mode, at least the lamps are switched on, allowing the restricted lighting to be compensated or overcompensated by the backlight alone. Accordingly, the backlight can be switched on or off in public viewing mode. In this case, the transmissive screen is positioned upstream of the lighting device.

[0046] The invention also encompasses a screen that can be operated in at least two operating modes: B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle or solid angle range that is restricted for a viewer compared to the free view mode. Such a screen initially comprises a planar, light-emitting backlight with an optical film, as described above, in the embodiment with a switchable liquid crystal layer that can be switched between two states. The backlight can optionally be directly illuminated, for example, as a so-called "direct matrix backlight." A linear polarization filter is arranged in front of the backlight in the viewing direction. This can optionally also correspond to the second polarization layer of the optical film.In this way, light emanating from the backlight and penetrating the linear polarization filter is restricted in its propagation direction. A transmissive image display device is arranged in front of the backlight in the viewing direction. The linear polarization filter can be part of the image display device and is then arranged within the transmissive image display device. However, it can also be implemented separately, in which case it is arranged as close as possible to the image display device in the stack of optical elements. A typical image display device has a linear polarizer above and below an LC layer in the viewing direction; the above statements refer to the linear polarizer arranged below in the viewing direction. The linear polarizer arranged above is essential for the privacy application.In operating mode B2, the liquid crystal layer, which can be switched between at least two states, is in the first switching state and in operating mode B2 in the second switching state, as described above.

[0047] Finally, the invention also encompasses a further screen that can be operated in at least two operating modes: B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle or solid angle range that is restricted for a viewer compared to the free view mode. Such a screen comprises an image display device, which can be of the OLED, microLED, or LCD type, for example, and an optical film arranged in front of the image display device in the viewing direction, which film has the liquid crystal layer described above that can be switched between at least two states. This liquid crystal layer is in the first switching state in operating mode B2 and in the second switching state in operating mode B2, corresponding to the above definition of first and second switching states.

[0048] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations indicated, but also in other combinations or in isolation, without departing from the scope of the present invention. Short description of the drawings

[0049] The invention is explained in more detail below using exemplary embodiments with reference to the accompanying drawings, which also disclose features essential to the invention. These exemplary embodiments are for illustrative purposes only and are not to be interpreted as restrictive. For example, a description of an embodiment with a large number of elements or components should not be interpreted to mean that all of these elements or components are necessary for implementation. Rather, other embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different exemplary embodiments may be combined with one another unless otherwise stated. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary embodiments.To avoid repetition, identical or corresponding elements in different figures are designated by the same reference numerals and are not explained more than once. They show:

[0050] Fig. 1A-1C different layer structures of an optical film for controlling and limiting the viewing angle of a viewer,

[0051] Fig. 2 the limitation of the viewing angle range,

[0052] Fig. 3 an example of the improvement of the visual protection by the Invention,

[0053] Fig. 4A-4B the privacy effect for a first embodiment of the optical Films,

[0054] Fig. 5A-5B the privacy effect for a second embodiment of the optical film,

[0055] Fig. 6A-6B the privacy effect for a third embodiment of the optical Films,

[0056] Fig. 7A-7G Polarization ellipses for the passage through the optical film,

[0057] Fig. 8 shows a switchable embodiment of the optical film,

[0058] Fig. 9A-9B an illumination device with a non-switchable optical film in two operating states,

[0059] Fig. 10 shows a screen with a switchable optical film, and

[0060] Fig. 11 shows another screen with a switchable optical film. Detailed description of the drawings

[0061] Figs. 1A-1C show various schematic diagrams of a layer structure of an optical film for controlling and limiting a viewer's viewing angle range. The observer (not shown) is located above the uppermost layer, which has a surface with a surface normal, which here runs parallel to the longer edge of the sheet in the plane of the sheet. The uppermost layer, viewed from the direction of the observer, in all three representations in Figs. 1A-1C is a first polarization layer 1. The first polarization layer 1 has a first absorption axis, which forms an angle of 0° to 30° with the surface normal of the optical film. If the angle is If the angle is 0°, the first polarization layer 1 is a Z-polarizer; for angles other than this, the term "Z*-polarizer" is used. An angle of 0° is suitable, for example, for notebooks where the user is directly in front of the screen. An angle of 30° can be advantageous in a motor vehicle, for example, where the screen is positioned midway between the driver and passenger seats, to make information visible only to the driver.

[0062] The bottom layer in Figs. 1A-1C is a second polarization layer 2 with a second absorption axis oriented parallel to the surface of the optical film. The second polarization layer 2 is therefore a linear polarizer.

[0063] Between the first polarization layer 1 and the second polarization layer 2, at least one phase-shifting compensation layer is arranged to improve the limitation of the viewing angle range. Depending on the type of compensation layer, a single compensation layer or multiple compensation layers are used. The compensation layers are, for example, uniaxially or biaxially birefringent polymer films. The layers are expediently firmly bonded to one another, for example, by optical adhesive or other material-to-material connections. Connections by ultrasonic welding are also conceivable, for example. A connection solely by adhesion when extremely smooth surfaces are placed or pressed together is also conceivable, optionally with an anti-reflection layer.To improve the privacy effect, there are various ways to implement one or more compensation layers, which are explained below. Alternatively—not shown in the drawing—the second polarization layer 2 can be arranged as the topmost layer in the viewing direction, with the first polarization layer 1 behind it. However, the at least one phase-shifting compensation layer is always located between the first polarization layer 1 and the second polarization layer 2.

[0064] In a first embodiment, which is shown in Fig. 1 A and is also referred to below as the first alternative or alternative i., a first B* compensation layer 3 is arranged between the first polarization layer 1 and the second polarization layer 2. The first B* compensation layer 3 is It is a spatially homogeneous layer made of a biaxially birefringent material. Accordingly, the material, or the first B* compensation layer 3, has two optical axes and three principal refractive axes. Each of the three principal refractive axes is uniquely a refractive index n. x , n y , n z These are typical properties of a biaxially birefringent material. The indices "x," "y," and "z" correspond to the axes of a Cartesian coordinate system. However, to achieve a privacy-protecting effect, it is essential that either the principal refractive axis, which corresponds to the lowest refractive index, or the principal refractive axis, which corresponds to the highest refractive index, is parallel to the first absorption axis.

[0065] The position of the first absorption axis determines the positions of all other absorption axes or principal refractive axes of all types of compensation layers: If the first polarization layer is, for example, a Z-polarizer, whose first absorption axis is parallel to the surface normal or perpendicular to the surface of the film, this means that the corresponding principal refractive axis, to which the smallest or largest refractive index corresponds, is also parallel to the surface normal. Accordingly, the other two principal refractive axes then lie in the plane of the surface of the optical film. The optical axes of the first B* compensation layer 3, which in this case is a B compensation layer, then lie in a plane that intersects the surface of the optical film perpendicularly. In this case, the first B* compensation layer 3 can be designed in two ways.

[0066] Firstly, it can be designed as a -B compensation layer. Since in this case we are talking about a Z polarizer, in a fictitious Cartesian coordinate system in which the principal refractive axes and the optical axes of the B* compensation layer are defined, the z direction is associated with the direction perpendicular to the surface of the optical film. In this notation, the -B compensation layer satisfies the condition n x > n y > n z . The smallest refractive index corresponding to the principal axis perpendicular to the surface of the optical film is therefore denoted by n z In addition, the principal refractive axis, to which the largest refractive index n x corresponds, parallel to the second absorption axis of the second polarization layer 2.

[0067] Alternatively, it can be designed as a +B compensation layer. Here, too, the z-direction is associated with the direction perpendicular to the surface of the optical film. In this notation, the +B compensation layer satisfies the condition n z > n x > n y . The largest refractive index corresponding to the principal axis perpendicular to the surface of the optical film is therefore denoted by n z In addition, the principal refractive axis, to which the smallest refractive index n y corresponds, parallel to the second absorption axis of the second polarization layer 2 (see above).

[0068] For all embodiments of the first alternative, the first B* compensation layer 3 must meet the condition

[0070] where d denotes the thickness of the first B* compensation layer 3, Aph a phase shift caused by the first compensation layer, and A a fundamentally arbitrarily specified wavelength at which the condition should be met. This defines an upper limit for the phase delay of the first B* compensation layer 3, which indirectly also determines a maximum thickness.

[0071] In a second embodiment, the basic structure of which is shown in Fig. 1B and which is also referred to below as the second alternative or alternative ii., at least two compensation layers made of uniaxially birefringent materials are arranged between the first polarization layer 1 and the second polarization layer 2. These are a first A* compensation layer 4 and a second A* compensation layer 5. Both A* compensation layers 4, 5 are spatially homogeneous in the sense defined at the beginning. The first A* compensation layer 4 consists of a first uniaxially birefringent material with a first optical axis and two first principal refractive axes, wherein the first optical axis is perpendicular or parallel to the first absorption axis of the first polarization layer 1.Arranged behind it from the viewer's perspective is the second A* compensation layer 5, which consists of a second uniaxially birefringent material with a second optical axis and two second principal refractive axes. The position of the second optical axis of the second A* compensation layer 5 is determined depending on the position of the first optical axis of the first. A* compensation layer 4 is determined by fulfilling the condition that the second optical axis is perpendicular to the first optical axis. Each of the two A* compensation layers 4, 5 fulfills the condition

[0072] ph = — d ■ \n e — n01 < 2n ,

[0073] where d is the thickness of the respective compensation layer, n eis the extraordinary refractive index, and n0 is the ordinary refractive index. Aph denotes a phase shift caused by the first and second A* compensation layers 4, 5, respectively, and A denotes a fundamentally arbitrarily specified wavelength at which the condition is to be met. Both A* compensation layers 4, 5 can be made of the same material and / or have the same thickness, which simplifies production.

[0074] Fig. 1C shows a further development of the second alternative: In order to define the solid angle range even more sharply, i.e., to achieve an even greater drop in luminance within compared to the luminance outside the specified solid angle range and / or greater flexibility in the selection of components for the compensation layers, it is advantageous in the second alternative if a third C* compensation layer 6, which in turn is also spatially homogeneous, is arranged between the first and second A* compensation layers. The third C* compensation layer 6 is constructed from a third uniaxially birefringent material with a third optical axis and two third principal refractive axes, wherein the third optical axis is parallel to the first absorption axis of the first polarization layer 1.Here too, the position of the first absorption axis of the first polarization layer 1 determines the position of the optical axis of the material of the third C* compensation layer 6.

[0075] If the first polarization layer 1 is configured as a Z-polarizer, and the first absorption axis is thus perpendicular to the surface of the optical film, either the first A* compensation layer 4 is configured as a +A compensation layer and the second A* compensation layer 5 as a -A compensation layer, or vice versa. If a third C* compensation layer 6 is present, this is configured as a -C or +C compensation layer.

[0076] In both the first and the second alternative, the materials and the thicknesses d of the compensation layers are specified in such a way that - based on a Spherical coordinate system, which has its origin on the surface of the film and in the plane of the film - the luminance is minimal only in a given solid angle range R. The solid angle range R covers only a part of the possible, perceivable half-space, namely, on the one hand, azimuthal angles cp lying in the surface of the film, for which |q>| or |180° -<p| kleiner als der Betrag eines vorgegebenen Grenzazimutalwinkels q> ij mis measured with respect to a preferred direction in the plane of the film's surface. The choice of preferred direction is essentially arbitrary, but depends on the application of the optical film. If, for example, this optical film is used in a screen with a fixed orientation, such as in a vehicle, the preferred direction will be chosen so that it runs parallel to an imaginary line between the eyes of an upright driver, i.e. usually horizontally. The specification of the limit azimuthal angle (piim) depends on the application for which the film is intended. Typical values ​​are between 30° and 40° around the preferred direction, for example for notebooks that are protected from lateral viewing in trains, etc.are to be protected - here the preferred direction is usually parallel to the longer edge of the screen and the first absorption axis is parallel to the normal of the screen, so that the drop in luminance is symmetrical in all directions.

[0077] Secondly, the solid angle range for which the luminance is minimal includes polar angles 0 whose magnitude is greater than a predetermined limiting polar angle 0üm, measured with respect to the first absorption axis and in a plane spanned by the surface normal and the first absorption axis, or measured only with respect to the surface normal if the first absorption axis is parallel to it. The limiting polar angle 0üm is preferably between 40° and 50°, depending on the application. If the optical film is used in a display screen as described above by way of example, the optical film effectively restricts visibility, since the luminance is minimal in the specified solid angle range.A viewer who is in a position which lies within the specified solid angle range with respect to the spherical coordinate system of the optical film will, ideally, not perceive any content on the screen or at least not be able to recognize it due to the minimal luminance in this range.

[0078] The visibility-restricting effect in the given solid angle range, i.e. the drop in luminance to the minimum in this solid angle range, can preferably be achieved if the individual components of the optical Films are matched so that in the given solid angle range R the loss function C2= f In T( <p, 0) d£l minimal ist, wobei T(cp, 0) die winkelabhängige Transmission ist und Q den Raumwinkelbereich bezeichnet. Es wird also der natürliche Logarithmus der winkelaufgelösten Transmission berechnet und über den Raumwinkelbereich, in welchem der Sichtschutz optimiert werden soll, integriert. Auf diese Weise werden nicht nur Blickwinkel einbezogen, die auf der Horizontalen liegen, sondern auch Blickwinkel, die von der vertikalen Blickrichtung von 0° - entlang der Oberflächennormalen - des eigentlichen Betrachters abweichen, also nach oben oder unten. Das schließt beispielweise dritte Beobachter mit ein, die seitlich neben dem sitzenden Nutzer des Geräts im Sichtschutzmodus stehen. Im Ergebnis erhält man auch für diese Blickwinkel einen gegenüber dem Stand der Technik stark verbesserten Sichtschutz.The logarithm is used to weight the values ​​over different orders of magnitude, but linear weighting or other weightings are also conceivable.

[0079] This is illustrated by way of example in Fig. 2 and Fig. 3. Fig. 2 shows a projection of a given solid angle range R into the plane of the optical film as black areas, also referred to as a conoscopic image. In this area, the privacy should be improved so that the luminance there is as low as possible. In this example, the solid angle range R is specified such that the limiting azimuthal angle q>ij mis at 40° – this is how the black areas above and below the horizontal axis arise – and the limiting polar angle θiim is also at 40° – this corresponds to the areas left out to the right and left of the center point. The inner concentric circle corresponds to a polar angle of θ = 40°. For this given solid angle range, the loss function, i.e., the integral of the logarithmic transmission, is minimized. Commercially available optical design programs, such as those mentioned above as examples, can be used to match the components to one another while fulfilling this condition.

[0080] As a result, a sideways improved privacy protection effect is obtained even for vertical viewing angles that deviate from 0° - corresponding to a perpendicular view of the surface of the optical film - as shown in Fig. 3 for a vertical viewing angle of 30° and an optical film according to the second alternative with a +A compensation layer and a -A compensation layer. is shown, and secondly with a further -C compensation layer. The first polarization layer 1 is designed as a Z-polarizer, so that the first absorption axis is parallel to the surface normal of the optical film. The privacy effect is shown in arbitrary units, i.e. the luminance normalized to an angle of 0°, as a function of the horizontal viewing angle in degrees, from which it can be seen how bright the screen is in the privacy-protected angular range compared to the non-privacy-protected angular range. The preferred direction was chosen as an example to be parallel to the horizontal direction, whereby this refers to a viewer's reference system, i.e. the horizontal direction corresponds to an imaginary connection between the viewer's eyes and the vertical direction is correspondingly perpendicular to this.The solid line corresponds to a privacy effect that can only be achieved in the state of the art with a Z-polarizer and without spatially homogeneous compensation layers at a vertical viewing angle of 30°. The dashed line shows the privacy effect for the combination with two A* compensation layers of type -A and +A, and the dash-dotted line shows the privacy effect for the combination additionally with a -C compensation layer. Since the A* compensation layers were not fully optimized in this example, no improvement is achieved for the combination with the -C compensation layer. However, the use of C* compensation layers usually results in improved privacy at angles around 30°. The improved privacy to the sides up to angles of around 60° is clearly visible.The reduced—but nevertheless improved—visual protection at very large angles of more than 60° is exaggerated due to the logarithmic scale, but is not noticeable in practice. At a vertical viewing angle of 0°, not shown here, an optical film according to the state of the art and an optical film with additional compensation layers, as described above and below, deliver approximately the same results, corresponding roughly to the dashed or dash-dotted curve.

[0081] The solid angle range R shown in Fig. 2 is only an example and can be adjusted as needed, for example, to improve privacy from vertical viewing angles where the horizontal viewing angle is 0°—for the example of a notebook, corresponding to viewers standing directly behind a seated user. In this case, for example, the black solid angle range in Fig. 2 would completely enclose the concentric circle at 40°.

[0082] Further examples are shown in Figs. 4A-4B, 5A-5B, and 6A-6B, where the solid angle range R is specified according to Fig. 2. Figs. 4A, 5A, and 6A each show the privacy protection effect for a vertical viewing angle of 0°, and Figs. 4B, 5B, and 6B each show it for a vertical viewing angle of 30°. The curves with solid lines always correspond to the optical film with only a first Z* polarization layer without additional, spatially homogeneous compensation layers.

[0083] Fig. 4A and Fig. 4B show the privacy effect for an optical film with a structure according to the first alternative shown in Fig. 1 A, with a first B-compensation layer 3. Since for biaxially birefringent layers for a variety of combinations of n x , n y and n zthe same optical functionality is achieved, such layers are classified by two other parameters that take this into account, namely the parameter

[0084] R e = (n x - ny) ■ d and the parameter

[0085] N z = (n x -n y )

[0086] The dotted curve is obtained for a thickness d = 5.25 pm, R e = 132 and N z = 3.84 at a wavelength of A = 550 nm in the green, mid-visible wavelength range, in which the human eye has the highest sensitivity. From N z one can derive how the refractive indices must behave towards each other. For example, this applies to n x = 1 ,6246, n y = 1 ,6 and n z= 1 .5287. To produce correspondingly calculated layers with precisely these refractive indices, numerous manufacturing processes are known in the state of the art, allowing for high control over the refractive indices. Once a material has been selected that has the desired refractive index ratio, the thickness d is adjusted so that N z is fulfilled. In addition to the parameters mentioned above, a noticeable improvement can also be achieved with other combinations; this results in the behavior of the privacy effect shown by the dot-dash curve for R e = 75 and N z = 3.84. These values ​​are only examples; tolerances of + / - 20% are possible and encompassed in any case, without noticeably reducing the privacy effect.

[0087] Fig. 5A and Fig. 5B show the privacy effect for an optical film with a structure according to the second alternative shown in Fig. 1C, with a first A* compensation layer 4 and a second A* compensation layer 5, as well as an additional, third C* compensation layer 6 between these two layers. The first absorption axis of the first polarization layer 1 is again perpendicular to the surface, thus representing a Z-polarizer. Accordingly, the first A* compensation layer 4 is configured as a +A compensation layer with positive birefringence, the second A* compensation layer 5 as a -A compensation layer with negative birefringence, and the third C* compensation layer 6 as a -C compensation layer with negative birefringence.Alternatively, the first A* compensation layer 4 can be designed as a -A compensation layer with negative birefringence and the second A* compensation layer 5 as a +A compensation layer with positive birefringence and the third C* compensation layer 6 as a +C compensation layer with negative birefringence, with negative birefringence being the case n. e < n0 is meant and with positive birefringence the case n e > n0.

[0088] While at a vertical viewing angle of 0°, Fig. 5A, corresponding to a direct top view of the film along the surface normal, the use of the three additional compensation layers in addition to the Z-polarizer does not result in any improvement, the improved privacy effect is clearly visible at a vertical viewing angle of 30°, Fig. 5B. The improvement shown in Fig. 5A and Fig. 5B can be achieved with a series of compensation layers, which for the first +A compensation layer satisfy the condition d-(n e - n0) = 264 nm and for the second -A compensation layer the condition d-(n0- n e ) = -264 nm, each with a tolerance of 20%. For the third -C compensation layer, the condition d-(n e- n0) = -22 nm, whereby the tolerance of + / - 10 nm is larger. In general, for the comparatively thin C* compensation layer, the tolerance is either + / - 10 nm or 20%, whichever is greater. In the alternative design with a first -A compensation layer, the signs are reversed accordingly.

[0089] In Fig. 6A and Fig. 6B, the privacy effect for an optical film with a structure according to the second alternative shown in Fig. 1C with a first A* compensation layer 4 and a second A* compensation layer 5 as well as an additional, third C* compensation layer 6 between these two layers shown. In contrast to Fig. 5A, Fig. 5B, the first absorption axis of the first polarization layer is inclined by 20° relative to the surface normal, thus resulting in a Z* polarizer. This orientation then also determines how the optical axes of the A* and C* compensation layers must be positioned to achieve the privacy effect. Photoalignment and polymerization of LC mesogens can be used, for example, to produce such inclined compensation layers. Without loss of generality, the first A* compensation layer 4 is designed as a -A* compensation layer and the second A* compensation layer 5 as a +A* compensation layer; correspondingly, the third C* compensation layer 6 is designed as a -C* compensation layer. The improvement shown in Fig. 6A and Fig. 6B can be achieved with a series of compensation layers, which for the first +A* compensation layer satisfy the condition d-(ne - n0) = 264 nm and for the second -A* compensation layer the condition d-(n0- n e ) = -264 nm, each with a tolerance of 20%. For the third -C* compensation layer, the condition d-(n e - n0) = -82 nm, also with a tolerance of 20%. In the alternative design with a first -A* compensation layer, the signs are reversed accordingly.

[0090] 7A to 7F show in detail the effect of the individual layers using polarization ellipses using the example of an optical film according to the second alternative shown in Fig. 1C, wherein the first absorption axis of the first polarization layer 1 is aligned parallel to the surface normal of the optical film. For comparison, Fig. 7G shows the polarization of light with a layer structure according to the prior art without additional compensation layers. The viewing direction of a fictitious observer is along the surface normal of the optical film. Each of Figs. 7A to 7G shows a plurality of polarization ellipses distributed in a circle around the origin of a coordinate system. The position of each polarization ellipse corresponds to a viewing angle on the surface of the optical film.The viewing angle at the origin of the coordinate system corresponds to the surface normal, i.e., the top view perpendicular to the surface of the optical film. Without loss of generality, the direction parallel to the shorter edge of the plane of the sheet is called the x-direction, and the direction perpendicular to it, along the long edge of the plane of the sheet, is called the y-direction. The x-direction also corresponds to the preferred direction and is parallel to an imaginary connecting line between the observer's eyes. Hence also referred to as the horizontal direction below. On the x-axis of the coordinate system in Fig. 7A to Fig. 7G there are therefore polarization ellipses that correspond to viewing angles that deviate from zero exclusively horizontally, corresponding to an observer who moves only sideways from the origin. On the y-axis there are polarization ellipses for which the viewing angle deviates from zero only vertically, corresponding to an observer who moves vertically up or down from the original position. A “vertical” movement or displacement is not to be understood as a viewer moving away from the optical film along the surface normal, i.e. it does not mean a displacement along the surface normal in the horizontal plane spanned by the surface normal and the horizontal direction between the eyes of a viewer. Rather, it means a displacement perpendicular to this plane.For example, if a seated first observer looks directly along the surface normal at the optical film, the viewing angle of a second observer standing directly behind the first observer is only vertically offset on the y-axis. For clarity, two concentric circles are shown in each of Figures 7A to 7G. The inner circle defines a viewing angle cone of viewing angles up to 25° in each direction, while the outer circle defines a viewing angle cone of viewing angles up to 45° in each direction. The 90° viewing angles—not perceivable in reality—are located at the very outer edges.

[0091] Fig. 7A shows circularly polarized light radiated by a backlight onto the optical film. The light initially strikes the second polarization layer 2 with a second absorption axis, which linearly polarizes it. This second absorption axis of the second polarization layer 2 is oriented parallel to the surface of the film, here along the horizontal direction or, more generally, an imaginary connecting line between the observer's eyes, which simultaneously also corresponds to the preferred direction.

[0092] After passing through the second polarization layer, the linearly polarized light strikes the second A* compensation layer 5, which is designed as an -A compensation layer. While the polarization remains almost unchanged, especially in the horizontal direction, but also in the vertical direction, in the viewing angle ranges with deviating angles, s-polarized light is obtained for large areas after passing through - in plan view - as shown in Fig. 7C, i.e. light whose vector of the associated electric field is perpendicular to the plane of incidence - the plane spanned by the surface normal and the direction of incidence - oriented. Fig. 7D shows the angle-resolved polarization after passing through the next layer, a third C* compensation layer 6. This is designed here as a +C compensation layer; however, the changes are barely visible in the image due to the low birefringence and the fact that it is a C* compensation layer. Fig. 7E shows the polarization of the light after passing through the first A* compensation layer 4, which is designed here as a +A* compensation layer. The light is approximately p-polarized over large parts, i.e. the vector of the electric field is parallel to the plane of incidence, so that the absorption of light with non-perpendicular propagation directions is increased by the subsequent first polarization layer 1, the Z polarization layer. This is achieved by the interaction of the three compensation layers mentioned. The privacy effect is thereby significantly increased.

[0093] Finally, Fig. 7F shows the polarization of the light after passing through the first polarization layer 1, the Z-polarization layer. For comparison, Fig. 7G shows the polarization of the light according to the prior art, in which the Z-polarization layer is directly adjacent to the second polarization layer. The closer the polarization ellipses approach the point shape, the lower the luminance. Here, it can be clearly seen that the luminance in the area highlighted in black in Fig. 2 has been further reduced compared to the prior art, and the privacy effect has thus been improved. This is achieved by not minimizing the luminance over the entire half-space - outside a narrow viewing cone - as in the prior art, but only over a real part of this half-space outside the viewing cone, the specified solid angle range R.Specifically, the loss function is in the given solid angle range R.

[0095] minimized, where T(cp, 0) is the angle-dependent transmission and Q denotes the solid angle. This results in a significantly improved reduction in luminance in the desired areas and thus better privacy.

[0096] Fig. 8 shows a design of an optical film with a first A* compensation layer 4 and a second A* compensation layer 5 analogous to Fig. 1 B, but in which an additional polarization layer 2 and the second A liquid crystal layer 7 is arranged between the A* compensation layer 5, which liquid crystal layer is switchable between at least two states. The liquid crystal layer 7 is designed such that, in a first switching state, it transmits light transmitted by the second polarization layer 2 with unchanged polarization or with a polarization rotated by 90°, and, in a second state, it transmits light transmitted by the second polarization layer 2 with circular or elliptical polarization. Of course, such a switchable liquid crystal layer 7 can also be used in the other possible embodiments of the optical film, in particular in those shown in Fig. 1A and Fig. 1C.

[0097] In this way, for example, the screens shown in Fig. 10 and Fig. 11 can be realized. Fig. 10 shows a screen that can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range that is restricted for a viewer compared to the free view mode. To switch between the two operating modes, the screen comprises a backlight 8 that is extensive and contains an optical film - not shown separately here - with the liquid crystal layer 7 that can be switched between at least two states and emits light, which is symbolized here by a number of light sources 9.However, this is only a sketch; it could, for example, be a surface radiator or an edge-lit light guide with structured surfaces, which can optionally contain additional optical layers such as diffuser films or prism louvre films. Optionally, the backlight can also be designed to be directly illuminated, for example as “direct matrix backlight dimming”. Arranged in front of the backlight 8 in the viewing direction, a linear polarization filter 10 is arranged, which limits the propagation directions of light emanating from the backlight 8 and penetrating the linear polarization filter 10. Arranged in front of the backlight 8 in the viewing direction, a transmissive image display device 11 is arranged. The linear polarization filter 10 is arranged behind the transmissive image display device 11 with respect to the viewing direction.It should be arranged as close to the latter as possible, i.e., there should be no additional layers between the two. Preferably, however, the linear polarization filter 10 is arranged in the transmissive image display device 11, i.e., a part of it or at least integrated into it. In operating mode B2, the liquid crystal layer 7, which can be switched between at least two states, is in the first switching state, and in the In operating mode B1, the liquid crystal layer 7, which can be switched between at least two states, is in the second switching state. The switchable liquid crystal layer thus enables switching between a public operating mode, in which the image content displayed on the screen can be viewed without restriction from a variety of viewing angles, and a privacy mode, in which the displayed image content is visible with sufficient brightness only within a narrow viewing angle range in a cone around the first absorption axis of the first polarization layer 1.

[0098] A further embodiment of a screen that can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range that is restricted for a viewer compared to the free view mode, is shown in Fig. 11. This comprises an image display device 12, as is known in the art and which can be configured, for example, as an actively luminous image display device 12 based on OLED or microLED or as a passively luminous, i.e. illuminated image display device 12, for example based on LCD. An optical film with a liquid crystal layer 7 that can be switched between at least two states is arranged in front of the image display device 12 in the viewing direction.The optical film is constructed here, by way of example, according to the first alternative, wherein a first, spatially homogeneous, B* compensation layer 3 made of a biaxially birefringent material is arranged between the first polarization layer 1 and the second polarization layer 2. The second polarization layer 2 can, however, also be designed, for example, as the rear polarizer of an LC display of the image display device 12. Of course, all other previously described embodiments of optical films with a switchable liquid crystal layer 7 can also be used. As with the previously described screen, the liquid crystal layer 7, which can be switched between at least two states, is in the first switching state in operating mode B2 and in the second switching state in operating mode B1. This embodiment is particularly well suited for retrofitting existing screens.

[0099] Alternatively, it is also possible to produce illumination devices for screens using an optical film which does not have a switchable liquid crystal layer 7, which can be configured in such a way that they can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode in which light in a solid angle range that is limited compared to the free view mode. An example of such an illumination device is shown in Fig. 9A and Fig. 9B in both operating modes. If the illumination device is combined with an image display device arranged upstream in the viewing direction, on which image content can be displayed, a screen is obtained that can be switched between the two operating modes B1 and B2.

[0100] The lighting device shown in Fig. 9A and Fig. 9B comprises a planar backlight 13 into which a static, i.e. non-switchable, optical film, as shown by way of example in Figs. 1A-1C, is integrated. Arranged in front of the backlight 13 in the viewing direction is a plate-shaped light guide 14 which has output coupling elements on at least one of the large surfaces and / or within its volume. In the example shown here, output coupling elements 15 are arranged in the volume of the light guide 14. Arranged in front of the backlight 13 or in front of the light guide 14 in the viewing direction. A linear polarization filter 16 is arranged. Light emanating from the backlight 13, thus penetrating the optical film, and then penetrating the linear polarization filter 16, is thus fundamentally restricted in its propagation directions.Optionally, the linear polarization filter 16 can also take on the function of the second polarization layer 2, i.e., correspond to it. Illuminants 17 are arranged laterally on at least one narrow side of the light guide 14—here, on two narrow sides—which, when switched on, radiate light into the light guide 14. The light radiated by the illuminants 17 is reflected back and forth in the light guide 14 by total internal reflection until it strikes output coupling elements 15, which redirect the light so that it passes outward through the surface of the light guide 14 toward the viewer. The output coupling elements 15 are designed such that they redirect light almost exclusively in this direction and allow light originating from the background lighting 13 to pass through almost unhindered.

[0101] Fig. 9A shows the illumination device in operating mode B2 for the limited view mode, in which only a small, usually conical solid angle range is illuminated, indicated by the arrows above the surface of the light guide 14. In this case, only the background lighting 13 is switched on, The lamps 17 must be switched off. Fig. 9B, however, shows the lighting device in operating mode B1 for the public viewing mode, in which light is emitted into a significantly larger or wider solid angle range than in operating mode B2, again indicated by the arrows above the light guide 14. The lamps 17 must be switched on in this case; their light, which is radiated into the light guide 14 and output by the output elements 15, ensures the widening of the illuminated solid angle range. The backlight 13 can be switched on or off in operating mode B1. A more homogeneous illumination of the solid angle range in operating mode B1 is usually achieved when the backlight 13 is switched off.

[0102] In cooperation with a passive image display device which is illuminated from behind by means of the lighting device shown in Fig. 9A and Fig. 9B, the viewer of image content displayed on the image display device then experiences the restricted viewing mode B2 or the public viewing mode B1, depending on whether the lighting means 17 are switched on or off.

[0103] The optical film described above, in conjunction with an image display device, possibly with a separate lighting device, can be advantageously used wherever confidential data must be displayed and / or entered, such as when entering a PIN, displaying data at ATMs or payment terminals, entering passwords, or reading emails on mobile devices. The invention can also be used in particular in motor vehicles to selectively withhold distracting image content from the driver or passenger. List of reference symbols 1 first polarization layer second polarization layer first B* compensation layer first A* compensation layer 5 second A* compensation layer 6 third C* compensation layer 7 Liquid crystal layer 8 Backlight 9 Light source 10 linear polarizing filters 11 Image display device 12 Image display device 13 Backlight 14 light guides 15 Decoupling element 16 polarizing filters 17 light bulbs R solid angle range

Claims

Patent claims 1. A backlight (13) which is extensive, emits light and has an optical film for controlling and restricting a viewing angle range of a viewer, the optical film comprising a first polarization layer (1) with a first absorption axis which encloses an angle of 0° to 30° inclusive with a surface normal of the optical film, at least one phase-shifting compensation layer for improving the restriction of the viewing angle range and a second polarization layer (2) with a second absorption axis which is oriented parallel to the surface of the optical film, characterized in that between the first polarization layer (1) and the second polarization layer (2) in a first alternative i.a first B* compensation layer (3) is arranged, which is spatially homogeneous and consists of a first, biaxially birefringent material, with two optical axes and three main refractive axes,. • where each of the principal refractive axes has a unique refractive index n x , n y , n z is assigned, • where either the principal refractive axis to which the smallest refractive index corresponds or the principal refractive axis to which the largest refractive index corresponds is parallel to the first absorption axis, and • where for the first B* compensation layer (3) the condition is fulfilled, with the thickness d of the first B* compensation layer (3), the phase shift Aph and a predetermined wavelength A, ii. in a second alternative, at least two compensation layers made of uniaxially birefringent materials are arranged, wherein • a first, spatially homogeneous “A*” compensation layer (4) is constructed from a first uniaxially birefringent material having a first optical axis and two first mutually different principal refractive axes, wherein the first optical axis is perpendicular or parallel to the first absorption axis of the first polarization layer (1), and from the direction of an observer behind • a second, spatially homogeneous “A*” compensation layer (5) is arranged, which consists of a second uniaxially birefringent material with a second optical axis and two second principal refractive axes, wherein the second optical axis is perpendicular to the first optical axis, • where for each of the compensation layers the condition is fulfilled, with the thickness of a compensation layer d, the extraordinary refractive index n eand the ordinary refractive index n0, the phase shift Aph and a given wavelength A, and wherein in both alternatives i. and ii. the materials and thicknesses d of the compensation layers are specified such that, measured in a spherical coordinate system with its origin on the surface of the film and in the plane of the surface of the film, exclusively in a given solid angle range R, which azimuthal angle<p, für welche |q> | and |180° — <p| kleiner als der Betrag eines vorgegebenen Grenzazimutalwinkels (pi im is measured with respect to a preferred direction in the plane of the surface of the film, and polar angle 0, the magnitude of which is greater than a given limiting polar angle 0i imis measured with respect to the surface normal or, if the first absorption axis is not parallel to the surface normal, measured with respect to the first absorption axis and in a plane spanned by the surface normal and the first absorption axis, the luminance is minimal.

2. Backlight (13) according to claim 1, alternative ii., characterized in that between the first and second “A*” compensation layer a third, spatially homogeneous “C*” compensation layer (6) is arranged, which is constructed from a third uniaxially birefringent material with a third optical axis and two third principal refractive axes, wherein the third optical axis parallel to the first absorption axis of the first polarization layer (1).

3. Backlight (13) according to claim 1 or 2, characterized in that the first absorption axis is oriented perpendicular to the surface of the film.

4. Backlight (13) according to claim 3 and alternative i., wherein the main refractive axis, to which the smallest refractive index corresponds, is parallel to the first absorption axis, characterized in that the first "B*" compensation layer (3) is designed as a "-B" compensation layer, with n x > n y > n z , , with the principal refractive axis, to which the smallest refractive index n z , corresponds to, parallel to the surface normal and with the principal refractive axis, to which the largest refractive index n x corresponds, parallel to the second absorption axis of the second polarization layer (2).

5. Backlight (13) according to claim 3 and alternative i., wherein the main refractive axis, to which the largest refractive index corresponds, is parallel to the first absorption axis, characterized in that the first "B*" compensation layer (3) is designed as a "+B" compensation layer, with n z > n x > n y , with the main refractive axis, to which the largest refractive index n z , corresponds to, parallel to the surface normal and with the principal refractive axis, to which the smallest refractive index n y corresponds, parallel to the second absorption axis of the second polarization layer (2).

6. Backlight (13) according to claim 3 and alternative ii., characterized in that the first "A*" compensation layer (4) is designed as a "+A" compensation layer, the second "A*" compensation layer (5) is designed as a "-A" compensation layer, or vice versa and, if a third "C*" compensation layer (6) is present, this is designed as a "-C" or "+C" compensation layer.

7. Backlight (13) according to claim 1, alternative ii., characterized in that the first and the second “A*” compensation layer (4, 5) are identical.

8. Backlight (13) according to claim 1 or 2, characterized in that a liquid crystal layer (7) which can be switched between at least two states is arranged between the second polarization layer (2) and the compensation layer arranged closest to it, which liquid crystal layer is designed to transmit light transmitted by the second polarization layer (2) with unchanged polarization or rotated by 90° in a first switching state and to transmit it with circular or elliptical or linear polarization in a second switching state.

9. Backlight (13) according to claim 1 or 2, characterized in that a liquid crystal layer (7) which can be switched between at least two states is arranged between the first polarization layer (1) and the compensation layer arranged closest to it, which liquid crystal layer is designed to transmit light transmitted by the first polarization layer (1) with unchanged polarization or rotated by 90° in a first switching state and to transmit it with circularly or elliptically or linearly polarized light in a second switching state.

10. Background lighting (13) according to claim 1 or 2, characterized in that in the predetermined solid angle range R the loss function C2= J R ln T(< >, e) dn is minimal, where T(cp, 0) is the angle-dependent transmission and Q denotes the solid angle range.

11. Background lighting (13) according to claim 1 or 2, characterized in that the limit azimuthal angle cpiim is between 30° and 40° around a preferred direction and / or the limit polar angle 0i im between 40° and 50°.

12. Lighting device for a screen, which is configured so that it can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a solid angle range that is restricted compared to the free view mode, comprising - a backlight (13) according to one of claims 1 to 7, 10 or 11, - a plate-shaped light guide (14) located in front of the background lighting (13) in the viewing direction, which has output coupling elements (15) on at least one of the large surfaces and / or within its volume, - illuminating means (17) arranged laterally on at least one narrow side of the light guide (14), and - a linear polarization filter (16) arranged in the viewing direction in front of the background illumination (13) or in front of the light guide, whereby light emanating from the background illumination (13) and penetrating the linear polarization filter (16) is restricted in its propagation directions, - wherein in operating mode B2 the background lighting (13) is switched on and the lamps (17) are switched off, and wherein in operating mode B1 at least the lamps (17) are switched on.

13. A screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - a backlight (8) according to claim 8 or 9 with a liquid crystal layer (7) switchable between at least two states, - a linear polarization filter (10) arranged in front of the backlight (8) in the viewing direction, whereby light emanating from the backlight (8) and penetrating the linear polarization filter (10) is restricted in its propagation directions, and - a transmissive image display device (11) which is arranged in front of the background illumination (8) in the viewing direction and in or behind which the linear polarization filter (10) is arranged, - wherein in operating mode B2 the liquid crystal layer (7) which can be switched between at least two states is in the first switching state, and wherein in operating mode B1 the liquid crystal layer (7) which can be switched between at least two states is in the second switching state.

14. A screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - an image display device (12), - in the viewing direction in front of the image display device (12) an optical film with a liquid crystal layer (7) switchable between at least two states, - wherein in operating mode B2 the liquid crystal layer (7) is in the first switching state, and wherein in operating mode B1 the liquid crystal layer (7) is in the second switching state, the optical film comprising a first polarization layer (1) with a first absorption axis which encloses an angle of 0° to 30° inclusive with a surface normal of the optical film, at least one phase-shifting compensation layer for improving the limitation of the viewing angle range and a second polarization layer (2) with a second absorption axis which is oriented parallel to the surface of the optical film, wherein between the first polarization layer (1) and the second polarization layer (2) in a first alternative i.a first B* compensation layer (3) is arranged, which is spatially homogeneous and consists of a first, biaxially birefringent material, with two optical axes and three main refractive axes,. • where each of the principal refractive axes has a unique refractive index n x , n y , n z is assigned, • where either the principal refractive axis to which the smallest refractive index corresponds or the principal refractive axis to which the largest refractive index corresponds is parallel to the first absorption axis, and • where for the first B* compensation layer (3) the condition is fulfilled, with the thickness d of the first B* compensation layer (3), the phase shift Aph and a predetermined wavelength A, ii. in a second alternative, at least two compensation layers made of uniaxially birefringent materials are arranged, wherein • a first, spatially homogeneous “A*” compensation layer (4) is constructed from a first uniaxially birefringent material having a first optical axis and two first mutually different principal refractive axes, wherein the first optical axis is perpendicular or parallel to the first absorption axis of the first polarization layer (1), and from the direction of an observer behind • a second, spatially homogeneous “A*” compensation layer (5) is arranged, which consists of a second uniaxially birefringent material with a second optical axis and two second principal refractive axes, wherein the second optical axis is perpendicular to the first optical axis, • where for each of the compensation layers the condition is fulfilled, with the thickness of a compensation layer d, the extraordinary refractive index n e and the ordinary refractive index n0, the phase shift Aph and a given wavelength A, - where in both alternatives i. and ii. the materials and thicknesses d of the compensation layers are specified such that, measured in a spherical coordinate system with its origin on the surface of the film and in the plane of the surface of the film, exclusively in a given solid angle range R, which azimuthal angle φ, for which |q>| and 1180° - <p | kleiner als der Betrag eines vorgegebenen Grenzazimutalwinkels q)| im is measured with respect to a preferred direction in the plane of the surface of the film, and polar angle 0, the magnitude of which is greater than a given limiting polar angle 0i im is measured with respect to the surface normal or, if the first absorption axis is not parallel to the surface normal, measured with respect to the first absorption axis and in a plane spanned by the surface normal and the first absorption axis, the luminance is minimal, - and where • either a liquid crystal layer (7) which can be switched between at least two states is arranged between the second polarisation layer (2) and the compensation layer arranged closest thereto, which liquid crystal layer (7) is designed to transmit light transmitted by the second polarisation layer (2) with unchanged polarisation or with a polarisation rotated by 90° in a first switching state and to transmit it with circular or elliptical or linear polarisation in a second switching state, • or a liquid crystal layer (7) which can be switched between at least two states is arranged between the first polarization layer (1) and the compensation layer arranged closest to it, which liquid crystal layer (7) is designed to transmit light transmitted by the first polarization layer (1) with unchanged polarization or with a polarization rotated by 90° in a first switching state and to transmit it with circular or elliptical or linear polarization in a second switching state.

15. Screen according to claim 14, alternative ii., characterized in that between the first and second "A*" compensation layer there is arranged a third, spatially homogeneous "C*" compensation layer (6), which is constructed from a third uniaxially birefringent material with a third optical axis and two third principal refractive axes, wherein the third optical axis is parallel to the first absorption axis of the first polarization layer (1).

16. An optical film for controlling and restricting a viewer's viewing angle range, comprising a first polarization layer (1) having a first absorption axis which encloses an angle of 0° to 30° inclusive with a surface normal of the optical film, at least one phase-shifting compensation layer for improving the restriction of the viewing angle range, and a second polarization layer (2) having a second absorption axis which is oriented parallel to the surface of the optical film, characterized in that between the first polarization layer (1) and the second polarization layer (2) in a first alternative i. a first B* compensation layer (3) is arranged, which is spatially homogeneous and consists of a first, biaxially birefringent material, with two optical axes and three principal refractive axes, • where each of the principal refractive axes has a unique refractive index nx , n y , n z is assigned, • where either the principal refractive axis, to which the smallest refractive index corresponds, or the principal refractive axis, to which the largest refractive index corresponds to, lies parallel to the first absorption axis, and • where for the first B* compensation layer (3) the condition is fulfilled, with the thickness d of the first B* compensation layer (3), the phase shift Aph and a predetermined wavelength A, ii. in a second alternative, at least two compensation layers made of uniaxially birefringent materials are arranged, wherein • a first, spatially homogeneous “A*” compensation layer (4) is constructed from a first uniaxially birefringent material having a first optical axis and two first mutually different principal refractive axes, wherein the first optical axis is perpendicular or parallel to the first absorption axis of the first polarization layer (1), and from the direction of an observer behind • a second, spatially homogeneous “A*” compensation layer (5) is arranged, which consists of a second uniaxially birefringent material with a second optical axis and two second principal refractive axes, wherein the second optical axis is perpendicular to the first optical axis, • where for each of the compensation layers the condition is fulfilled, with the thickness of a compensation layer d, the extraordinary refractive index n eand the ordinary refractive index n0, the phase shift Aph and a given wavelength A, and wherein in both alternatives i. and ii. the materials and thicknesses d of the compensation layers are specified such that, measured in a spherical coordinate system with its origin on the surface of the film and in the plane of the surface of the film, exclusively in a given solid angle range R, which azimuthal angle<p, für welche |q> | and |180° — <p| kleiner als der Betrag eines vorgegebenen Grenzazimutalwinkels (pi im is measured with respect to a preferred direction in the plane of the surface of the film, and polar angle 0, the magnitude of which is greater than a given limiting polar angle 0i im is measured with respect to the surface normal or, if the first absorption axis is not parallel to the surface normal, measured with respect to the first absorption axis and in a plane spanned by the surface normal and the first absorption axis, the luminance is minimal.

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