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

Abstract

The invention relates to a first switchable light filter (5), comprising a linear polarization filter (P), a phase plate (PP), a quarter-wave plate (4), a cholesteric liquid crystal layer (3), and means for selectively generating at least a first electric field (EF1) or a second electric field (EF2). The first electric field (EF1) or the second electric field (EF2) at least temporarily acts on the cholesteric liquid crystal layer (3), and the state of the cholesteric liquid crystal layer changes between a focal-conical texture and a planar cholesteric texture or vice versa on the basis of the electric field such that the transmission properties of the 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. The invention further relates to a second switchable light filter (5a) and to lighting devices and screens which use the switchable light filters (5, 5a) described above.

Description

title

[0001] Switchable light filter as well as lighting device and screen with such a switchable light filter Technical field of the invention

[0002] In recent years, great strides have been made in widening the viewing angle of LCDs. However, there are often situations where the very large viewing area 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 – 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 – in a private mode – if they want to keep the image information confidential.

[0003] 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 these while driving. Therefore, a screen that can switch between the corresponding display modes is required.

[0004] 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

[0005] 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.

[0006] 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.

[0007] US 2012 / 0235891 A1 describes a very complex backlight in a screen. According to Figs. 1 and 15, not only multiple light guides are used, but also other complex optical elements such as microlens elements 40 and prism structures 50, which transform 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 being converted at great expense into light with a wide illumination angle. This complex conversion, as already mentioned above, significantly reduces brightness.

[0008] US 2013 / 0308185 A1 describes a special light guide with steps that emits light over a large area 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, a screen that can be switched between free and restricted view modes can be created. A disadvantage here is, among other things, 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. In addition, even in restricted view mode, residual light is still visible from blocked viewing angles.

[0009] WO 2015 / 121398 A1 of 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. The scattering particles selected therein from However, polymer-based light guides generally have the disadvantage that light is coupled out of both large surfaces, resulting in approximately half of the useful light being emitted in the wrong direction, namely toward the backlight, where it cannot be adequately recycled due to their design. Furthermore, the polymer-based scattering particles distributed throughout the volume of the light guide can, under certain circumstances, especially at higher concentrations, lead to scattering effects that reduce the privacy effect in the protected operating mode.

[0010] 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 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.

[0011] DE 10 2021 128 224 B3 of the applicant describes a method and an arrangement, as well as the use of an arrangement, for selectively influencing the propagation directions of light. Two types of circular polarization are used. It does not disclose how the reflection behavior can be configured to be switchable for specific directions.

[0012] Furthermore, DE 10 2024 110 269 B3 of the applicant describes a switchable light filter, a lighting device, and a display. This document also does not disclose how the reflection behavior can be configured to be switchable for specific directions.

[0013] Finally, DE 29 05 970 A1 describes a liquid crystal display device addressing the topic of plate spacing in LC structures. This document does not describe how to design a switchable light filter in which the light transmission can be influenced depending on the angle, while also enabling switching between at least two operating states.

[0014] The aforementioned methods and arrangements generally share 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. Furthermore, many of the aforementioned approaches are not suitable for self-illuminating screens. Description of the invention

[0015] The object of the invention is therefore to describe light filters in which the transmission of light can be influenced depending on the angle, with the possibility of switching between at least two operating states. In particular, the transmission behavior and optionally also the reflection behavior should be switchable for specific directions. Furthermore, the solution should be usable for self-illuminating screens. Furthermore, a screen and lighting device with such light filters should be described.

[0016] This object is achieved according to the invention by a first switchable light filter, comprising (preferably viewed from the direction of an observer in this order) a linear polarization filter which has its transmission maximum for linear polarization oriented 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 a first electric field EF1 or a second electric field EF2, wherein optionally the first electric field EF1 or the second electric field EF2, or no field, acts on the cholesteric liquid crystal layer at least temporarily, and the latter changes its state between a focal conic texture (which is scattering) and a planar cholesteric texture (which ensures Bragg reflection) or vice versa, wherein these two aforementioned texture states are stable, ie the electric field EF1 or EF2 can be eliminated again after the 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, in that in the first operating mode B1, the cholesteric liquid crystal layer scatters light which is incident on the side facing away from the viewer into the first switchable light filter, is at least partially scattered upon passing through the cholesteric liquid crystal layer, and then penetrates the quarter-wave plate and the phase plate without any significant influence on its polarization and is finally linearly polarized upon passing through the linear polarization filter before it leaves the 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 that in the second operating mode B2 the cholesteric liquid crystal layer reflects light which is incident on the side facing away from the viewer into the switchable light filter with respect to a circular polarization and transmits it with respect to the complementary circular polarization, and then the said transmitted light is converted into linearly polarized light by the quarter-wave plate, while due to the phase plate the polarization of the light is influenced depending on its propagation direction, whereby light which is along a selectable first preferred direction, wherein the first preferred direction is arranged at a predetermined angle a to the bisector of the first linear polarization filter, wherein the angle a in a selectable first plane,which contains said central perpendicular, is measured, propagates, and has said first direction of linear polarization, can ultimately penetrate the linear polarization filter, while light propagating at at least an angle greater than 30° relative to the first preferred direction onto the cholesteric liquid crystal layer is absorbed by the linear polarization 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 privacy screen 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.)

[0017] To better understand their mode of operation, some information about cholesteric liquid crystals is provided below. Their focal-conical texture is scattering and stable. It is created when an electric field is applied along the surface. The planar-cholesteric texture, on the other hand, is reflective and stable for circular polarization. Combined 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 polarization-sensitive.

[0018] The homeotropic texture, on the other hand, is unstable and develops when an electric field is applied between the interfaces. When the electric field is removed, the homeotropic texture transforms into the planar cholesteric texture.

[0019] Thus, a layer of cholesteric liquid crystals can optionally polarize, scatter, or transmit light essentially unchanged by applying an electric field.

[0020] By way of example, for a cholesteric liquid crystal layer used within the scope of the invention, its design wavelength should be 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 slope per turn. An exemplary design wavelength A 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 because the optical path difference increases for inclined light incidence angles, which is partially compensated for by the aforementioned relationship. The following applies: 0.5 < p * n / A < 1 .0.

[0021] Furthermore, the phase plate is preferably designed such that its extraordinary refractive index axis is located in the zV plane or along the V-axis of the first switchable light filter, where the V-axis denotes the vertical axis of the first switchable light filter from the perspective of a standing or sitting observer, and the "z-axis" denotes a perpendicular direction to the respective first switchable light filter. For the phase plate, 300 nm < d * n < 2000 nm applies, where d is the thickness of the phase plate and An is the refractive index difference between the ordinary and extraordinary refractive index axes of the phase plate.

[0022] Furthermore, the first switchable light filter with the above-described structure ensures that light incident on the first switchable light filter from a viewer side in the first operating mode B1 is not reflected up to a predeterminable tolerance value, and in the second operating mode B2 is not reflected up to a predeterminable tolerance value when incident along the first preferred direction, and in the second operating mode B2 when incident at angles which deviate by 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 due to the superposition of residual light still present from lateral viewing angles with the light reflected at such angles.

[0023] The means for selectively generating at least a first electric field EF1 or a 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. In special embodiments, these means for selectively generating at least a first electric field EF1 or a second electric field EF2 can also be located entirely 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. In this case, particular advantage is taken of the fact that the two textures used for the invention are stable, ie the desired texture is retained even without an electric field.To switch between the two operating modes, for example, the means for selectively generating at least a first electric field EF1 or a second electric field EF2 are briefly positioned in front of the first switchable light filter until its texture has changed accordingly. The means are then arranged elsewhere. This procedure allows, among other things, the use of so-called in-cell touch or on-cell touch systems, which can be located behind the first or second switchable light filter within an image display device because the means do not permanently disrupt or block the electrodynamics of the touch units.

[0024] A further advantageous embodiment of the first switchable light filter provides that an angle-limiting layer (e.g. a louvre filter / privacy filter) ter) or optionally a lenticular grid) is present, which permanently has an 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 which has penetrated the angle-limiting layer and is incident on it, so that the brightness is increased for viewing angles which differ from the first preferred direction (preferably by at least 10°), whereas in the second operating mode B2 such scattering does not occur, whereby the angle-limiting effect of the angle-limiting layer is maintained. The angle-limiting layer preferably has its maximum transmission with a tolerance of at most 10°, particularly preferably at most 5°, along the first preferred direction.

[0025] Preferably, however, the angle-limiting layer comprises at least a first optical element, which in turn comprises • a multitude 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 the first preferred direction or vary around it, at least in a first state, with a tolerance of a maximum of 20°, • so that light which is incident on the first optical element with a direction of incidence and a state of polarization is transmitted or at least partially absorbed depending on its direction of incidence relative to the first optical element and its state of polarization, whereby light incident in the first preferred direction is transmitted to a degree of at least 50% regardless of its polarization, and whereby light incident at angles of greater than 30° to the preferred direction is transmitted to a degree of at least 50% if it is s-polarized and absorbed to a degree of at least 80% if it is p-polarized. For example, p-polarized light can be absorbed by the first optical element to a degree of over 90% at angles of over 45° to the first preferred direction.

[0026] 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 possible operating modes B1 and B2 is possible.

[0027] The object of the invention is also achieved by a second switchable light filter comprising (preferably viewed from the direction of a viewer in this order) a linear polarization 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 a first electric field EF1 or a second electric field EF2, wherein optionally the first electric field EF1 or the second electric field EF2, or no field, acts at least temporarily on the guest-host liquid crystal layer, and the latter changes its state depending thereon between a first, 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 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 that in the first operating mode B1 the second guest-host liquid crystal layer 3a transmits light,which is incident on the side facing away from the viewer into the second switchable light filter, at least partially scatters it and is not significantly influenced in its type of polarization, wherein the light subsequently penetrates the phase plate and is finally linearly polarized upon passing through the linear polarization 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 that in the second operating mode B2 the second guest-host liquid crystal layer does not significantly scatter light which is incident on the side facing away from the viewer into the second switchable light filter and linearly along a second, Direction (this second direction is preferably at an angle of approximately 90° to the first direction), wherein the polarization of the light is subsequently influenced by the phase plate as a function of its propagation directions, whereby light which propagates along a selectable second preferred direction (this can differ from the perpendicular bisector, but preferably corresponds to the perpendicular bisector) and has the said first direction of linear polarization can finally penetrate the linear polarization filter, while light which propagates at least an angle greater than 30° with respect to the perpendicular bisector on the second guest-host liquid crystal layer is absorbed by the linear polarization filter to the extent of 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 is produced.(This can create a privacy screen, for example, by using a second switchable light filter with a screen.)

[0028] Linearly polarized or non-polarized light can enter the second switchable light filter. The second switchable light filter operates similarly to the first switchable light filter, although it does not use circularly polarized light, thus eliminating the need for a quarter-wave plate in the setup.

[0029] 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 switchability between the respectively possible operating modes B1 and B2 is enabled.

[0030] Of course, suitable controls, e.g. control electronics, are available for a first or second switchable light filter 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.

[0031] Furthermore, the invention comprises a screen with a first or second switchable light filter and an image display device located behind it in the viewing direction, wherein preferably the luminance curve of the light emitted or transmitted by the image display device is plotted along at least one Cutting plane has a maximum. Such a design supports the 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 propagation directions or increases the light intensity in a preferred direction. For an OLED panel, it is also conceivable for the light to radiate essentially in one direction. For this purpose, the layer thicknesses and refractive indices of the individual layers in the OLED are optimized to achieve this radiation behavior.

[0032] Alternatively, it is possible, for example, to use crossed prism screens in the backlight of an LCD panel as an image display device to achieve the aforementioned effect. In the case of an LCD panel in conjunction with a first switchable light filter, it should be noted that the linearly polarized light emitted by the LCD panel should first be converted into circularly polarized light using a retarder (e.g., a quarter-wavelength 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.

[0033] In the case of a screen as described above, the first or second switchable light filter can be subsequently attached by a user and / or reversibly to the image display device.

[0034] In a screen as described above, a light guide can also be arranged in the viewing direction in front of the first switchable light filter or the second switchable light filter, which is exposed to light via at least one narrow side in operating mode B2, wherein the light guide couples out light over at least one of its large surfaces. The maximum light coupling of the 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.

[0035] Finally, the invention also comprises a lighting device for a screen which can be used in at least a first operating mode B1 for a free view mode and a second operating mode 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 viewing mode, comprising - a large area backlight that emits light and - a first switchable light filter arranged in front of the background light in the viewing direction or a second switchable light filter as described above.

[0036] A light filter, lighting device, or screen as described above is advantageously used in a mobile device, a motor vehicle, aircraft, or watercraft, in a payment terminal, or in an access system. Switching between the aforementioned operating modes is possible to protect sensitive data, i.e., to display it in a way that is perceptible to only one viewer, or alternatively, to display image content simultaneously for multiple viewers.

[0037] In principle, the performance of the invention is maintained if the parameters described above are varied within certain limits.

[0038] 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

[0039] The invention will be explained in more detail below using exemplary embodiments with reference to the attached drawings, which also disclose features essential to the invention. These exemplary embodiments are for illustrative purposes only and are not to be interpreted as limiting. For example, a description of an exemplary embodiment with a plurality of elements or components should not 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 of different exemplary embodiments may be combined with one another unless otherwise stated. Modifications and variations which are suitable for a The features described in connection with one of the embodiments may also be applicable to other 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: Fig.1 a schematic diagram of the construction of an exemplary first switchable light filter, Fig.2 a schematic diagram of the construction of an exemplary second switchable light filter, Fig.3 a schematic diagram of the construction of an exemplary lighting device with a switchable light filter, Fig.4 is a schematic diagram of the operation of an exemplary first switchable light filter in transmission in a first operating mode B1, Fig.5 is a schematic diagram of the operation of an exemplary first switchable light filter in transmission in a second operating mode B2, Fig.6 is a schematic diagram of the operation of an exemplary first switchable light filter in reflection in a first operating mode B1, as well as Fig.7 is a schematic diagram of the operation of an exemplary first switchable light filter in reflection in a second operating mode B2. Detailed description of the drawings

[0040] The drawings are not to scale and only represent schematic diagrams.

[0041] Fig.1 shows a schematic diagram of the construction of an exemplary first switchable light filter 5. This comprises (preferably seen from the direction of an observer in this order) a linear polarization filter P, which has its transmission maximum for linear polarization oriented in a first direction (the first direction is, for example, the horizontal direction from the viewpoint of a standing or sitting observer), a phase plate PP, a quarter-wave plate 4, a cholesteric liquid crystal layer 3, (not shown in the drawing) means for selectively generating at least a first electric field EF1 or a second electric field EF2, e.g. two ITO layers with an electronic control, wherein optionally the first electric field EF1 or the second electric field EF2, or no field, acts on the cholesteric liquid crystal layer 3 at least temporarily, and the cholesteric liquid crystal layer 3 changes its state between a focal-conic texture (which is scattering and stable) and a planar-cholesteric texture (which ensures Bragg reflection) or vice versa, wherein these two aforementioned texture states are stable, ie the electric field EF1 or EF2 can be omitted again after the texture change, so that the transmission properties of the first switchable light filter 5 can be switched between a first operating mode B1,in which the cholesteric liquid crystal layer 3 has a focal-conic 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, the cholesteric liquid crystal layer 3 scatters light which is incident on the side facing away from the viewer into the first switchable light filter 5, is at least partially scattered upon passing through the cholesteric liquid crystal layer 3, and then penetrates the quarter-wave plate 4 and the phase plate PP without any significant influence on its polarization and is finally linearly polarized upon passing through the linear polarization 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 produced,and in that in the second operating mode B2 the cholesteric liquid crystal layer 3 reflects light which is incident on the side facing away from the viewer into the switchable light filter 5 with respect to a circular polarization and transmits it with respect to the complementary circular polarization, and then the said transmitted light is converted into linearly polarized light by the quarter-wave plate 4, while due to the phase plate PP the polarization of the light is influenced as a function of its propagation direction, whereby light which is along a selectable first preferred direction, wherein the first preferred direction is arranged at a predetermined angle a to the bisector of the first linear polarization filter P and the angle a at a selectable, first plane, which contains said central perpendicular, and has said first direction of linear polarization, can ultimately penetrate the linear polarization filter P, while light that propagates at least an angle greater than 30° relative to the first preferred direction onto the cholesteric liquid crystal layer 3 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 first switchable light filter 5 is created. (Angles of incidence always refer to the propagation direction / angle of incidence in a vacuum.)

[0042] Thus, by using a first switchable light filter 5 together with an image display device, a switchable visual protection can be created.

[0043] To better understand their mode of operation, some information about cholesteric liquid crystals is provided below. Their focal-conical texture is scattering and stable. It is created when an electric field is applied along the surface. The planar-cholesteric texture, on the other hand, is reflective and stable for circular polarization. Combined 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 polarization-sensitive.

[0044] The homeotropic texture, on the other hand, is unstable and develops when an electric field is applied between the interfaces. When the electric field is removed, the homeotropic texture transforms into the planar-cholesteric texture.

[0045] Thus, a layer of cholesteric liquid crystals can optionally polarize, scatter, or transmit light essentially unchanged by applying an electric field.

[0046] By way of example, for a cholesteric liquid crystal layer 3 used within the scope of the invention, its design wavelength should be 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 slope per turn. An exemplary design wavelength A 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 because the optical path difference increases for inclined light incidence angles, which is partially compensated for by the aforementioned relationship. The following applies: 0.5 < p * n / A < 1 .0.

[0047] Furthermore, the phase plate PP is preferably designed such that its extraordinary refractive index axis is located in the zV plane or along the V-axis of the first switchable light filter 5, wherein the V-axis denotes the vertical axis of the first switchable light filter 5 from the perspective of a standing or sitting observer, and the "z-axis" denotes a perpendicular to the respective first switchable light filter 5. For the phase plate PP, 300 nm < d * n < 2000 nm applies, wherein d is the thickness of the phase plate PP and An is the refractive index difference between the ordinary and extraordinary refractive index axes of the phase plate PP.

[0048] Furthermore, the first switchable light filter 5 with the structure described above ensures that light which is incident on the first switchable light filter 5 from a viewer side is not reflected in the first operating mode B1 up to a predeterminable tolerance value, and in the second operating mode B2 when incident along the first preferred direction is not reflected up to a predeterminable tolerance value, and in the second operating mode B2 when incident at angles which deviate by 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 due to the superposition of residual light still present from lateral viewing angles with the light reflected at such angles.

[0049] The operation of a first switchable light filter 5 is explained in more detail below with reference to the drawings Fig. 4 to Fig. 7, whereby only representative light rays are shown here. In reality, a large number of light rays are present. Fig. 4 shows a schematic diagram of the operation of an exemplary first switchable light filter in transmission in a first operating mode B1, and Fig. 5 in a second operating mode B2. In addition, Fig. 6 shows a schematic diagram of the operation 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 right represents linearly parallel polarized light, and c. a circle with an arrow represents circularly polarized light.

[0050] First, it is assumed that unpolarized light, for example from an OLED panel, is incident on the first switchable light filter 5 from behind. The conditions according to Fig. 4 therefore represent the first operating mode B1, i.e. the cholesteric liquid crystal layer 3 has a focal-conical texture. The cholesteric liquid crystal layer 3 thus at least partially scatters light which is incident on 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 influence on its polarization and is finally linearly polarized upon passing through the linear polarization 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 produced. In Fig.This can be seen in Figure 4, since both the vertical and the oblique beam are continuous up to above the linear polarization filter P.

[0051] In contrast, as assumed for Fig. 5, in the second operating mode B2, the cholesteric liquid crystal layer 3 has a planar texture. Light incident on the side facing away from the viewer into the switchable light filter 5 is reflected with a circular polarization (see the two downward arrows) and transmitted with the complementary circular polarization. The transmitted light is then converted into linearly polarized light by the quarter-wave plate 4, while the phase plate PP influences the polarization of the light depending on its propagation direction. The thus altered polarization properties are shown in Fig. 5.Here, the polarization properties between the phase plate PP and the linear polarization filter P are "perpendicularly linearly polarized" in the perpendicular direction (the perpendicular direction corresponds to the selectable first preferred direction) and "parallel linearly polarized" in the oblique direction. Accordingly, the linear polarization filter P only allows perpendicular light rays to pass through. As a result, light propagating along the first preferred direction and exhibiting said first direction of linear polarization can ultimately penetrate the linear polarization filter P, while light impinging on the cholesteric liquid crystal layer 3 at an angle greater than 30° relative to the first preferred direction 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 first switchable light filter 5 is produced.

[0052] In addition, Fig. 6 shows a schematic diagram of the mode of operation of an exemplary first switchable light filter 5 in reflection in a first operating mode B1, i.e. the cholesteric liquid crystal layer 3 has a focal-conical texture. In this case, light which is incident on the first switchable light filter 5 from a viewer side is not reflected except for a predeterminable tolerance value, as shown in Fig. 6. While the light changes its polarization properties or is partially absorbed when passing through the first switchable light filter 5 depending on the direction (along the first preferred direction, i.e. here perpendicular or oblique to it), there is no dedicated or intended function for reflection at any point in the structure - apart from unavoidable reflections, e.g. in the context of refractive index transitions.

[0053] In other words, light coming from the viewer side along the first preferred direction is not affected in its polarization state by the phase plate PP. The linear polarization is converted into circular polarization by the quarter-wave plate 4, which passes unhindered through the cholesteric liquid crystal layer 3, which acts as a reflective polarization filter.

[0054] The situation is different in operating mode B2, which is shown as a schematic diagram 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 viewer side is not reflected when incident along the first preferred direction except for a predeterminable tolerance value, whereas light incident at angles deviating by at least 10° from the first preferred direction has its polarization influenced 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 due to 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 behind from the observer's perspective, which should ideally be completely absorbed at angles greater than 30° to the first preferred direction, which is generally not achieved for technical reasons.

[0055] In other words: Light originating from the viewer side and not from the first preferred direction is influenced in its polarization state by the phase plate PP. As a result, the light is subsequently partially converted by the quarter-wave plate 4 into left- and right-handed polarized light. One type of polarization is transmitted by the cholesteric liquid crystal layer 3 in its capacity as a reflective polarization filter, and the other type of polarization is reflected. The reflected portion passes through the structure again and reaches the side of the first switchable light filter 5 facing the viewer, penetrates the polarization filter P, and thereby intentionally reduces the contrast perceivable by the viewer, always from angles that deviate from the first preferred direction.

[0056] 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 possibly a lenticular screen) is also present, which permanently has an 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 the angle-limiting layer 1D and is incident on it, so that the brightness is increased for viewing angles which differ from the first preferred direction (preferably by at least 10°), whereas in the second operating mode B2 such scattering by the cholesteric liquid crystal layer 3 does not take place, whereby the angle-limiting effect of the angle-limiting layer 1D is maintained.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. In the drawings Fig. 1 and Fig. 2, such an angle-limiting layer 1D is shown as optional.

[0057] Preferably, the angle-limiting layer 1 D comprises at least one first optical element 1 , which in turn comprises • a multitude 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 the first preferred direction or vary around it, at least in a first state, with a tolerance of a maximum of 20°, • so that light which is incident on the first optical element 1 with a direction of incidence and a state of polarization is transmitted or at least partially absorbed depending on its direction of incidence relative to the first optical element 1 and its state of polarization, whereby light incident in the first preferred direction is transmitted to a degree of at least 50% regardless of its polarization, and whereby light incident at angles of greater than 30° to the preferred direction is transmitted to a degree of at least 50% if it is s-polarized and absorbed to a degree of at least 80% if it is p-polarized. For example, p-polarized light can be absorbed by the first optical element 1 to a degree of over 90% at angles of over 45° to the first preferred direction.

[0058] 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 a locally different switchability between the respectively possible operating modes B1 and B2 is enabled.

[0059] Fig. 2 shows a schematic diagram of the construction of an exemplary second switchable light filter 5a, comprising (preferably viewed from the direction of a viewer in this order) a linear polarization 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 contains, in addition to cholesteric liquid crystals, a proportion of one or more dichroic dyes, (not shown in the drawing) means for selectively generating at least a first electric field EF1 or a second electric field EF2, wherein optionally the first electric field EF1 or the second electric field EF2, or no field, acts on the guest-host liquid crystal layer 3a at least temporarily, 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, 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 enters the second switchable light filter 5a on the side facing away from the observer and is not significantly influenced in its polarization type, wherein the light subsequently penetrates the phase plate PP and is finally linearly polarized upon passing through the linear polarization filter P before it leaves the switchable light filter 5,such 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 that in the second operating mode B2, the second guest-host liquid crystal layer 3a does not significantly scatter light which is 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 approximately 90° to the first direction), wherein the polarization of the light is subsequently influenced as a function of its propagation directions due to the phase plate PP, whereby light which propagates along a selectable second preferred direction (this can differ from the perpendicular bisector, but preferably corresponds to the perpendicular bisector) and has the said first direction of linear polarization,Finally, the linear polarization filter P can penetrate, while light propagating at an angle greater than 30° relative to the perpendicular bisector of 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 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.)

[0060] 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 a second electric field Field EF2 can be divided into several separately switchable segments so that local switching between the possible operating modes B1 and B2 is possible.

[0061] 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 are disclosed, wherein the luminance curve of the light emitted or transmitted by the image display device 9 preferably has a maximum along at least one sectional plane. Such a configuration supports the privacy-protection 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 microstructure restricts the propagation directions or increases the light intensity in a preferred direction. For an OLED panel, it is also conceivable for the same light to emit substantially in one direction. For this purpose, the layer thicknesses and refractive indices of the individual layers in the OLED are optimized to achieve such emission behavior.

[0062] Furthermore, it is possible, for example, in the case of an LCD panel as the image display device 9, to use crossed prism louvre films in its backlight to achieve the above-described effect. In the case of an LCD panel, it should be noted that 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., quarter-wavelength filter) 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.

[0063] To explain the previously described screens, Fig. 1 and Fig. 2 can be used with schematic diagrams of the construction of the first and second switchable light filters 5, 5a, respectively. Possible positions for the image display device 9 are shown there. The explanations given above apply analogously here and will therefore not be repeated.

[0064] 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 used in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode. Viewing mode, in which light is emitted into a viewing angle range that is restricted for a viewer compared to the free viewing mode, can be operated, includes - a surface-like backlight 8 which emits light and - a first switchable light filter 5 or a second switchable light filter 5a arranged in front of the background lighting 8 in the viewing direction, as described above.

[0065] The invention solves the stated problem: A light filter has been described in which the transmission of light can be influenced depending on the angle – optionally perpendicular to a seated or standing observer – while switching between at least two operating states. In particular, the transmission behavior and optionally also the reflection behavior can be switched for specific directions. The presented solution is also suitable for self-luminous screens, such as OLED panels. Furthermore, a screen and lighting device with such light filters have been described.

[0066] The invention described above, in conjunction with an image display device, can be advantageously used wherever confidential data is displayed and / or entered, such as when entering a PIN or displaying data at ATMs or payment terminals, entering a password, or reading emails on mobile devices. As described above, the invention can also be used in cars to selectively withhold distracting image content from the driver or passenger.

Claims

Patent claims 1 . A first switchable light filter (5) comprising a linear polarization filter (P) having its transmission maximum for linear polarization oriented 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 the first electric field (EF1) or the second electric field (EF2) selectively acts on the cholesteric liquid crystal layer (3) at least temporarily, and the latter changes its state between a focal-conic texture and a planar-cholesteric texture or vice versa, as a function thereof, so that the transmission properties of the switchable light filter (5) differ between a first operating mode B1, in which the cholesteric liquid crystal layer (3) has a focal-conic 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, the cholesteric liquid crystal layer (3) transmits light which is incident on the side facing away from the viewer into the switchable light filter (5),is at least partially scattered when passing through the cholesteric liquid crystal layer (3), and then penetrates the quarter-wave plate (4) and the phase plate (PP) without any significant influence on its polarization and is finally linearly polarized when passing through the linear polarization 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 that in the second operating mode B2 the cholesteric liquid crystal layer (3) reflects light which is incident on the side facing away from the viewer into the switchable light filter (5) with respect to a circular polarization and transmits it with respect to the complementary circular polarization, and then 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, 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, and having said first direction of linear polarization, can finally penetrate the linear polarization filter (P), while light propagating at at least one angle greater than 30° relative to the first preferred direction onto the cholesteric liquid crystal layer (3) is absorbed by the linear polarization filter (P) to 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., 2. First switchable light filter (5) according to claim 1, characterized in that light which is incident on the first switchable light filter (5) from an observer side is not reflected in the first operating mode B1 up to a predeterminable tolerance value, and in the second operating mode B2 is not reflected when incident along the first preferred direction up to a predeterminable tolerance value, and in the second operating mode B2 when incident at angles which deviate by 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).

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 permanently has an 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 the angle-limiting layer (1D) and is incident on it, so that the brightness for viewing angles which are different from the first preferred direction is increased while in the second operating mode B2 such scattering does not occur, whereby the angle-limiting effect of the angle-limiting layer (1 D) is maintained.

4. First switchable light filter (5) according to claim 3, characterized in that the angle-limiting layer (1 D) comprises at least one first optical element (1 ), which in turn comprises • a multitude of light-absorbing transition dipole moments arranged in a layer at least 0.2 micrometers thick, • wherein the majority of the transition dipole moments are aligned parallel to the first preferred direction or vary around it, at least in a first state, with a tolerance of a maximum of 20°, • so that light which is incident into the first optical element (1) with a direction of incidence and a state of polarization is transmitted or at least partially absorbed depending on its direction of incidence relative to the first optical element (1) and its state of polarization, whereby light incident in the first preferred direction is transmitted to at least 50% regardless of its polarization, and whereby light incident at angles of greater than 30° to the preferred direction is transmitted to at least 50% if it is s-polarized and is absorbed to at least 80% if it is p-polarized.

5. 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 a plurality of separately switchable segments, so that local switchability between the respectively possible operating modes B1 and B2 is enabled.

6. Second switchable light filter (5a), comprising a linear polarization filter (P) having 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, Means for selectively generating at least a first electric field (EF1) or a second electric field (EF2), wherein selectively the first electric field (EF1) or the second electric field (EF2) acts at least temporarily 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, as a function thereof, 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) transmits light,which enters the second switchable light filter (5a) on the side facing away from the viewer, at least partially scatters it and is not significantly influenced in its polarization type, wherein the light subsequently penetrates the phase plate (PP) and is finally linearly polarized upon passing through the linear polarization filter (P) before it leaves 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 switchable light filter (5) is produced, and in that in the second operating mode B2 the second guest-host liquid crystal layer (3a) does not significantly scatter light which enters the switchable light filter (5) on the side facing away from the viewer and linearly polarizes it along a second direction, wherein subsequently due to the phase plate (PP) the polarization of the light is influenced depending on its propagation directions, whereby light,which propagates along a selectable second preferred direction and has said first direction of linear polarization, can finally penetrate the linear polarization filter (P), while light propagating at at least an angle greater than 30° relative to the mid-perpendicular to the second guest-host liquid crystal layer (3a) is absorbed by the linear polarization filter (P) to 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 generated.

7. 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 a plurality of separately switchable segments, so that a local switchability between the respectively possible operating modes B1 and B2 is enabled.

8. 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, 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 sectional plane.

9. Screen according to claim 8, characterized in that furthermore in the viewing direction in front of the first switchable light filter (5) or the second switchable light filter (5a) a light guide is arranged, which in the operating mode B2 is exposed to light via at least one narrow side, wherein the light guide couples out light over at least one of its large surfaces and the maximum light coupling is directed in one or more directions which differ by at least 10° from the first or the second preferred direction.

10. Lighting device for a screen which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode 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 surface-like backlight (8) which emits light and - a first switchable light filter (5) according to one of claims 1 to 5 arranged in front of the background lighting (8) in the viewing direction, or a second switchable light filter (5a) according to claim 6 or 7.

Images