Switchable light filter, lighting device, and screen with such a switchable light filter

Abstract

First switchable light filter (5), comprising: - a linear polarizing filter (P) having its transmission maximum for linear polarization directed in a first direction, - a phase plate (PP), - a quarter-wave plate (4), - a cholesteric liquid crystal layer (3), - means for selectively generating at least one first electric field (EF1) or one second electric field (EF2), - wherein either the first electric field (EF1) or the second electric field (EF2) acts at least temporarily on the cholesteric liquid crystal layer (3), and the latter changes its state between a focal-conical texture and a planar-cholesteric texture or vice versa, depending thereon, - such that the transmission properties of the first switchable light filter (5) change between a first operating mode B1, in which the cholesteric liquid crystal layer (3) has a focal-conical texture, and a second operating mode B2,in which the cholesteric liquid crystal layer (3) has a planar texture, - in that in the first operating mode B1, light which, on the side facing away from an observer - who is located on the side of the first switchable light filter (5) on which the linear polarizing filter (P) is arranged - is at least partially scattered when passing through the cholesteric liquid crystal layer (3), and subsequently, without any significant effect on its polarization, passes through the quarter-wave plate (4) and the phase plate (PP) and is finally linearly polarized when passing through the linear polarizing filter (P) before it leaves the first switchable light filter (5), so that in the first operating mode B1 no restriction of the propagation directions of the light incident on the first switchable light filter (5) is produced,and- in the second operating mode B2, the cholesteric liquid crystal layer (3) reflects light incident on the side facing away from the viewer into the first switchable light filter (5) with respect to circular polarization and transmits it with respect to the complementary circular polarization, and subsequently converts said transmitted light into linearly polarized light by the quarter-wave plate (4), while the polarization of the light is influenced by the phase plate (PP) depending on its direction of propagation, whereby light propagating along a selectable first preferred direction, wherein the first preferred direction is arranged at a predetermined angle Σ to the perpendicular bisector of the first linear polarization filter (P), the angle Σ being measured in a selectable first plane containing said perpendicular bisector, and exhibiting said first direction of linear polarization,finally, light can penetrate the linear polarizing filter (P), while light propagating at an angle greater than 30° to the first preferred direction onto the cholesteric liquid crystal layer (3) is absorbed by the linear polarizing filter (P) by at least 80%, so that in the second operating mode B2 a restriction of the propagation directions of the light incident on the first switchable light filter (5) is produced.

Description

Technical field of the invention Significant progress has been made in recent years in widening the viewing angle of LCDs. However, there are often situations where this very wide viewing area can be a disadvantage. Information such as banking details, personal information, and sensitive data is increasingly accessible on mobile devices like laptops and tablets. Consequently, people need control over who can see this sensitive data; they must be able to choose between a wide viewing angle—a public mode—to share information on their display with others, for example, when viewing vacation photos or for advertising purposes. Conversely, they need a narrow viewing angle—a private mode—when they want to keep the image information confidential. A similar problem arises in vehicle manufacturing: The driver must not be distracted by visual content, such as digital entertainment programs, when the engine is running, while the passenger wants to consume these programs even while the vehicle is in motion. Therefore, a screen is needed that can switch between the corresponding display modes. Additional films based on microlouvers have already been used for mobile displays to achieve visual privacy. However, these films were neither switchable nor reversible; they always had to be manually applied and then removed. They also had to be transported separately from the display when not in use. Furthermore, a significant disadvantage of using such lamellar films is the associated loss of light transmission. State of the art US Patent 6,765,550 B2 describes such a privacy screen using micro-louvers. The biggest disadvantages are the mechanical removal and installation of the filter, as well as the light loss in the protected mode. In WO 2012 / 033583 A1, the switching between clear and restricted vision is achieved by controlling liquid crystals between so-called "chromonic" layers. This results in a loss of light and the technical effort is quite high. US 2012 / 0235891 A1 describes a very complex backlight in a screen. According to Figures 1 and 15, not only are multiple light guides used, but also other complex optical elements such as microlens elements 40 and prism structures 50, which reshape the light from the rear light source on its way to the front light source. This is expensive and technically complex to implement and also results in light loss. According to the variant shown in Figure 17 of US 2012 / 0235891 A1, both light sources 4R and 18 produce light with a narrow illumination angle, whereby the light from the rear light source 18 is first converted into light with a wide illumination angle in a complex process. This complex conversion—as already mentioned above—significantly reduces brightness. US patent 2013 / 0308185 A1 describes a special, stepped light guide that emits light in different directions across a large area, depending on the direction from which it is illuminated from one of its narrow sides. In conjunction with a transmissive display device, such as an LCD, this allows for the creation of a screen that can be switched between unobstructed and restricted viewing modes. A disadvantage of this design is that the restricted viewing effect can only be generated for either left / right or top / bottom simultaneously, but not for left / right / top / bottom simultaneously, as is necessary for certain payment transactions. Furthermore, even in restricted viewing mode, some residual light remains visible from obstructed viewing angles. WO 2015 / 121398 A1, filed by the applicant, 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 scattering particles chosen, made of a polymer, generally have the disadvantage that light is coupled out from both large areas, causing approximately half of the useful light to be emitted in the wrong direction, namely towards the backlight, where it cannot be sufficiently recycled due to the design. Furthermore, the scattering polymer particles distributed in the volume of the light guide can, under certain circumstances, especially at higher concentrations, lead to scattering effects that reduce the visual privacy effect in the protected operating mode. The approach of "electrical birefringence (EDB)" technology is based on the idea of ​​using the switchable liquid crystals of an additionally applied LC panel to "filter" all light rays that do not exit the imaging layer at a specific viewing angle. Disadvantages of this technology include high additional energy and cost consumption and the difficulty in altering the + / -40° sweet spot, i.e., the optimal viewing position. The absorption coefficient of the LC structures is also insufficient, as the attenuation of light intensity increases again for viewing angles larger than the sweet spot, resulting in a light intensity of up to 3% of the maximum light intensity for viewing angles greater than + / -40°. The applicant's German patent application DE 10 2021 128 224 B3 describes a method and an arrangement, as well as the use of an arrangement, for selectively influencing the propagation directions of light. Two circular polarization types are used. However, it is not disclosed how the reflection behavior can be made switchable for specific directions. Furthermore, the applicant's DE 10 2023 110 269 B3 describes a switchable light filter, a lighting device, and a screen. However, it does not disclose how the reflection behavior can be made switchable for specific directions. Finally, DE 28 05 970 A1 describes a liquid crystal display device that addresses the issue of plate spacing in LC setups. This document does not describe how to design a switchable light filter in which the transmission of light can be influenced depending on the angle, and which also enables switching between at least two operating states. The aforementioned methods and arrangements generally share the disadvantage that they significantly reduce the brightness of the primary 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. Furthermore, many of the aforementioned approaches are not suitable for self-illuminating screens. Description of the invention The object of the invention is therefore to describe light filters in which the transmission of light can be influenced depending on the angle, and in which it is possible to switch between at least two operating states. In particular, the transmission behavior and, optionally, the reflection behavior should be switchable for specific directions. Furthermore, the solution should be usable for self-illuminating displays. Finally, the screen and lighting device with such light filters should be described. This problem is solved according to the invention by a first switchable light filter comprising (preferably viewed from the direction of an observer in this order): - a linear polarizing filter, which has its transmission maximum for linear polarization directed in a first direction (the first direction is, for example, the horizontal direction from the perspective of a standing or sitting observer), - a phase plate, - a quarter-wave plate, - a cholesteric liquid crystal layer, - means for selectively generating at least one first electric field EF1 or one second electric field EF2, - wherein either the first electric field EF1 or the second electric field EF2, or no field, acts at least temporarily on the cholesteric liquid crystal layer.and this changes its state depending on the texture, between a focal-conical texture (which is scattering) and a planar-cholesteric texture (which ensures Bragg reflection) or vice versa, whereby these two aforementioned texture states are stable, i.e., the electric field EF1 or EF2 can disappear again after a texture change, so that the transmission properties of the first switchable light filter differ between a first operating mode B1, in which the cholesteric liquid crystal layer has a focal-conical texture, and a second operating mode B2, in which the cholesteric liquid crystal layer has a planar texture, by the fact that in the first operating mode B1, light which enters the first switchable light filter on the side facing away from an observer - who is located on the side of the first switchable light filter on which the linear polarizing filter is arranged -when passing through the cholesteric liquid crystal layer, the light is at least partially scattered, and subsequently, without any significant effect on its polarization, passes through the quarter-wave plate and the phase plate, and is finally linearly polarized when passing through the linear polarizing filter before leaving the first switchable light filter, so that in the first operating mode B1 no restriction of the propagation directions of the light incident on the first switchable light filter is produced, and—in the second operating mode B2, the cholesteric liquid crystal layer reflects light incident on the side facing away from the viewer with respect to a circular polarization and transmits it with respect to the complementary circular polarization, and subsequently the transmitted light is converted into linearly polarized light by the quarter-wave plate.While the polarization of the light is influenced by the phase plate depending on its direction of propagation, light propagating along a selectable first preferred direction, wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first linear polarizing filter, the angle α being measured in a selectable first plane containing said perpendicular bisector, and exhibiting said first direction of linear polarization, can ultimately penetrate the linear polarizing filter, while light propagating at an angle greater than 30° to the first preferred direction onto the cholesteric liquid crystal layer is absorbed by the linear polarizing filter by at least 80%.so that in the second operating mode B2, a restriction of the propagation directions of the light incident on the first switchable light filter is created. (This can create a visual barrier when using a first switchable light filter with a screen. Angles of incidence always refer to the propagation direction / angle of incidence in a vacuum.) To better understand the mechanism of action, the following section describes cholesteric liquid crystals. Their focal-conical texture is scattering and stable. It forms when an electric field is applied along the surface. In contrast, the planar-cholesteric texture is reflective and stable for circular polarization. Together with a quarter-wave plate, the layer can be used as a linearly reflecting polarization filter. The reflection behaves analogously to a conventional Bragg mirror, but the reflection is sensitive to polarization. The homeotropic texture, on the other hand, is unstable and arises when an electric field is applied between the interfaces. When the electric field is removed, the homeotropic texture transforms into the planar cholesteric texture. Thus, a layer of cholesteric liquid crystals can selectively polarize, scatter, or transmit light essentially unchanged by applying an electric field. For example, for a cholesteric liquid crystal layer used within the scope of the invention, its design wavelength is greater than the product p * n, where n corresponds to the average refractive index of the cholesteric liquid crystal layer and p is the average rise per turn. An exemplary design wavelength λ can be, for example, 580 nm. As a rule, the aforementioned product p * n should therefore be smaller than the design wavelength of the cholesteric liquid crystal layer, since the optical path difference increases for oblique angles of incidence, which is partially compensated for by the aforementioned relationship. In this context: 0.5 < p * n / λ < 1.0. Furthermore, the phase plate is preferably designed such that its extraordinary refractive index axis lies in the zV plane or along the V-axis of the first switchable light filter, where V-axis denotes the vertical axis of the first switchable light filter from the perspective of a standing or seated observer, and z-axis denotes a direction perpendicular to the first switchable light filter in question. For the phase plate, 300 nm < d ∗ Δn < 2000 nm, where d is the thickness of the phase plate and Δn is the refractive index difference between the ordinary and extraordinary refractive index axes of the phase plate. Furthermore, the first switchable light filter with the above-described structure ensures that light incident on the first switchable light filter from one side of a viewer is not reflected in the first operating mode B1, up to a predefinable tolerance value; and in the second operating mode B2, when incident along the first preferred direction, is not reflected up to a predefinable tolerance value; and in the second operating mode B2, when incident at angles that deviate at least 10° from the first preferred direction, its polarization is influenced by the phase plate and the quarter-wave plate in such a way that it is at least partially reflected by the cholesteric liquid crystal layer, so that any intended privacy effect is enhanced by the superposition of residual light still present from lateral viewing angles with the light reflected at such angles. The means for selectively generating at least one first electric field EF1 or one second electric field EF2 can, for example, be one or more ITO layers between which a potential difference can be set by means of a control signal. In particular embodiments, these means for selectively generating at least one first electric field EF1 or one second electric field EF2 can also be located wholly or partially outside the first switchable light filter, e.g., in a shell that can be mechanically folded onto or in front of the cholesteric liquid crystal layer. This particularly exploits the fact that the two textures used for the invention are stable, i.e., the desired texture is retained even without an electric field.To switch between the two operating modes, the means for selectively generating at least one first electric field EF1 or one second electric field EF2 are briefly positioned in front of the first switchable light filter until its texture has changed accordingly. Afterwards, these means are repositioned. This approach allows, among other things, the use of so-called in-cell touch or on-cell touch systems, which can be located within an image display device behind the first or second switchable light filter, because these means do not permanently interfere with or block the electrodynamics of the touch units. A further advantageous embodiment of the first switchable light filter provides that an angle-limiting layer (e.g., a lamellar filter / privacy filter or, if applicable, a lens array) is also present, which has a permanent angle-limiting transmission and which is preferably located behind the cholesteric liquid crystal layer in the viewing direction, wherein in the first operating mode B1 the cholesteric liquid crystal layer at least partially scatters light that has penetrated and is incident on the angle-limiting layer, so that the brightness is increased for viewing angles that differ from the first preferred direction (preferably by at least 10°), while in the second operating mode B2 such scattering does not take place, thereby maintaining the angle-limiting effect of the angle-limiting layer.Preferably, the angle-limiting layer has its maximum transmission with a tolerance of at most 10°, particularly preferably at most 5°, along the first preferred direction. Preferably, however, the angle-limiting layer comprises at least one first optical element, which in turn comprises: • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.2 micrometers thick, • wherein the majority of these transition dipole moments are aligned parallel to or vary around the first preferred direction with a tolerance of at most 20° in at least one first state, • such that light incident on the first optical element with an incidence direction and a polarization state is transmitted or at least partially absorbed depending on its incidence direction relative to the first optical element and its polarization state, whereby at least 50% of the light incident in the first preferred direction is transmitted regardless of its polarization.and whereby light incident at angles greater than 30° to the preferred direction is transmitted at least 50% if it is s-polarized, and absorbed at least 80% if it is p-polarized. For example, p-polarized light at angles greater than 45° to the first preferred direction can be absorbed by the first optical element at an angle of more than 90%. Advantageously, the liquid crystal layer and / or the means for selectively generating at least one first electric field EF1 or one second electric field EF2 are divided into several separately switchable segments, so that local switching between the respective possible operating modes B1 and B2 is enabled. The object of the invention is also achieved by a second switchable light filter comprising (preferably viewed from the direction of an observer in this order): - a linear polarizing filter having its transmission maximum for linear polarization oriented in a selectable first direction; - a phase plate; - a second guest-host liquid crystal layer containing, in addition to cholesteric liquid crystals, a proportion of one or more dichroic dyes; - means for selectively generating at least one first electric field EF1 or one second electric field EF2; - wherein either the first electric field EF1 or the second electric field EF2, or no field, selectively acts on the guest-host liquid crystal layer, and the latter, depending thereon, changes its state between a first, scattering and non-polarizing state and a second,- changes between non-scattering and polarizing states or vice versa, - so that the transmission properties of the second switchable light filter differ between a first operating mode B1, in which the second guest-host liquid crystal layer is in the first state, and a second operating mode B2, in which the second guest-host liquid crystal layer is in the second state, - in the first operating mode B1, the second guest-host liquid crystal layer at least partially scatters light that enters the second switchable light filter on the side facing away from the observer - who is located on the side of the second switchable light filter on which the linear polarizing filter is arranged - and is not significantly affected in its polarization, whereby the light then penetrates the phase plate and is finally linearly polarized when passing through the linear polarizing filter,before it leaves the switchable light filter, so that in the first operating mode B1 no (significant) restriction of the propagation directions of the light incident on the switchable light filter is produced, and—in the second operating mode B2, the second guest-host liquid crystal layer does not significantly scatter light that occurs on the side facing away from the viewer into the second switchable light filter and polarizes it linearly along a second direction (this second direction is preferably at an angle of about 90° to the first direction), wherein the polarization of the light is subsequently influenced by the phase plate depending on its propagation directions, whereby light that propagates along a selectable second preferred direction (this can differ from the perpendicular, but preferably corresponds to the perpendicular) and exhibits the said first direction of linear polarization,Finally, light can penetrate the linear polarizing filter, while light propagating at an angle greater than 30° to the perpendicular center of the second guest-host liquid crystal layer is absorbed by the linear polarizing filter by at least 80%, thus creating a restriction in the propagation directions of the light incident on and penetrating the second switchable light filter in the second operating mode B2. (This can, for example, create a privacy screen when a second switchable light filter is used with a screen.) Linearly polarized or non-polarized light can enter the second switchable light filter. The operating principle of the second switchable light filter is similar to that of the first, except that it does not use circularly polarized light, which is why a quarter-wave plate is not required in the setup. Furthermore, in such a second switchable light filter, the second guest-host liquid crystal layer and / or the means for selectively generating at least one first electric field EF1 or one second electric field EF2 can be divided into several separately switchable segments, so that local switching between the respective possible operating modes B1 and B2 is enabled. Naturally, suitable control systems, e.g. control electronics, are available for the liquid crystal layer of the first switchable light filter or for the second guest-host liquid crystal layer of the second switchable light filter. The invention further comprises a screen with a first or second switchable light filter and an image display device located behind it in the viewing direction, wherein the luminance curve of the light emitted or transmitted by the image display device preferably exhibits a maximum along at least one cross-sectional plane. Such a configuration supports the visual privacy effect in the second operating mode B2 and can be implemented, for example, by applying a microstructure to the image display device, such as an OLED panel, which restricts the directions of propagation or increases the light intensity in a preferred direction. For an OLED panel, it is also conceivable that it emits light essentially in one direction. For this purpose, the layer thicknesses and refractive indices of the individual layers in the OLED are optimized to achieve such a radiation pattern. Alternatively, for example in the case of an LCD panel as an image display device, crossed prism grid films can be used in its backlight to achieve the effect described above. It should be noted that, in the case of an LCD panel in conjunction with a first switchable light filter, the linearly polarized light emitted by the LCD panel should first be converted into circularly polarized light by means of a retarder (e.g., a quarter-wave retarder) so that s- and p-polarized light components are available for the further light path. Alternatively, at least partial depolarization or other configurations would also be conceivable. In a screen as described above, the first or second switchable light filter can be subsequently attached to the image display device by a user and / or reversibly. In a screen as described above, a light guide can be arranged in the viewing direction in front of the first or second switchable light filter. In operating mode B2, this light guide is illuminated on at least one narrow side, with the light guide emitting light across at least one of its large surfaces. The maximum light output of said light guide can be directed in one or more directions that differ by at least 10° from the first or second preferred direction. Such a configuration can be particularly advantageous for nighttime applications because the reflection component is then greater in the second operating mode B2. Finally, the invention also includes a lighting device for a screen which can be operated in at least one first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode in which light is emitted into a viewing angle area that is restricted for a viewer compared to the free viewing mode, comprising: - an area-like extended backlight which emits light and - a first switchable light filter or a second switchable light filter arranged in the viewing direction in front of the backlight, as described above. A previously described light filter, lighting device, or screen is advantageously used in a mobile device, a motor vehicle, aircraft, or watercraft, in a payment terminal, or in an access control system. It is possible to switch between the aforementioned operating modes to protect sensitive data, i.e., to display it perceptibly to only one viewer, or alternatively, to display image content simultaneously to multiple viewers. In principle, the performance of the invention is maintained if the parameters described above are varied within certain limits. It is understood that the features mentioned above and those to be explained below can be used not only in the combinations given, but also in other combinations or on their own, without leaving the scope of the present invention. Brief description of the drawings The invention is explained in more detail below with reference to exemplary embodiments and the accompanying drawings, which also disclose essential features of the invention. These exemplary embodiments serve only for illustration and are not to be interpreted as limiting. For example, a description of an exemplary embodiment with a plurality of elements or components is not to be interpreted as meaning that all of these elements or components are necessary for implementation. Rather, other exemplary embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components from different exemplary embodiments may be combined with one another unless otherwise specified. 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 with the same reference numerals and are not explained multiple times. Figure 1 shows a schematic diagram of the construction of an exemplary first switchable light filter, Figure 2 shows a schematic diagram of the construction of an exemplary second switchable light filter, Figure 3 shows a schematic diagram of the construction of an exemplary lighting device with a switchable light filter, Figure 4 shows a schematic diagram of the operation of an exemplary first switchable light filter in transmission in a first operating mode B1, Figure 5 shows a schematic diagram of the operation of an exemplary first switchable light filter in transmission in a second operating mode B2, Figure 6 shows a schematic diagram of the operation of an exemplary first switchable light filter in reflection in a first operating mode B1, and Figure 6 shows...7 a schematic diagram of the operating principle of an exemplary first switchable light filter in reflection in a second operating mode B2. Detailed description of the drawings The drawings are not to scale and only represent schematic diagrams. Figure 1 shows a schematic diagram of the construction of an exemplary first switchable light filter 5. This comprises (preferably viewed from the perspective of an observer, in this order): - a linear polarizing filter P, which has its transmission maximum for linear polarization directed in a first direction (the first direction is, for example, the horizontal direction from the perspective of a standing or sitting observer), - a phase plate PP, - a quarter-wave plate 4, - a cholesteric liquid crystal layer 3, - means (not shown in the drawing) for selectively generating at least one first electric field EF1 or one second electric field EF2, e.g., two ITO layers with an electronic control, - wherein either the first electric field EF1 or the second electric field EF2, or no field, acts at least temporarily on the cholesteric liquid crystal layer 3.and the cholesteric liquid crystal layer 3 changes its state depending on the condition, between a focal-conical texture (which is scattering and stable) and a planar-cholesteric texture (which ensures Bragg reflection) or vice versa, whereby these two aforementioned texture states are stable, i.e., the electric field EF1 or EF2 can disappear again after a texture change, so that the transmission properties of the first switchable light filter 5 differ between a first operating mode B1, in which the cholesteric liquid crystal layer 3 has a focal-conical texture, and a second operating mode B2, in which the cholesteric liquid crystal layer 3 has a planar texture, in that in the first operating mode B1, light which is incident on the side of the first switchable light filter 5 facing away from the observer is at least partially scattered when passing through the cholesteric liquid crystal layer 3,and subsequently, without any significant influence on its polarization, penetrates the quarter-wave plate 4 and the phase plate PP and is finally linearly polarized upon passing through the linear polarizing filter P before leaving the switchable light filter 5, so that in the first operating mode B1 no restriction of the propagation directions of the light incident on the first switchable light filter 5 is created, and—in the second operating mode B2, the cholesteric liquid crystal layer 3 reflects light incident on the side facing away from the viewer with respect to a circular polarization and transmits it with respect to the complementary circular polarization, and subsequently the said transmitted light is converted into linearly polarized light by the quarter-wave plate 4, while the polarization of the light is influenced by the phase plate PP depending on its propagation direction.This allows light propagating along a selectable first preferred direction, wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first linear polarizing filter P, and the angle α is measured in a selectable first plane containing said perpendicular bisector, and exhibiting said first direction of linear polarization, to eventually penetrate the linear polarizing filter P, while light propagating at an angle greater than 30° to the first preferred direction onto the cholesteric liquid crystal layer 3 is absorbed by the linear polarizing filter P by at least 80%, so that in the second operating mode B2 a restriction of the propagation directions of the light incident on the first switchable light filter 5 is generated. (Angles of incidence always refer to the propagation direction / angle of incidence in a vacuum.) Thus, by using a first switchable light filter 5 together with an image display device, a switchable privacy screen can be created. To better understand the mechanism of action, the following section describes cholesteric liquid crystals. Their focal-conical texture is scattering and stable. It forms when an electric field is applied along the surface. In contrast, the planar-cholesteric texture is reflective and stable for circular polarization. Together with a quarter-wave plate, the layer can be used as a linearly reflecting polarization filter. The reflection behaves analogously to a conventional Bragg mirror, but the reflection is sensitive to polarization. The homeotropic texture, on the other hand, is unstable and arises when an electric field is applied between the interfaces. When the electric field is removed, the homeotropic texture transforms into the planar cholesteric texture. Thus, a layer of cholesteric liquid crystals can selectively polarize, scatter, or transmit light essentially unchanged by applying an electric field. For example, for a cholesteric liquid crystal layer 3 used within the scope of the invention, its design wavelength is greater than the product p * n, where n corresponds to the average refractive index of the cholesteric liquid crystal layer 3 and p is the rise per turn. An exemplary design wavelength λ can be, for example, 580 nm. As a rule, the aforementioned product p * n should therefore be smaller than the design wavelength of the cholesteric liquid crystal layer 3, since the optical path difference increases for oblique angles of incidence, which is partially compensated for by the aforementioned relationship. In this context: 0.5 < p * n / λ < 1.0. Furthermore, the phase plate PP is preferably designed such that its extraordinary refractive index axis lies in the zV plane or along the V-axis of the first switchable light filter 5, where V-axis denotes the vertical axis of the first switchable light filter 5 from the perspective of a standing or seated observer, and z-axis denotes a perpendicular to the first switchable light filter 5. For the phase plate PP, 300 nm < d*Δn < 2000 nm, where d is the thickness of the phase plate PP and Δn is the refractive index difference between the ordinary and extraordinary refractive index axes of the phase plate PP. Furthermore, the first switchable light filter 5 with the above-described structure ensures that light incident on the first switchable light filter 5 from a viewing side is not reflected in the first operating mode B1, except to a predefinable tolerance value, and in the second operating mode B2, when incident along the first preferred direction, is not reflected except to a predefinable tolerance value, and in the second operating mode B2, when incident at angles that deviate at least 10° from the first preferred direction, its polarization is influenced by the phase plate PP and the quarter-wave plate 4 in such a way that it is at least partially reflected by the cholesteric liquid crystal layer 3, so that any intended privacy effect is enhanced by the superposition of residual light still present from lateral viewing angles with the light reflected at such angles. The operating principle of a first switchable light filter 5 is explained in more detail below with reference to the drawings Fig. 4, Fig. 5, Fig. 6 to Fig. 7, where only representative light rays are shown. In reality, a multitude of light rays are present. Thus, Fig. 4 shows a schematic diagram of the operating principle of an exemplary first switchable light filter in transmission in a first operating mode B1, and Fig. 5 in a second operating mode B2. Furthermore, Fig. 6 shows a schematic diagram of the operating principle of an exemplary first switchable light filter in reflection in a first operating mode B1, and Fig. 7 in a second operating mode B2. With regard to the polarization properties of light rays, a. a circle represents linearly perpendicularly polarized light, b. an arrow pointing to the right represents linearly parallel polarized light, and c. a circle with an arrow represents circularly polarized light. Initially, it is assumed that unpolarized light, for example from an OLED panel, falls onto the first switchable light filter 5 from behind. Under the conditions shown in Fig. 4, the first operating mode B1 is present, i.e., the cholesteric liquid crystal layer 3 has a focal-conical texture. Thus, the cholesteric liquid crystal layer 3 scatters at least partially the light that falls onto the first switchable light filter 5 on the side facing away from the viewer. The light then passes through the quarter-wave plate 4 and the phase plate PP without any significant effect on its polarization, and is finally linearly polarized upon passing through the linear polarizing filter P before exiting the switchable light filter 5. Therefore, in the first operating mode B1, no restriction of the propagation directions of the light incident on the first switchable light filter 5 is created. In Fig.This can be seen in Figure 4, since both the vertical and the oblique rays extend above the linear polarization filter P. In contrast, as assumed for Fig. 5, in the second operating mode B2 the cholesteric liquid crystal layer 3 exhibits a planar texture. Light incident on the side facing away from the viewer into the switchable light filter 5 is now reflected with respect to circular polarization (see the two downward arrows) and transmitted with respect to the complementary circular polarization. Subsequently, the transmitted light is converted into linearly polarized light by the quarter-wave plate 4, while the polarization of the light is influenced by the phase plate PP depending on its direction of propagation. The resulting altered polarization properties are shown in Fig. 5.Here, the polarization properties between the phase plate PP and the linear polarizing filter P are "perpendicularly linearly polarized" in the perpendicular direction (the perpendicular direction corresponds here to the selectable first preferred direction) and "parallel linearly polarized" in the oblique direction. Accordingly, the linear polarizing filter P only allows perpendicular light rays to pass through.This allows light propagating along the first preferred direction and exhibiting the aforementioned first direction of linear polarization to eventually pass through the linear polarization filter P, while light striking the cholesteric liquid crystal layer 3 at an angle greater than 30° to the first preferred direction is absorbed by the linear polarization filter P by at least 80%, thus creating a restriction of the propagation directions of the light incident on the first switchable light filter 5 in the second operating mode B2. Furthermore, Fig. 6 shows a schematic diagram illustrating the operation of an exemplary first switchable light filter 5 in reflection mode B1, i.e., the cholesteric liquid crystal layer 3 has a focal-conical texture. Here, light incident on the first switchable light filter 5 from one side of the viewer is not reflected, except for a predefinable tolerance value, as shown in Fig. 6. While the light changes its polarization properties or is partially absorbed as it passes through the first switchable light filter 5, depending on its direction (along the first preferred direction, i.e., perpendicular to it, or obliquely to it), there is no dedicated or intended function for reflection at any point in the structure—except for unavoidable reflections, e.g., at refractive index transitions. In other words, light arriving from the viewer's side along the first preferred direction is not affected in its polarization state by the phase plate PP. The linear polarization is converted into circularly polarized light by the quarter-wave plate 4, which passes unimpeded through the cholesteric liquid crystal layer 3, acting as a reflective polarization filter. The situation is different in operating mode B2, which is shown in schematic form in Fig. 7. Here, the cholesteric liquid crystal layer 3 again has a planar texture, so that light incident on the first switchable light filter 5 from one side of the viewer is not reflected up to a predefinable tolerance value when incident along the first preferred direction. However, light incident at angles that deviate by at least 10° from the first preferred direction is influenced in its polarization by the phase plate PP and the quarter-wave plate 4 such that it is at least partially reflected by the cholesteric liquid crystal layer 3. This reinforces any intended privacy effect resulting from the superposition of residual light still present from lateral viewing angles with the light reflected at such angles.The assumed residual light corresponds to residual transmissions of light incident on the first switchable light filter 5 from the viewer's perspective from behind, which should ideally be completely absorbed at angles greater than 30° to the first preferred direction, which is usually not achieved due to technical reasons. In other words, light originating from the viewer's side and not from the first preferred direction is influenced in its polarization state by the phase plate PP. This causes the light to be partially converted into left- and right-hand polarized light by the quarter-wave plate 4. One polarization type is transmitted by the cholesteric liquid crystal layer 3, which acts as a reflective polarizing filter, while the other polarization type is reflected. The reflected component passes through the assembly again and reaches the viewer-facing side of the first switchable light filter 5, penetrates the polarizing filter P, and thereby—intentionally—reduces the contrast perceptible to the viewer, always from angles that deviate from the first preferred direction. An advantageous, optional embodiment of the first switchable light filter 5 provides that an angle-limiting layer 1D (e.g., a lamellar filter / privacy filter or, if applicable, a lens array) is also present, which permanently exhibits angle-limiting transmission and which is preferably located behind the cholesteric liquid crystal layer 3 in the viewing direction, wherein in the first operating mode B1 the cholesteric liquid crystal layer 3 at least partially scatters light that has penetrated and is incident upon the angle-limiting layer 1D, so that the brightness is increased for viewing angles that differ from the first preferred direction (preferably by at least 10°), while in the second operating mode B2 such scattering by the cholesteric liquid crystal layer 3 does not take place, thereby maintaining the angle-limiting effect of the angle-limiting layer 1D.Preferably, the angle-limiting layer exhibits its maximum transmission with a tolerance of at most 10°, particularly preferably at most 5°, along the first preferred direction. In Figures 1 and 2, such an angle-limiting layer 1D is shown as an option. Preferably, the angle-limiting layer 1D comprises at least one first optical element 1, which in turn comprises: • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.2 micrometers thick, • wherein the majority of these transition dipole moments are aligned parallel to or vary around the first preferred direction with a tolerance of at most 20° in at least one first state, • such that light incident on the first optical element 1 with an incidence direction and a polarization state is transmitted or at least partially absorbed depending on its incidence direction relative to the first optical element 1 and its polarization state, whereby light incident in the first preferred direction is transmitted at least 50% regardless of its polarization.and whereby light incident at angles greater than 30° to the preferred direction is transmitted at least 50% if it is s-polarized, and absorbed at least 80% if it is p-polarized. Thus, for example, p-polarized light at angles greater than 45° to the first preferred direction can be absorbed by the first optical element 1 at an angle of more than 90%. Advantageously, the liquid crystal layer 3 and / or the means for selectively generating at least one first electric field EF1 or one second electric field EF2 are divided into several separately switchable segments, so that locally different switching between the respective possible operating modes B1 and B2 is enabled. Fig. 2 shows a schematic diagram of the construction of an exemplary second switchable light filter 5a, comprising (preferably viewed from the perspective of an observer, in this order): - a linear polarizing filter P, which has its transmission maximum for linear polarization oriented in a selectable first direction; - a phase plate PP; - a second guest-host liquid crystal layer 3a, which, in addition to cholesteric liquid crystals, contains a proportion of one or more dichroic dyes; - (not shown in the drawing) means for selectively generating at least one first electric field EF1 or one second electric field EF2; - wherein either the first electric field EF1 or the second electric field EF2, or no field, selectively acts on the guest-host liquid crystal layer 3a, and this changes its state between a first,a scattering and non-polarizing state and a second, non-scattering and polarizing state, or vice versa, - so that the transmission properties of the second switchable light filter 5a differ between a first operating mode B1, in which the second guest-host liquid crystal layer 3a is in the first state, and a second operating mode B2, in which the second guest-host liquid crystal layer 3a is in the second state, - in that in the first operating mode B1 the second guest-host liquid crystal layer 3a at least partially scatters light which is incident on the side facing away from the observer into the second switchable light filter 5a and is not significantly affected in its polarization, wherein the light then penetrates the phase plate PP and is finally linearly polarized when passing through the linear polarizing filter P before it leaves the switchable light filter 5,so that in the first operating mode B1 no (significant) restriction of the propagation directions of the light incident on the switchable light filter 5 is produced, and—in the second operating mode B2, the second guest-host liquid crystal layer 3a does not significantly scatter light incident on the side facing away from the viewer into the second switchable light filter 5a and polarizes it linearly along a second direction (this second direction is preferably at an angle of about 90° to the first direction), wherein the polarization of the light is subsequently influenced by the phase plate PP depending on its propagation directions, whereby light propagating along a selectable second preferred direction (this may differ from the perpendicular, but preferably corresponds to the perpendicular) and exhibiting the aforementioned first direction of linear polarization,Finally, light can penetrate the linear polarizing filter P, while light propagating at an angle greater than 30° to the perpendicular to the second guest-host liquid crystal layer 3a is absorbed by the linear polarizing filter P by at least 80%, so that in the second operating mode B2, a restriction of the propagation directions of the light incident on and penetrating the second switchable light filter 5a is created. (This can, for example, create a switchable privacy screen when using a second switchable light filter with a screen.) Furthermore, in such a second switchable light filter 5a, the second guest-host liquid crystal layer 3a and / or the means for selectively generating at least one first electric field EF1 or one second electric field EF2 can be divided into several separately switchable segments, so that local switching between the respective possible operating modes B1 and B2 is enabled. Furthermore, a screen with a first or second switchable light filter 5, 5a and an image display device 9 located behind it in the viewing direction is disclosed, wherein the luminance curve of the light emitted or transmitted by the image display device 9 preferably exhibits a maximum along at least one cross-sectional plane. Such a configuration supports the visual privacy effect in the second operating mode B2 and can be implemented, for example, by applying a microstructure to the image display device 9, such as an OLED panel, which restricts the directions of propagation or increases the light intensity in a preferred direction. For an OLED panel, it is also conceivable that it emits light essentially in one direction. For this purpose, the layer thicknesses and refractive indices of the individual layers in the OLED are optimized to achieve such a radiation pattern. Furthermore, it is possible, for example in the case of an LCD panel as an image display device 9, to use crossed prism grid films in its backlight to achieve a previously described effect. It should be noted that, in the case of an LCD panel, when using a first switchable light filter 5, the linearly polarized light emitted by the LCD panel should first be converted into circularly polarized light by means of a retarder (e.g., a quarter-wave retarder) so that s- and p-polarized light components are available for the further light path. Alternatively, at least partial depolarization or other configurations would also be conceivable. To illustrate the screens described above, Figures 1 and 2, which show schematic diagrams of the construction of the first and second switchable light filters 5 and 5a respectively, can be used. Possible positions for the image display unit 9 are shown there. The explanations given above are analogous here and are therefore not repeated. Furthermore, Fig. 3 shows a schematic diagram of the construction of an exemplary lighting device with a switchable light filter 5, 5a. Such a lighting device for a screen, which can be operated in at least a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode in which light is emitted into a viewing angle area that is more limited than in the free viewing mode, comprises: - an area-like extended backlight 8 that emits light, and - a first switchable light filter 5 or a second switchable light filter 5a arranged in the viewing direction in front of the backlight 8, as described above. The invention solves the stated problem: A light filter is described in which the transmission of light can be influenced depending on the angle – optionally perpendicular to a seated or standing observer – and in which it is possible to switch between at least two operating states. In particular, the transmission behavior and, optionally, the reflection behavior can be switched for specific directions. The presented solution is also suitable for self-illuminating displays, such as OLED panels. Furthermore, a display and a lighting device incorporating such light filters are described. The invention described above can be advantageously used in conjunction with an image display device wherever confidential data is displayed and / or entered, such as during PIN entry or data display at ATMs or payment terminals, password entry, or when reading emails on mobile devices. As described above, the invention can also be used in cars to selectively shield the driver or passenger from distracting visual content.

Claims

First switchable light filter (5), comprising: - a linear polarizing filter (P) having its transmission maximum for linear polarization directed in a first direction, - a phase plate (PP), - a quarter-wave plate (4), - a cholesteric liquid crystal layer (3), - means for selectively generating at least one first electric field (EF1) or one second electric field (EF2), - wherein either the first electric field (EF1) or the second electric field (EF2) acts at least temporarily on the cholesteric liquid crystal layer (3), and the latter changes its state between a focal-conical texture and a planar-cholesteric texture or vice versa, depending thereon, - such that the transmission properties of the first switchable light filter (5) change between a first operating mode B1, in which the cholesteric liquid crystal layer (3) has a focal-conical texture, and a second operating mode B2,in which the cholesteric liquid crystal layer (3) has a planar texture, - in that in the first operating mode B1, light which, on the side facing away from an observer - who is located on the side of the first switchable light filter (5) on which the linear polarizing filter (P) is arranged - is at least partially scattered when passing through the cholesteric liquid crystal layer (3), and subsequently, without any significant effect on its polarization, passes through the quarter-wave plate (4) and the phase plate (PP) and is finally linearly polarized when passing through the linear polarizing filter (P) before it leaves the first switchable light filter (5), so that in the first operating mode B1 no restriction of the propagation directions of the light incident on the first switchable light filter (5) is produced,and- in the second operating mode B2, the cholesteric liquid crystal layer (3) reflects light incident on the side facing away from the viewer into the first switchable light filter (5) with respect to circular polarization and transmits it with respect to the complementary circular polarization, and subsequently converts said transmitted light into linearly polarized light by the quarter-wave plate (4), while the polarization of the light is influenced by the phase plate (PP) depending on its direction of propagation, whereby light propagating along a selectable first preferred direction, wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first linear polarization filter (P), wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, exhibits said first direction of linear polarization,finally, light can penetrate the linear polarizing filter (P), while light propagating at an angle greater than 30° to the first preferred direction onto the cholesteric liquid crystal layer (3) is absorbed by the linear polarizing filter (P) by at least 80%, so that in the second operating mode B2 a restriction of the propagation directions of the light incident on the first switchable light filter (5) is produced. First switchable light filter (5) according to claim 1, characterized in that light incident on the first switchable light filter (5) from the viewer side is not reflected in the first operating mode B1 up to a predefinable tolerance value, and in the second operating mode B2, when incident along the first preferred direction, is not reflected up to a predefinable tolerance value, and in the second operating mode B2, when incident at angles that deviate at least 10° from the first preferred direction, is influenced in its polarization by the phase plate (PP) and the quarter-wave plate (4) such that it is at least partially reflected by the cholesteric liquid crystal layer (3). First switchable light filter (5) according to one of the preceding claims, characterized in that an angle-limiting layer (1D) is further provided which has a permanent angle-limiting transmission and which is preferably located behind the cholesteric liquid crystal layer (3) in the viewing direction, wherein in the first operating mode B1 the cholesteric liquid crystal layer (3) at least partially scatters light which has penetrated and is incident on the angle-limiting layer (1D), so that the brightness is increased for viewing angles that differ from the first preferred direction, while in the second operating mode B2 such scattering does not take place, whereby the angle-limiting effect of the angle-limiting layer (1D) is maintained. First switchable light filter (5) according to claim 3, characterized in that the angle-limiting layer (1D) comprises at least one first optical element (1) which in turn comprises: • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.2 micrometers thick, • wherein the plurality of the transition dipole moments is aligned parallel to or varies around the first preferred direction with a tolerance of at least 20° in at least one first state, • such that light incident on the first optical element (1) with an incident direction and a polarization state is transmitted or at least partially absorbed depending on its incident direction relative to the first optical element (1) and its polarization state, whereby light incident in the first preferred direction is transmitted at least 50% regardless of its polarization.and whereby light incident at angles greater than 30° to the preferred direction is transmitted at least 50% if it is s-polarized, and absorbed at least 80% if it is p-polarized. First switchable light filter (5) according to one of the preceding claims, characterized in that the liquid crystal layer (3) and / or the means for selectively generating at least one first electric field (EF1) or one second electric field (EF2) is / are divided into several separately switchable segments, so that local switching between the respective possible operating modes B1 and B2 is enabled. Second switchable light filter (5a), comprising: - a linear polarizing filter (P) having its transmission maximum for a linear polarization oriented in a selectable first direction, - a phase plate (PP), - a second guest-host liquid crystal layer (3a) which, in addition to cholesteric liquid crystals, contains a proportion of one or more dichroic dyes, - means for selectively generating at least one first electric field (EF1) or one second electric field (EF2), - wherein either the first electric field (EF1) or the second electric field (EF2) selectively acts on the guest-host liquid crystal layer (3a), and the latter changes its state between a first, scattering and non-polarizing state and a second, non-scattering and polarizing state, or vice versa, depending thereon.- so that the transmission properties of the second switchable light filter (5a) differ between a first operating mode B1, in which the second guest-host liquid crystal layer (3a) is in the first state, and a second operating mode B2, in which the second guest-host liquid crystal layer (3a) is in the second state, - in the first operating mode B1, the second guest-host liquid crystal layer (3a) at least partially scatters light which enters the second switchable light filter (5a) on the side facing away from an observer - who is located on the side of the second switchable light filter (5a) on which the linear polarizing filter (P) is arranged - and does not substantially influence its polarization, wherein the light then passes through the phase plate (PP) and is finally linearly polarized when passing through the linear polarizing filter (P),before it leaves the second switchable light filter (5a), so that in the first operating mode B1 no restriction of the propagation directions of the light incident on the second switchable light filter (5a) is created, and—in the second operating mode B2, the second guest-host liquid crystal layer (3a) does not significantly scatter light incident on the side facing away from the viewer into the second switchable light filter (5a) and polarizes it linearly along a second direction, whereby the polarization of the light is subsequently influenced by the phase plate (PP) depending on its propagation directions, whereby light propagating along a selectable second preferred direction and exhibiting the aforementioned first direction of linear polarization can finally pass through the linear polarization filter (P), while light,which propagates at an angle greater than 30° to the perpendicular center onto the second guest-host liquid crystal layer (3a), is absorbed by the linear polarization filter (P) by at least 80%, so that in the second operating mode B2 a restriction of the propagation directions of the light incident on the second switchable light filter (5a) is produced. Second switchable light filter (5a) according to claim 6, characterized in that the second guest-host liquid crystal layer (3a) and / or the means for selectively generating at least one first electric field (EF1) or one second electric field (EF2) is / are divided into several separately switchable segments, so that local switching between the respective possible operating modes B1 and B2 is enabled. Screen comprising a first switchable light filter (5) according to one of claims 1 to 5 or a second switchable light filter (5a) according to claim 6 or 7 and an image display device (9) located behind it in the viewing direction of a viewer - who is located on the side of the first switchable light filter (5) or the second switchable light filter (5a) on which the linear polarizing filter (P) is arranged - characterized in that the luminance curve of the light emitted or transmitted by the image display device (9) has a maximum along at least one cross-sectional plane. Screen according to claim 8, characterized in that a light guide is further arranged in the viewing direction in front of the first switchable light filter (5) or the second switchable light filter (5a), which in operating mode B2 is supplied with light over at least one narrow side, wherein the light guide couples out light over at least one of its large surfaces and the maximum light output is directed in one or more directions which differ by at least 10° from the first or the second preferred direction. Lighting device for a screen which can be operated in at least a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode in which light is emitted into a viewing angle area that is restricted for a viewer compared to the free viewing mode, comprising: - an area-like extended backlight (8) which emits light and - a first switchable light filter (5) arranged in the viewing direction in front of the backlight (8) according to one of claims 1 to 5 or a second switchable light filter (5a) according to claim 6 or 7.

Images