Switchable optical filter, lighting device and screen

Abstract

The invention relates to a switchable optical filter (5) comprising a first optical element (1) comprising on its side a plurality of light absorption transition dipole moments such that light incident into the first optical element (1) is transmitted or at least partially absorbed depending on its direction of incidence with respect to the first optical element (1) and its polarization state; means for generating at least two different electric fields; a liquid crystal layer (3), on which a respective electric field acts, such that a transmission characteristic of the switchable optical filter (5) can be changed by switching the applied electric field between a first operating mode B1 and a second operating mode B2, the respective relative transmittances in both operating modes B1 and B2 are each described by a transmittance TB1 ([beta]) for operating mode B1 or a transmittance TB2 ([beta]) for operating mode B2, which are each normalized to values such that TB1 ([alpha]) = 1 and TB2 ([alpha]) = 1 are suitable for the transmittances. According to the operating mode, the light filter (5) influences how the light is transmitted according to the angle of incidence and the polarization state. The invention further discloses a lighting device and a screen using the switchable light filter (5).

Description

TECHNICAL FIELD

[0001] In recent years, great progress has been made in the case of LCDs for widening the viewing angle. However, there are often situations in which such a very large viewing area of the screen can be disadvantageous. Information is also increasingly becoming available on mobile devices such as laptops and tablet PCs, such as bank data or other personal data and sensitive data. In response thereto, it is necessary to control who can see this sensitive data; one must be able to choose between a wide viewing angle (public mode) in order to share information on their screen with others, for example when looking at holiday photos or also for advertising purposes. On the other hand, if one wants to handle image information confidentially, one needs a small viewing angle (i.e. a private mode).

[0002] Similar problems arise in the manufacture of vehicles: there, the driver must not be distracted by image content such as, for example, digital entertainment programs in the case of a running engine, while fellow passengers want to consume these image contents even during driving. Therefore, a screen is required which can be switched between the corresponding representation modes.

[0003] Microflake-based additional films have already been used for mobile displays in order to achieve their visual data protection. However, these films are not switchable or not convertible; the films always have to be put in place by hand and then removed again. The films also have to be transported separately from the display when they are not in use. One significant disadvantage of using such flake films is furthermore associated with a light loss occurring. BACKGROUND

[0004] US 6,765,550 B2 describes such a privacy protection by means of microflakes. The greatest disadvantage is the mechanical removal or mechanical addition of the filter and the light loss in the protection mode.

[0005] The use of a film is described in US 5,993,940 A, which has small strip-shaped prisms arranged uniformly on its surface in order to achieve a privacy mode, i.e. a restricted field of view mode with a small viewing angle range. The development and manufacture are technically quite costly.

[0006] In WO 2012 / 033583 A1, a transition between a free field of view and a restricted field of view is produced by means of manipulating the liquid crystals between so-called "chromonischen" layers. A light loss occurs in this case and the technical effort is quite high.

[0007] US 2012 / 0235891 A1 describes a very costly backlight in a screen - a background illumination device. There, according to Figures 1 and 15, not only a plurality of light guides are used, but also other complex optical elements, such as, for example, micro-lens elements 40 and prism structures 50, which transform the light from the rear illumination into front illumination on the way. This can be implemented in an expensive and technically costly manner and is likewise associated with light losses. According to the variant according to Figure 17 in US 2012 / 0235891 A1, two light sources 4R and 18 produce light with a narrow illumination angle, wherein the light from the rear light source 18 is first costiy transformed into light with a large illumination angle. As has already been mentioned further above, such a costly transformation is a great reduction in brightness.

[0008] In US 2013 / 0308185 A1 a special stepped light guide is described, which radiates light in different directions over a large area depending on from which direction the light guide is illuminated from the narrow side. Interactively with a transmissive image reproduction device, for example an LCD display, a screen can be produced which is switchable between a free field of view mode and a restricted field of view mode. It is particularly disadvantageous in this case that the restricted field of view effect can only be produced for left / right or for up / down, but not simultaneously for left / right / up / down, as is necessary, for example, for specific payment processes. In addition, it is additionally the case that even in the restricted field of view mode residual light is still visible from obstructed peep angles.

[0009] The applicant's WO 2015 / 121398 A1 describes a screen with two operating modes, wherein scattering particles are present in the volume of the corresponding light guide for the purpose of switching the operating mode. However, the scattering particles selected there from the polymerization product generally have the disadvantage that light is outcoupled from both large areas, whereby approximately half of the useful light can be radiated in the wrong direction, i.e. towards the background illumination device, and cannot be recycled there to a sufficient extent due to the structure. Furthermore, scattering particles from the polymerization product distributed in the volume of the light guide can in some cases, especially at higher concentrations, lead to scattering effects which reduce the privacy protection effect in the protected operating mode.

[0010] The concept of the "Electrically-Induced Birefringence (EDB)" technology is based on the idea of "filtering" all light beams that do not exit from the imaging layer at a certain angle of radiation using a switchable liquid crystal of an additionally applied LC panel. The disadvantages of this technology are the high additional energy and cost expenditure and the difficult-to-change + / - 40° sweet spot, i.e. the optimal gaze position. The absorption of the LC structure is also insufficient, because for viewing angles greater than the sweet spot the light intensity again increases, so that for viewing angles greater than + / - 40° the light intensity is up to 3% of the maximum light intensity.

[0011] A disadvantage common to the above-mentioned methods and devices is generally that the methods and devices significantly reduce the brightness of the underlying screen, and / or require costly and expensive optical elements for mode conversion, and / or reduce the resolution in the freely viewable public mode, and / or have visual artifacts in the case of very high-resolution displays. SUMMARY

[0012] It is therefore the task of the present application to describe a filter with optical elements, wherein light incident into the optical elements is transmitted or partially or completely absorbed depending on its direction of incidence and its polarization properties (not primarily, but depending on its position). The filter using the optical elements should influence the transmission of light in an angle-dependent manner (optionally vertically with respect to a sitting or standing observer), wherein a conversion between at least two operating states is possible. In particular here, the transmission behavior should be switchable for certain directions.

[0013] According to the application, the task is solved in a first design variant by a filter, which comprises: - a first optical element, which comprises on its side: • a plurality of light-absorbing transition dipole moments, which are arranged in a layer of at least 0.2 micrometers in thickness, wherein the transition dipole moments that absorb are preferably composed of dichroic dyes, wherein the dye mass density is typically greater than 1% or greater than 10%, • wherein the majority of the transition dipole moments is oriented parallel to or fluctuates around a first preferred direction selectable for the first optical element with a tolerance of maximum 20° (other tolerance values, e.g. 5° or 10° are also possible) which is arranged in a pre-defined angle a to the median line of the first optical element, e.g. with a = 0°, a = + / - 2° or a value (a) > 2°, wherein the angle a is measured in a selectable first plane containing the median line and the angle a is preferably measured parallel to an edge, e.g. the upper edge, of the first optical element, • such that light incident into the first optical element in 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 and its polarization state, - means for selectively generating a first electric field EF1 or a second electric field EF2, - a liquid crystal layer arranged along the viewing direction before or after the first optical element, the first electric field EF1 or the second electric field EF2 acting on the liquid crystal layer and the liquid crystal layer thereby influencing the polarization state of light penetrating the liquid crystal layer, - if the liquid crystal layer is arranged before the first optical element along the viewing direction, a first linear polarization filter before the liquid crystal layer along the viewing direction, wherein the preferred polarization transmission direction of the first polarization filter is preferably oriented parallel to an edge, preferably the lower edge, of the first optical element, - such that the transmission characteristic of the switchable filter differs between a first operating mode B1 (wherein the first electric field EF1 (e.g. with a field strength of 0 V / pm) is applied) and a second operating mode B2 (wherein the second electric field EF2 (e.g. with a field strength unequal to 0 V / pm, approximately in the order of 1 V / pm, e.g. as a square wave with 10 kHz) is applied), wherein in both operating modes B1 and B2, at least one point on the first optical element (preferably at multiple points, particularly advantageously over at least half of the area of the optical element), the respective relative transmission is described by a first transmission T B1 (β) for the first operating mode B1 or a second transmission T B2 (β) for the second operating mode B2, which is dependent on the angle β and is normalized such that for the value of the transmission for a pre-defined angle a, T B1 (α) = 1 and T B2(α) = 1 applies, - in such a way that, if light • is incident into the optical element 1 with an angle β, wherein or then, in a first operating mode B1 with application of a first electric field EF1, an s-polarized component of said light is transmitted with at least a first normalized transmittance value T B1 (β) ≥ 0.25 (preferably T B1 (β) ≥ 0.3) and • is incident into the optical element 1 with an angle β, wherein or then, in a second operating mode B2 with application of a second electric field EF2, a p-polarized component of said light is transmitted with at most a second normalized transmittance value T B2 (β) ≤ 0.2 (preferably T B2 (β) ≤ 0.1, particularly preferably T B2 (β) ≤ 0.05 and advantageously also for all angles or .

[0014] Here, s-polarized light denotes light whose electric field is oriented perpendicular to the plane of incidence onto the medium, while p-polarized light denotes light whose electric field is oriented parallel to the plane of incidence. The plane of incidence is spanned by the normal to the interface of the medium and the direction of incidence. Here, the essential means-effect relationship consists in the fact that by switching between a first operating mode B1, in which a first electric field EF1 (for example with a field strength of 0 V / μm) is applied, and a second operating mode B2, in which a second electric field EF2 (for example with a field strength not equal to 0 V / μm, approximately in the order of magnitude of 1 V / μm, for example as a square wave with 10 kHz) is applied, only in the operating mode B2 s-polarized light incident onto the liquid crystal layer is converted substantially into p-polarized light, which subsequently impinges onto the first optical element, and vice versa. The above-mentioned transmittance changes for T B1 (β) or T B2 (β) result in the above-mentioned operating modes in interaction with at least a 0.2-μm-thick layer of the first optical element with a transition dipole moment which absorbs. If the transition dipole moment which absorbs is furthermore composed of a dichroic dye, wherein the dye mass density is typically greater than 1 %, this is added in an auxiliary manner.

[0015] Further operating modes B3, B4, etc. with electric fields EF3, EF4, etc. different from the electric fields EF1 and EF2 can also be explicitly provided. Furthermore, the operating modes B1, B2, etc. can also differ locally on the switchable filter.

[0016] With regard to the normalization of the transmittance T B1(α) = 1 and T B2 (α) = 1, it should be noted that the angles β and α are measured in the same plane as mentioned above. It is also possible in principle that T B1 (α) > 1 and / or T B2 (α) > 1 applies for angles β ≠ α. In many cases, T B1 (α) < 1 and / or T B2 (α) < 1 applies again for angles β ≠ α.

[0017] In the case of a changeable transition dipole moment, for example by means of a so-called guest-host liquid crystal cell, such a guest-host liquid crystal cell can directly correspond to the above-mentioned liquid crystal layer, but this is not necessarily the case.

[0018] In a preferred design, the light penetrating the liquid crystal layer is transmitted substantially in an unchanged manner upon application of a first electric field EF1, while upon application of a second electric field EF2 the incident light is circularly or elliptically polarized or the polarization of the light is rotated by 90°. By substantially, it is meant in this respect that at the interface the orientation of the liquid crystal molecules is determined by the electric field and the surface-induced forces, so that the liquid crystal molecules are not ideally oriented, which leads to an undesired small change in the polarization.

[0019] It applies for the design with TN liquid crystals that at a defined large area of the liquid crystal layer the orientation of the liquid crystal molecules differs by 90° in a directed manner. Such an orientation is supported by PMI or PVA and additionally a mechanical or optical treatment of the surface. Furthermore, it generally applies for TN liquid crystal layers that when switching between the electric fields EF1 and EF2 the majority of the liquid crystals in the liquid crystal layer are rotated out of the plane by 75 to 90 degrees. In the case of IPS and FFS liquid crystal layers, the rotation of the LC molecules is less than 45°, typically about 20° to 30°.

[0020] If the liquid crystal layer is arranged behind the first optical element in the viewing direction, linearly or elliptically polarized light is incident, with a ratio of the major semi-axis to the minor semi-axis of at least 4:1 (preferably at least 5:1 or more). This can be achieved, for example, by a linear polarization filter in the light path, or in the case of circularly polarized light incidence by a λ / 4 layer.

[0021] It is advantageously applicable that the first optical element (and each other such optical element, if present) and / or the liquid crystal layer is divided into a plurality of individually switchable segments in order to enable a local switching between the respectively possible operating states.

[0022] In another design variant, the switchable filter comprises at least two first optical elements, between which optionally a retarder is arranged. Furthermore, the at least two first optical elements can, if necessary, have mutually different thicknesses of the layers each comprising a plurality of light-absorbing transition dipole moments, but this is not a necessity.

[0023] It can be appropriate in a particular design variant of the application that there is at least one first specific angle β1 for which the transmissivity of the p-polarized component of the light is not the same as the transmissivity of the s-polarized component of the light. This condition preferably applies to the entire angular range of the angle β1 and particularly preferably even to all angles α ≠ β.

[0024] It can be appropriate in yet other design variants of the application that in both operating modes B1 and B2 there is at least one second specific angle β2 for which the transmissivity of the s-polarized component of the light is greater than the transmissivity of the p-polarized component of the light. This condition preferably applies to the entire angular range of the angle β2 and particularly preferably even to all angles α ≠ β.

[0025] The application also comprises an illumination device for a screen in the first design variant, which screen can be operated in at least one first operating mode B1 for a free-view mode and a second operating mode B2 for a restricted-view mode in which light is radiated into a viewing angle range that is restricted for an observer compared to the free-view mode, which illumination device comprises - a planarly extended background illumination device that radiates light and is optionally constructed in a direct-light-emitting manner, for example with a LED matrix, and - a switchable filter according to the application arranged in front of the background illumination device in the viewing direction, as described above.

[0026] In addition to this, the application likewise comprises a screen in the first design variant, which screen can be operated in at least one first operating mode B1 for a free-view mode and a second operating mode B2 for a restricted-view mode in which light is radiated into a viewing angle range that is restricted for an observer compared to the free-view mode, which screen comprises - an illumination device as described above, which illumination device in particular comprises a switchable filter having the structural features and properties as described above, - and, if no first linear polarization filter is arranged in the switchable filter of the illumination device, a second linear polarization filter arranged in front of the background illumination device in the viewing direction, whereby the light that emerges from the background illumination device and penetrates the second linear polarization filter is restricted in its propagation direction, and - a transmissive image reproduction device, which is arranged in front of the switchable filter along the viewing direction, - wherein a second electric field EF2 is applied in the second operating mode B2, and wherein a first electric field EF1 is applied in the first operating mode B1.

[0027] It is preferred here that the first or second linear polarizing filter P is arranged in or is part of the transmissive image reproduction device.

[0028] Furthermore, the application comprises a screen in a second design, which can be operated in at least one first operating mode B1 for a free field of view mode and in a second operating mode B2 for a restricted field of view mode, in which the light is radiated into a viewing angle range which is restricted for the observer compared to the free field of view mode, which comprises - an image reproduction device, wherein here in principle any type of image reproduction device can be considered, such as LC panels, OLED, microLED and other image reproduction devices, - a switchable filter according to the application with the structural features and properties as further described previously, which is arranged in front of the image reproduction device along the viewing direction, - wherein a second electric field EF2 is applied in the second operating mode B2, and wherein a first electric field EF1 is applied in the first operating mode B1.

[0029] It can optionally be applicable in this case that the switchable filter is retrofitted by the user and / or reversibly installed at the image reproduction device. In this case, the filter can be marketed as a so-called "aftermarket product".

[0030] Furthermore, the application comprises a screen in a third and fourth design, which can be operated in at least one first operating mode B1 for a free field of view mode and in a second operating mode B2 for a restricted field of view mode, in which the light is radiated into a viewing angle range which is restricted for the observer compared to the free field of view mode, which comprises - a transmissive image reproduction device, preferably an LC panel, - a background illumination device which is arranged downstream of the transmissive image reproduction device along the viewing direction, wherein the background illumination device has a luminance density profile, the peak luminance of which is radiated in a direction which forms an angle of at least 3°, advantageously even 5° or 8° with the first preferred direction (the background illumination device can have a permanent or variable luminance curve), - a switchable filter according to the application with the structural features and properties as further described previously, which is arranged in front of (third design) or behind (fourth design) the image reproduction device along the viewing direction, - wherein a second electric field EF2 is applied in the second operating mode B2, and wherein a first electric field EF1 is applied in the first operating mode B1.

[0031] If a background lighting device with such characteristics still includes a machined film and / or a partially mirrored or asymmetrical prism grating above a light conductor having a diffuser, BEF, or DBEF, then, for example, the aforementioned background lighting device can be produced.

[0032] For screens of the third or fourth design scheme—specifically for at least one partial surface—it continues to be advantageously applicable that, in the second operating mode B2, for at least (Preferably even for) or , until the maximum Regarding the angular range, for all angles β included within the said angular range, the following applies (with the first transmittance T) B1 (β) The second transmittance T is exactly the same as that related to the angle. B2 (β), the angle-dependent light density L of the background lighting device at angle β V (β) and the angle-dependent transmittance T of the image reproduction device at angle β. BW The product of (β) deviates from the value of the product for the angle α=β by at most + / -10%.

[0033] By following this product—within the aforementioned tolerance range—perceived uniformity is improved for the observer because, for example, when viewing the screen from the side, the corrected optical density distribution based on the background illumination device is compensated again when the observer is centered and looking directly at the screen, in cases of strong transmittance reduction of optical elements or image reproduction devices, such as when viewing the screen from the side.

[0034] Alternatively, for screens of the third or fourth design scheme—specifically for at least one portion of the surface—it may be applicable, in the second operating mode B2, for at least (Preferably even for) or until the maximum The second transmittance T is applicable to the angular range of all angles β included within the angular range. B2 (β) and the light density L of the background lighting device at angle β V The product of (β) deviates from the value of the product for angle α=β by at most + / -10%.

[0035] Here the above-mentioned compensation effect based on the confusing light density distribution of the backlighting device also applies, however, the transmission behavior of the image reproduction device is ignored.

[0036] Finally, the application also comprises a screen in a fifth design variant, wherein the screen can be operated in at least a first operating mode B1 for a free field of view mode and a second operating mode B2 for a restricted field of view mode, in which the light is radiated into a viewing angle range which is restricted for the observer compared to the free field of view mode, which screen comprises - a transmissive image reproduction device, preferably an LC panel, - a background lighting device arranged downstream of the transmissive image reproduction device in the viewing direction, wherein the background lighting device has an asymmetric light density distribution, wherein the asymmetry preferably exists with respect to a direction which is horizontal from the perspective of the observer, - a switchable filter according to the application with the structural features and properties as described further above before or after the image reproduction device in the viewing direction, - wherein a second electric field EF2 is applied in the second operating mode B2 and wherein a first electric field EF1 is applied in the first operating mode B1.

[0037] Here, the background lighting device is designed such that it has essentially no light density distribution which is symmetrical (for example around a center line which runs - from the perspective of the observer - vertically, perpendicular to the imaginary connecting line between the eyes), but instead embodies an asymmetric light density distribution (for example on the horizontal line). In other words: the background lighting device 8a has an asymmetric light density distribution, wherein the asymmetry preferably exists with respect to a direction which is horizontal from the perspective of the observer. Such a design variant is possible, for example, in the case of the use of deterministic light output coupling light guide bodies which shift the peak luminance and / or turned films.

[0038] This variant is advantageous for use cases in a vehicle, because then, in particular by designing the background lighting device, it is possible to significantly reduce the light which would be radiated in the direction of the co-passenger window from approximately a horizontal angle of 25 degrees or more (with respect to the median line), for example to less than 10% of the peak luminance - preferably less than 2.5% - while there is desirably a high light density in the direction of the driver. In this way, disturbing reflections in the co-passenger window or, if necessary, on the outer rear-view mirror closest to the co-passenger are reduced or even avoided. Nonetheless, based on the filter arranged before the image reproduction device, the screen can be selectively operated such that either only the co-passenger can see the image content (operating mode B2), for example for moving images, or both the driver and the co-passenger can see the image content (operating mode B1), for example for navigation map material.

[0039] For some of the aforementioned screens of the first to fourth design, it is advantageous if a second optical element is arranged in front of the transmissive image reproduction device along the viewing direction, which second optical element comprises: • a plurality of light-absorbing transition dipole moments; in this case, the dye mass density is greater than 1% or even greater than 10%.

[0040] • wherein the majority of the transition dipole moments are oriented at least in the first state parallel to or fluctuating around a second preferred direction selectable for the second optical element with a tolerance of a maximum of 20° (alternatively 10°), wherein the second preferred direction is arranged at an angle a1 to the median of the second optical element (in this case, for example, a1 = 0°, a1 = + / - 2° or a value (a1) > 2° can apply), wherein the angle a1 is measured in a selectable second plane containing the median; the second plane is preferably perpendicular to the first plane of the first optical element, • such that light incident into the second optical element is transmitted or at least partially absorbed depending on its direction of incidence relative to the second optical element and its polarization state.

[0041] The last-mentioned design advantageously leads to a reduced transmission in the vertical direction and thus can reduce or completely avoid reflections of the image content represented on the screen on the windshield in the vehicle.

[0042] Furthermore, the invention comprises an illumination device for a screen in a second design, which illumination device can be operated in at least one first operating mode B1 for a free field of view mode and a second operating mode B2 for a restricted field of view mode, in which restricted field of view mode light is radiated into an angular range which is restricted compared to the free field of view mode, which illumination device comprises - a planarly extending background illumination device, which background illumination device radiates light into a restricted angular range and is optionally constructed in a direct light-emitting manner, and - a plate-shaped light guide placed in front of the background illumination device along the viewing direction, which light guide has output coupling elements on at least one of its large areas and / or within its volume, - a light-emitting device arranged laterally at at least one narrow side of the light guide, and - optionally a linear polarization filter, - a switchable filter according to the invention with the structural features and properties as described further above arranged in front of this background illumination device along the viewing direction (this also includes a position in front of the screen with which the illumination device is used), - wherein in the second operating mode B2 the background illumination device is switched on and the light emission device is switched off, and wherein in the first operating mode Bl at least the light emission device is switched on, and - wherein in the second operating mode B2 a second electric field EF2 is applied, and wherein in the first operating mode Bl a first electric field EF1 is applied.

[0043] Within the scope of the present invention, and in particular with respect to the background illumination device, a "limited angular range" means that the corresponding light density is at least 80% or 90% concentrated within a defined angular range, whereas it is still completely possible that residual light exists outside the defined limited angular range, which is typically technically caused. Ideally, such residual light is minimal and decreases towards larger angles. In order to achieve a particularly strong minimization, in addition to the background illumination device which radiates light into a limited angular range, a corresponding light filter is used. This also applies to the variant with a light guide described hereinafter, which couples out light predominantly into a limited angular range. Unlike this design of the present invention, the light density curve of the background illumination device typically is bell-shaped over a particularly horizontal (if necessary also vertical) angular range, whereas a true concentration of the light density around a small angular range does not necessarily have to exist.

[0044] Finally, the present invention comprises a lighting device for a screen in a third design, which lighting device can be operated in at least one first operating mode Bl for a free field of view mode and in a second operating mode B2 for a limited field of view mode, in which limited field of view mode light is radiated into a limited angular range compared to the free field of view mode, - a planarly extending background illumination device, which radiates light into an unlimited angular range and is optionally constructed in a directly light-emitting manner (for example by means of a locally dimmable LED matrix lighting unit), and - a plate-like light guide placed in front of the background illumination device in the viewing direction, which light guide has output coupling elements on at least one of its large areas and / or within its volume, wherein the output coupling elements couple out the light predominantly (i.e. more than half, preferably more than 80% or 90%) of the light which is input laterally into at least one narrow side of the light guide into a limited angular range, - a light emission device arranged laterally at the at least one narrow side of the light guide, and - optionally a linearly polarizing light filter, - a switchable light filter according to the present invention with the structural features and properties as described further above, arranged in front of the background illumination device in the viewing direction, preferably in front of the light guide, - wherein in the second operating mode B2 the background illumination device is switched off and the light emission device is switched on, and wherein in the first operating mode B1 at least the background illumination device is switched on (additionally, the light emission device can optionally also be switched on), and - wherein in the second operating mode B2 a second electric field EF2 is applied, and wherein in the first operating mode B1 a first electric field EF1 is applied.

[0045] Advantageously, the illumination devices of the first, second and third design are combined with a transmissive screen, for example an LC panel, in order to produce a screen which can be operated in at least two operating modes B1 (for a free field of view mode) and B2 (for a limited field of view mode), in which the light is radiated into an angular range which is limited compared to the free field of view mode.

[0046] It is advantageously applicable that the transition dipole moment of the first optical element (and, if necessary, also of the second or further optical elements, if present) is configured as one or more dichroic dye, which is mixed with the liquid crystal in a guest-host arrangement. For permanent transition dipole moments, the liquid crystal can preferably be fixed by a curing process.

[0047] In contrast thereto, the transition dipole moment can also be embedded in the liquid crystal layer as a guest-host arrangement without being fixed, so that the transition dipole moment can be changed in its orientation and / or in its value between a first state and at least one second state depending on the influence of the liquid crystal layer.

[0048] The dichroic dye molecules are usually oriented parallel to the liquid crystal molecules.

[0049] Alternatively, the first optical element can be configured as a laminate of layers of a polymer film polarizer.

[0050] Furthermore, it is ensured by means of the above-mentioned design that the first optical element is constructed structurally as non-periodic. This is very advantageous, because then there is no risk of artefacts such as the Moire effect in interaction with the pixel structure of the screen.

[0051] The first preferred direction may, for example, enclose an angle between 0° and 45° with the surface normal of the first optical element, respectively. It is furthermore possible that the first preferred direction varies over the surface of the first optical element. In the sense of the present invention, then an average weighted preferred direction is applicable.

[0052] It is furthermore possible that at least two such preferred directions differ by more than 10° in the selectable plane and / or that the respective preferred direction of the transition dipole moment can be selected depending on its position in the first optical element.

[0053] Furthermore, it can be advantageous if the first optical element is divided into different regions (A1, A2,...) along a selectable reference line, wherein for each region (A1, A2,...) a selectable region-preferred direction is applied, which is suitable for all transition dipole moments located within the region (A1, A2,...), wherein all region-preferred directions are pairwise different and, except for a tolerance of maximum + / - 10 degrees, point into the direction of the observer 3. This arrangement has the advantage that in the limited field of view mode the observer perceives the screen with the filter as uniformly illuminated.

[0054] The transition dipole moment (also called transition matrix element) is a quantum mechanical vector quantity and is assigned to a specific transition between an initial state (usually a ground state) and a final state (usually an excited state) of a system (i.e. an atom, molecule or solid) and corresponds to the electric dipole moment associated with this transition. The direction of this vector defines the polarization of the transition, which determines on its side how the system interacts with an electromagnetic wave of a specific polarization, e.g. in the transition from the ground state to the excited state, light of the corresponding polarization is absorbed. The numerical value of this vector corresponds to the strength of the interaction or to the transition probability.

[0055] Here, the first (second) preferred direction corresponds to the orientation of the transition dipole moment of the first (second) optical element in a predefined propagation direction of the light, wherein the absorption is the same for arbitrary polarizations of the light.

[0056] The first and second preferred directions can also be identical or only differ by a few degrees (max. 10°) in their orientation and both are in particular perpendicular to the associated optical element. This is the preferred case. However, it can also be the case, depending on the application, that the first and second preferred directions differ from each other by more than 10°.

[0057] Furthermore, the filter can comprise a polarization filter arranged upstream or downstream of the first or second optical element as seen in the direction of incidence. Alternatively or additionally, a A / 4 layer is also conceivable, e.g. upon circularly polarized light incidence, which is converted into (substantially) linearly polarized light on the basis of the layer.

[0058] A first exemplary manufacturing variant of the first or second optical element in the case of utilization of the guest-host principle is based on a mixture of a dichroic dye or a mixture of dichroic dyes with a mixture of liquid crystals or compounds and comprises the following steps for the manufacture (cf. US 9,481,658 B2 or WO 2021 / 177308 A1 paragraph 37 ff) - coating of a low birefringent or non-birefringent substrate with a film which determines the orientation of the molecules with respect to the surface, usually parallel or perpendicular to the surface. For this purpose polymers are used, preferably polyvinyl alcohol or polyimide.

[0059] - optionally: optical or mechanical treatment of the surface in order to improve the later amount of molecular orientation.

[0060] - application of a mixture of dichroic dyes and thermotropic liquid crystal compounds or polymers.

[0061] - local coagulation of the side chains by light irradiation, so that the side chains induce birefringence along the surface.

[0062] An alternative second manufacturing variant uses thermotropic liquid crystal dichroic dyes (cf. JP 2011-237513 A) and comprises the following steps: - manufacture of the corresponding dyes and addition of polar groups.

[0063] - application of the dye mixture and photoinduced orientation of the dye mixture by means of polarized light and solidification.

[0064] For the different manufacturing variants, the following materials can be considered, for example, where the list does not claim completeness: - as a polymer substrate with low birefringence or no birefringence: preferably TAC, - as dichroic substance or mixture: dichroic dyes (preferably azo dyes) or dichroic metal nanoparticles (preferably gold, silver, copper and aluminum); here generally a single dye or typically a mixture of up to three different dyes in order to be able to realize an absorption over the entire spectrum, - for surface treatment by orientation of the dye or liquid crystal substance: polymers, preferably polyvinyl alcohol or polyimide, - for thermotropic liquid crystal compounds or polymers, cf. JP 2011-237513 A.

[0065] - as chemical groups for cross linking in combination (cross linking) with thermotropic liquid crystal compounds or polymers: methacryl, epoxy, oxetane and styrene groups, preferably methacryl. Alternatively, it can be a polymerizable liquid crystal compound as described in JP 6268730 B2.

[0066] - polymerizable liquid crystal dichroic dyes, for example azo dyes.

[0067] The at least one dye consists of dye molecules, wherein a transition dipole or a transition dipole moment is advantageously associated with each dye molecule, i.e. each dye molecule corresponds to one transition dipole or transition dipole moment. The dye typically has a mass fraction of at least 0.01 %, preferably 1 % to 15 %, of the material of the respective layer of the optical element. In special cases, the concentration can even amount to 95 % in the case of liquid-crystalline dichroic dyes. The thickness of the layer is preferably in the range of 0.2 μιη to 50 μιη, preferably in the range of 0.5 μιη to 20 μιη, including all limiting values, respectively. The dye or dye mixture used for the different layers within the optical element can be different, but need not be.

[0068] The above filter, illumination device or such screen is advantageously used in mobile devices, motor vehicles, aircraft or ships, payment terminals or access control systems. Here, a conversion between the above operating modes can take place in order to protect sensitive data, i.e. to represent in a manner perceptible to only one observer, or alternatively to represent the image content simultaneously for multiple observers.

[0069] If the above parameters are changed within certain limits, the performance of the present application is maintained in principle.

[0070] It goes without saying that the above features, as well as the features still to be set out below, can be used not only in the combinations indicated, but also in other combinations or individually, without departing from the scope of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0071] The application is explained below in more detail with reference to the accompanying drawings, which likewise show essential features of the application. These embodiments are merely exemplary and are not to be interpreted as limiting. For example, a description of an embodiment as having a number of elements or components is not to be interpreted as requiring these elements or components to be present in such embodiment. Rather, other embodiments can include fewer or additional elements or components. Elements or components of different embodiments can be combined with each other, unless otherwise indicated. Modifications and changes to the embodiments described herein can be made without departing from the scope of the application. In order to avoid repetition, elements or components of different embodiments are marked with the same reference numerals in the different figures, and are not repeatedly explained. Among others: Figure 1a a schematic diagram showing the mode of action of an exemplary switchable filter in operating modes B1 and B2, Figure 1b an exemplary normalized transmission map for an optical element in combination with a linear polarizing filter, without retardation plate (B plate) on the left-hand side and with retardation plate (B plate) on the right-hand side, Figure 2schematic diagram showing an exemplary structure of a switchable filter in a first design, Figure 3 schematic diagram showing an exemplary normalized transmittance plot (principle sketch based on measurements) over a horizontal angular range for a switchable filter in a first operating mode B1, Figure 4 schematic diagram showing an exemplary normalized transmittance plot (principle sketch based on measurements) over a horizontal angular range for a switchable filter in a second operating mode B2, Figure 5 schematic diagram showing an exemplary structure of a switchable filter in a second design, Figure 6 schematic diagram showing an exemplary structure of a lighting device in a first design with a switchable filter, Figure 7 schematic diagram showing an exemplary structure of a screen in a first design with a switchable filter, which screen can be operated in at least two operating modes B1 and B2, Figure 8 schematic diagram showing an exemplary structure of a screen in a second design with a switchable filter, which screen can be operated in at least two operating modes B1 and B2, Figure 9a schematic diagram showing an exemplary structure of a screen in a third design with a switchable filter, which screen can be operated in at least two operating modes B1 and B2, Figure 9b schematic diagram showing an exemplary structure of a screen in a fourth design with a switchable filter, which screen can be operated in at least two operating modes B1 and B2, Figure 9c schematic diagram showing an exemplary normalized luminance distribution of a background lighting device as can be used, for example, in a screen of the third or fourth design, Figures 10a to 10d schematic diagram showing an exemplary luminance or transmittance plot with respect to a screen of, in particular, but not exclusively, the third or fourth design, Figure 11 schematic diagram showing an exemplary structure of a screen in a fifth design with a switchable filter, which screen can be operated in at least two operating modes B1 and B2, Figure 12 schematic diagram showing an exemplary structure of a lighting device in a second design with a switchable filter, Figure 13A principle sketch showing an exemplary structure of a lighting device in a third design with a switchable filter. DETAILED DESCRIPTION

[0072] The figures are not drawn to scale and only reproduce principle sketches. For the reproduction of e.g. one- or two-dimensional angle-dependent exemplary transmittance representations of a filter Figure 1b , 3 , 4, 9c and 10b to 10d it applies that the numbers assigned to the lines reproduce the transmittance normalized to "1" in the case of the corresponding angle pair in polar coordinates. Thereby e.g. in the diagram Figure 1b the number "0.90" corresponds to a transmittance of 90% for the corresponding direction.

[0073] Figure 1a A principle sketch showing the mode of action of an exemplary switchable filter 5 in a first operating mode B1 and a second operating mode B2. The exemplary filter 5 comprises - a first optical element 1, comprising on its side: • a plurality of light-absorbing transition dipole moments, which are arranged in a layer of at least 0.2 micrometer thickness, wherein the transition dipole moments that absorb are preferably composed of dichroic dyes, wherein the dye mass density is greater than 1% or greater than 10%, • wherein a majority of the transition dipole moments of said transition dipole moments are oriented parallel to or fluctuate around a first preferred direction selectable for the first optical element 1 with a tolerance of maximum 20° (other tolerance values, e.g. 5° or 10° are also possible) at least in a first state (in combination with the embodiments described below according to the figures, the property "at least in a first state" should include exactly one possibility, namely that there is exactly one state; i.e. a permanent design), wherein the first preferred direction is arranged at a predefined angle a with respect to the median of the first optical element 1, e.g. wherein a = 0°, a = + / - 2° or a value (a) > 2°, wherein the angle a is measured in a selectable first plane containing said median, and the angle a is preferably measured parallel to an edge, e.g. the upper edge, of the first optical element 1, • such that light incident into the first optical element 1 in an incident direction and a polarization state is transmitted or at least partially absorbed depending on its incident direction with respect to the first optical element 1 and its polarization state, - a (not shown in the drawing) device for selectively generating a first electric field EFi or a second electric field EF2, - a liquid crystal layer 3 arranged behind the first optical element 1 in the observation direction in this example, a first electric field EF1 or a second electric field EF2 acting on said liquid crystal layer and said liquid crystal layer thereby influencing the polarization state of light penetrating through the liquid crystal layer, - such that the transmission characteristic of the switchable optical filter 5 differs between a first operating mode B1 in which the first electric field EF1 is applied (for example with a field strength of 0 V / pm) and a second operating mode B2 in which the second electric field EF2 is applied (for example with a field strength not equal to 0 V / pm, approximately in the order of magnitude of 1 V / pm, for example as a square wave with 10 kHz), wherein in both operating modes B1 and B2, at at least one point on the first optical element 1, preferably at multiple points, particularly advantageously over at least half of the area of the optical element, the respective relative transmission is described by a first transmission T B1 (β) for the first operating mode B1 or a second transmission T B2 (β) for the second operating mode B2, which is dependent on the angle β and is normalized such that for the value of the transmission for a pre-specified angle a, T B1 (α) = 1 and T B2 (α) = 1 applies, respectively, - in such a way that, if light • is incident into the optical element 1 with an angle β, wherein or then, in the first operating mode B1 with the first electric field EF1 applied, the s-polarized component of said light is transmitted with at least a first normalized transmission value T B1 (β) ≥ 0.25 (preferably T B1 (β) ≥ 0.3); and • is incident into the optical element 1 with an angle β, wherein or then, in the second operating mode B2 with the second electric field EF2 applied, the p-polarized component of said light is transmitted with at most a normalized second transmission value T B2 (β) ≤ 0.2 (preferably T B2 (β) ≤ 0.1, particularly preferably T B2 (β) ≤ 0.05, and advantageously also for all angles or .

[0074] With regard to the normalization of the transmission T B1 (α) = 1 and T B2 (α) = 1, it should be noted that, without further ado, the angles β and a are measured in the same plane as mentioned above. It is also possible in principle that T B1(α) > 1 and / or T B2 (α) > 1 applies for angles β ≠ α. In many cases, T B1 (α) < 1 and / or T B2 (α) < 1 also applies for angles β ≠ α.

[0075] Here, the important means-effect relationship consists in the fact that by switching between a first operating mode B1 in which a first electric field EF1 (for example with a field strength of 0 V / μm) is applied and a second operating mode B2 in which a second electric field EF2 (for example with a field strength not equal to 0 V / μm, approximately in the order of magnitude of 1 V / μm, for example as a square wave with 10 kHz) is applied, only in the operating mode B2 the s-polarized light incident on the liquid crystal layer 3 is converted substantially into p-polarized light, which then impinges on the first optical element 1. This is shown in Figure 1a on the left-hand side, where the s-polarized light component of the light incident on the liquid crystal layer 3 remains s-polarized after passing through the liquid crystal layer. In contrast, on the right-hand side of Figure 1a it is shown how the s-polarized light component becomes p-polarized after passing through the liquid crystal layer. Interactively with a layer of the first optical element with a transition dipole moment that absorbs, for T B1 (β) or T B2 (β) the above-mentioned transmission changes result in the above-mentioned operating modes. This is indicated by the arrows in Figure 1a on the left-hand side, i.e. in the operating mode B1, the s-polarized light also passes through the first optical element 1 at an angle of inclination, while on the right-hand side, i.e. in the operating mode B2, the p-polarized component is transmitted a little towards the side, so that the arrows are only schematically shown in the vertical direction.

[0076] In a preferred design, therefore, when the first electric field EF1 is applied, the light penetrating the liquid crystal layer 3 is transmitted substantially in an unaltered manner, while when the second electric field EF2 is applied, the incident light is circularly or elliptically polarized, or the polarization of the light is rotated by 90°. As mentioned above, Fig. 1 shows on the left-hand side the operating mode B1. Here, the first electric field E1 is applied, and the light incident from below, which is shown with "ss s", is not substantially changed in its polarization by the liquid crystal layer 3. Correspondingly, the s-polarized light can penetrate the optical element 1 with a transition dipole moment, i.e. (at least) in all directions indicated here in the plane of the paper. Although there is a loss in the transmission here, for the corresponding angles β, where or the above-mentioned conditions for the first transmission value T B1 (β) apply.

[0077] For a better overview, the angle a is plotted in Fig. 1 with values greater than 0°. However, for all the following considerations, in principle a = 0° should apply. The values a ≠ 0° allow the preferred transmission direction of the switchable filter 5 to be tilted.

[0078] Fig. 1 furthermore shows on the right-hand side the operating mode B2. Here, a second electric field E2 is applied, and the light incident from below, which is shown with "s s s", is changed in its polarization by the liquid crystal layer 3, i.e. is essentially converted into p-polarized light. Correspondingly, the p-polarized light now formed can only penetrate the optical element 1 with the transition dipole moment in a limited manner. Here, based on the orientation of the transition dipole moment, a limited transmission applies, in particular for the corresponding angle β, wherein or the above-mentioned conditions for the transmission value T B2 (β) are adhered to. Preferably, even for all angles or the conditions mentioned are adhered to.

[0079] Furthermore, in order to further understand the mode of action of the present application, Figure 1b an exemplary normalized transmission diagram for the optical element 1 in combination with a linear polarization filter is shown, without a retardation plate (B-plate) on the left-hand side and with a retardation plate (B-plate) on the right-hand side, it can be seen that, in particular for vertical angles with an absolute numerical value of approximately 15° or more, the use of a retardation plate (Retarder, as the B-plate calculated here) leads to a reduced transmission and is therefore advantageous.

[0080] The diagram is plotted in a polar coordinate system of a hemisphere into which light is incident. The straight lines correspond to the horizontal and vertical lines, respectively. The transmission is limited in particular in the horizontal direction. In the case of the circles shown, the polar angle is 25°, 45° and 90°, respectively, from the inside, i.e. in the intersection of the straight lines, to the outside.

[0081] It is advantageously applied that the transition dipole moment is configured as one or more dichroic dyes, which are mixed with the liquid crystal in a guest-host arrangement. For permanent transition dipole moments, the liquid crystal can be fixed by a curing process. The dichroic dye molecules are usually oriented parallel to the liquid crystal molecules.

[0082] Furthermore, by means of the design described above, it is ensured that the first optical element 1 is constructed structurally as non-periodic. This is very advantageous, because then there is no risk of artefacts such as the Moire effect in interaction with the pixel structure of the screen.

[0083] The first preferred direction may, for example, enclose an angle between 0° and 45° with the surface normal of the first optical element 1, respectively. It is furthermore possible that the first preferred direction varies over the surface of the first optical element 1. In the sense of the present application, then an average weighted preferred direction applies.

[0084] Furthermore, Figure 2 A schematic diagram of the principle of an exemplary structure of the switchable optical filter 5 in the first design variant is shown. Here, the liquid crystal layer 3 is arranged before the first optical element 1 in the viewing direction (from above). Furthermore, the first design variant comprises a first linear polarizing filter (X) which is located before the liquid crystal layer 3 in the viewing direction, the preferred polarization transmission direction of which is preferably oriented parallel to the edge, preferably the lower edge, of the optical element 1.

[0085] In Figure 3 An exemplary normalized transmittance diagram over the horizontal angular range for the switchable optical filter 5 in the first operating mode B1 with the first electric field EF1 applied is shown (as a measurement-based schematic diagram of the principle). Since a = 0° is assumed, it desirably applies here that if light is incident into the optical element 1 (and in principle into the switchable optical filter 5) at an angle β, wherein or then the s-polarized component of the light is transmitted with at least a normalized transmittance value T B1 (β) ≥ 0.25.

[0086] In contrast thereto, Figure 4 An exemplary normalized transmittance diagram over the horizontal angular range for the switchable optical filter 5 in the second operating mode B2 with the second electric field EF2 applied is shown (as a measurement-based schematic diagram of the principle).

[0087] It can be seen here that if light is incident into the optical element 1 (or into the switchable optical filter 5) at an angle β in the second operating mode B2 with the second electric field EF2 applied, wherein or then the p-polarized component of the light (the p-polarization here in particular being caused by the influence of the liquid crystal layer 3) is transmitted with at most a normalized transmittance value T B2 (β) ≤ 0.2 (in this case even T B2 (β) ≤ 0.15) is transmitted.

[0088] It can apply in a particular design variant of the present application that there is at least one angle β1 for which the transmittance of the p-polarized component of the light is not identical to the transmittance of the s-polarized component of the light. This condition preferably applies to the entire angular range of the angle β1 (for example and or ), and particularly preferably even for all angles a ≠ β.

[0089] Applicable in yet other design variants of the application is that in both operating modes B1 and B2 there is at least one angle β2 for which the transmittance of the s-polarized component of the light is greater than the transmittance of the p-polarized component of the light. This condition preferably applies to the entire angular range of the angle β2 (e.g. and or ), and particularly preferably even for all angles a ≠ β.

[0090] In Figure 5 a principle sketch of an exemplary structure of a switchable optical filter in the second design variant is reproduced. In this case, the switchable optical filter 5 comprises at least two first optical elements 1, 1', wherein here a retarder R is arranged between the at least two such first optical elements 1, 1'. The use of a retarder R is purely optional, but is here merely exemplary. Furthermore, the at least two first optical elements 1, 1' can, if necessary, have different thicknesses of the layers each comprising a plurality of light-absorbing transition dipole moments from one another, but this is not a necessity. For the case that a retarder R is arranged between the two first optical elements 1, 1', this retarder in particular allows an improved effect of the transmission limitation in the second operating mode B2.

[0091] In Figure 6 a principle sketch of an exemplary structure of an illumination device in the first design variant is shown, which has a switchable optical filter 5. This illumination device for a screen in the first design variant comprises - a planarly extended background illumination device 8, which radiates light and is optionally constructed in a direct-light-emitting manner (e.g. with a LED matrix for local dimming), and - a filter 5 as described above arranged in front of the background illumination device 8 along the viewing direction, the screen can be operated in at least two operating modes B1 (for a free-view mode) and B2 (for a limited-view mode), in which the light is radiated into a viewing angle range that is limited for an observer compared to the free-view mode.

[0092] Thus, such an illumination device in the first design variant can also be operated in the two operating modes B1 and B2 described above, if the switchable optical filter 5 used therein is used accordingly in the respective operating mode.

[0093] Furthermore, Figure 7 a principle sketch of an exemplary structure of a screen in the first design variant is reproduced, which can be operated in at least two operating modes B1 and B2, which has a switchable optical filter 5. Such a screen in the first design variant comprises - As mentioned above Figure 6 The described lighting device, -And if the first linear polarization filter X is not arranged in the switchable filter 5 of the illumination device, the second linear polarization filter P arranged in front of the background illumination device 8 along the viewing direction, thereby interacting with the optical element 1 of the switchable filter 5, restricts the light originating from the background illumination device and passing through the second linear polarization filter P in its propagation direction, and - A transmissive image reproduction device 11, which is arranged in front of the filter 5 along the observation direction. - Wherein, a second electric field EF2 is applied in operating mode B2, and where a first electric field EF1 is applied in operating mode B1. The screen can be operated in at least two operating modes, B1 (for free-view mode) and B2 (for limited-view mode), in which light is radiated into a viewing angle range that is limited for the observer compared to free-view mode.

[0094] Preferably, the first or second linear polarization filter P, X is arranged in or as part of the transmissive image reproduction device 11.

[0095] Furthermore, the present invention includes a screen in a second design embodiment, which can operate in at least two operating modes, B1 (for free-view mode) and B2 (for restricted-view mode), in which light is radiated into a viewing angle range that is limited for the observer compared to free-view mode. Therefore, Figure 8 A schematic diagram illustrating an exemplary structure of such a screen in the second design scheme is shown. The screen includes... - Image reproduction device 12, wherein any type of image reproduction device can be considered in principle, such as LC panel, OLED, microLED and other image reproduction devices. - As further described above, along the viewing direction, in front of the image reproduction device 12, filter 5, -In operation mode B2, a second electric field EF2 is applied, and in operation mode B1, a first electric field EF1 is applied.

[0096] In this case, it is optional to have the switchable filter 5 subsequently and / or reversibly installed at the image reproduction device 12 by the user. In this case, the filter 5 can be sold as a so-called "aftermarket product".

[0097] Figure 9a and Figure 9bExemplary structures of screens in the third design variant and in the fourth design variant are shown in principle sketches, which can be operated in at least two operating modes B1 and B2, with switchable light filters. Such screens comprise - a transmissive image reproduction device 11, preferably an LC panel, - a background illumination device 8a arranged downstream of the transmissive image reproduction device 11 in the viewing direction, wherein said background illumination device 8a has a luminance profile, the peak luminance of which is radiated in a direction forming an angle of at least 3°, advantageously even 5° or 8° with the first preferred direction (the background illumination device 8a can have a permanent or variable luminance profile), - a light filter 5, as described further above, before (see Figure 9a , third design variant) or after (see Figure 9b , fourth design variant) the image reproduction device 11 in the viewing direction, - wherein a second electric field EF2 is applied in the operating mode B2, and wherein a first electric field EF1 is applied in the operating mode B1.

[0098] The background illumination device 8a with such properties can be produced, for example, if it also still comprises a Turning-Film and / or a partially mirrored or asymmetric prismatic grating above the light conductor with diffuser, BEF or DBEF.

[0099] To this end, Figure 9c An exemplary normalized luminance profile of a background illumination device 8a as can be used, for example, in the screens of the third or fourth design variant is shown here: Here, the peak luminance is shifted by about 5° to the right in the horizontal direction.

[0100] For such screens of the third or fourth design variant, it can advantageously continue to apply for at least one partial surface that, in the second operating mode B2, for at least ((preferably even for or , up to a maximum ) an angle range, for all angles β contained in said angle range, it applies that T B2 (β) x L V (β) x T BW (β) of the image reproduction device 11 at the angle β deviates by at most + / - 10% from the value of said product for the angle α = β.

[0101] Due to the adherence to this product, within the above-mentioned tolerance range, the perceived uniformity for the observer is increased, since the corrected optical density distribution based on the background illumination device 8a is again compensated in the case of a strong angle-dependent transmittance drop of the optical element 1 or the image reproduction device 11 upon, for example, a present oblique viewing of lateral areas of the screen, when the observer looks centrally frontally at the screen.

[0102] In this regard, reference should be made to the attached figures Figures 10a to 10d , which show exemplary optical density or transmittance diagrams for the individual components or component groups of the screen with respect to, in particular, but not exclusively, the third or fourth design variant. Therein, two different parameter combinations for the optical density of the background illumination device 8a, the transmittance of the transmissive image reproduction device 11 and the transmittance of the optical element 1 are shown, respectively. For both parameter sets, the condition: T B2 (β) * L V (β) * T BW (β) is fulfilled for an angle β, wherein the difference between the angle β and the angle a is at most 10%. For the case represented by the solid line, the optical density L V (β) and the transmittance T BW (β) of the image reproduction device do not have an angle dependence.

[0103] For the second case (dashed line), the angle dependence of T B2 (β) is more pronounced, such that the condition is only fulfilled when the optical density L V (β) at least locally increases with the difference between the viewing angle β and the angle a.

[0104] Alternatively, for such a screen of the third design variant - for at least one partial area - it can optionally apply that, in the second operating mode B2, for at least (optimally even for or up to a maximum , for all angles β contained in said angle range, the product of T B2 (β) and the optical density L V (β) of the background illumination device 8a at the angle β deviates at most + / - 10% from the value of said product for the angle a = β.

[0105] The above-mentioned compensation effect based on the confusing optical density distribution of the backlighting device also applies here, however, disregarding the transmittance behavior T BW (β) of the image reproduction device 11.

[0106] Finally, the invention also comprises a screen in a fifth design variant, wherein the screen can be operated in at least two operating modes B1 (for a free field of view mode) and B2 (for a restricted field of view mode), in which restricted field of view mode light is radiated into a viewing angle range which is restricted for the observer compared to the free field of view mode. To this end, Figure 11 a schematic diagram showing an exemplary structure of such a screen in the fifth design variant, comprising - a transmissive image reproduction device 11, preferably an LC panel, - a background illumination device 8x arranged downstream of the transmissive image reproduction device 11 in the viewing direction, wherein the background illumination device 8x has an asymmetric light density distribution, wherein the asymmetry preferably exists with respect to a direction which is horizontal from the observer's point of view, - a light filter 5 as described further above, before or after the image reproduction device 11 in the viewing direction, - and, if no first linear polarization filter X is arranged in the switchable light filter 5, a linear polarization filter P arranged before the background illumination device 8x or in the image reproduction device 11 in the viewing direction, as realized in the shown embodiment, whereby the light which emanates from the background illumination device 8x and which penetrates the second linear polarization filter P is restricted in its propagation direction in interaction with the optical element 1 of the switchable light filter 5, - wherein a second electric field EF2 is applied in the operating mode B2, and wherein a first electric field EF1 is applied in the operating mode B1.

[0107] Here, the background illumination device 8x is designed such that it essentially has no (e.g. around a vertical center line from the observer's point of view) symmetric light density distribution, but rather embodies an asymmetric light density distribution (e.g. on a horizontal line) instead. In other words: the background illumination device 8a has an asymmetric light density distribution, wherein the asymmetry preferably exists with respect to a direction which is horizontal from the observer's point of view. Such a design variant is possible, for example, in the case of the use of deterministic light output coupling light guide bodies and / or turned films which shift the peak luminance.

[0108] This variant is advantageous for use in a means of transport, since then, in particular by designing the background illumination device, it is possible to significantly reduce the light that would be radiated in the direction of the co-passenger window from approximately a horizontal angle of 25 degrees or more (relative to the median line), for example to less than 20% of the peak brightness - preferably less than 2.5% - while there is desirably a high light density in the direction of the driver. In this way, disturbing reflections in the co-passenger window or, if necessary, on the outside mirror closest to the co-passenger are reduced or even avoided. Nonetheless, on the basis of the switchable filter 5 arranged before the image reproduction device 11, the screen can be operated selectively such that either only the co-passenger can see the image content (operating mode B2), for example for moving images, or both the driver and the co-passenger can see the image content (operating mode Bl), for example for navigation map data.

[0109] For some of the aforementioned screens of the first to fifth design variants, it is advantageous when, arranged before the transmissive image reproduction device 11 along the viewing direction, there is an optional second optical element 2 (see also Figure 11 ) which comprises: • a plurality of light-absorbing transition dipole moments; in this case, the dye mass density is greater than 1% or even greater than 10%.

[0110] • the majority of the transition dipole moments are oriented at least in the first state parallel to or fluctuating around a second preferred direction selectable for the second optical element 2 with a tolerance of a maximum of 20° (alternatively 10°), wherein the second preferred direction is arranged at an angle a1 relative to the median line of the second optical element 2 (in this case, for example, a1 = 0°, a1 = + / - 2° or a value (a1) > 2° can apply), wherein the angle a1 is measured in an optional second plane containing the median line; the second plane 2 is preferably perpendicular to the first plane of the first optical element 1, • such that light incident into the second optical element 2 is transmitted or at least partially absorbed depending on its angle of incidence relative to the second optical element 2 and its polarization state.

[0111] The last-mentioned design variant advantageously leads to a reduced transmission in the vertical direction and thus can reduce or completely avoid reflections of the image content represented on the screen on the windscreen in a means of transport.

[0112] Furthermore, the invention comprises an illumination device for a screen in a second design variant, which can be operated in at least two operating modes Bl (for a free-view mode) and B2 (for a restricted-view mode), in which the light is radiated into an angle range that is restricted compared to the free-view mode. To this end, Figure 12A schematic diagram showing an exemplary structure of a lighting device in such a second design variant. The lighting device comprises - a planarly extended background lighting device 8b which radiates light into a limited angular range and which is optionally constructed in a direct light-emitting manner, and - a plate-shaped light conductor 9 which is placed in front of the background lighting device 8b in the viewing direction, which has output-coupling elements on at least one of its large areas and / or within its volume, - a light-emitting device 10 which is arranged laterally at at least one narrow side of the light conductor 9, and - optionally a linearly polarizing filter P (not shown in the drawing), - a filter 5 which is arranged in front of the background lighting device 8b in the viewing direction (this also includes a position in front of a screen with which the lighting device is used), as described further above, - wherein in the operating mode B2 the background lighting device 8b is switched on and the light-emitting device 10 is switched off, and wherein in the operating mode B1 at least the light-emitting device 10 is switched on, and - wherein in the operating mode B2 a second electric field EF2 is applied, and wherein in the operating mode B1 a first electric field EF1 is applied.

[0113] Furthermore, there is a transmissive image reproduction device 11, for example an LC panel, which is still in the viewing direction here, i.e. from above in the drawing plane.

[0114] Within the scope of the invention, and in particular with respect to the background lighting device, "limited angular range" means that the corresponding light density is concentrated at least 80% or 90% within a defined angular range, whereas it is also entirely possible for residual light to exist outside the defined limited angular range, which is often technically caused. Ideally, such residual light is minimal and decreases towards larger angles. In order to achieve a particularly strong minimization, in addition to the background lighting device which radiates light into a limited angular range, a corresponding filter is also used. This also applies to the variant described below with a light conductor which radiates or out-couples light predominantly into a limited angular range. Unlike this design variant of the invention, the light density curve of the background lighting device typically has a bell shape over a particularly horizontal (if necessary also vertical) angular range, whereas a true concentration of the light density around a small angular range does not necessarily have to exist.

[0115] Finally, the application comprises a lighting device for a screen in a third design, which lighting device can be operated in at least two operating modes B1 (for a free field mode) and B2 (for a restricted field mode), in which restricted field mode light is radiated into an angular range which is restricted compared to the free field mode. To this end, Figure 13 a schematic diagram of the principle of an exemplary structure of a lighting device in the third design. The lighting device comprises - a planarly extended background lighting device 8c, which radiates light into an unrestricted angular range and is optionally constructed in a direct-lighting manner (for example by means of a locally dimmable LED matrix lighting unit), and - a plate-like light guide 9c which is placed in front of the background lighting device 8c in the viewing direction, which light guide has output-coupling elements on at least one of its large areas and / or within its volume, wherein the output-coupling elements output a majority (i.e. more than half, preferably 80% or 90% or more) of the light which is laterally input-coupled into at least one narrow side of the light guide 9c into a restricted angular range, - a lighting device 10 which is arranged laterally at the at least one narrow side of the light guide 9c, and - optionally a linearly polarizing filter P (not shown in the drawing), - a filter 5 which is arranged in front of the background lighting device 8c, preferably in front of the light guide 9c, in the viewing direction, as described previously, - wherein in the operating mode B2 the background lighting device 8c is switched off and the lighting device 10 is switched on, and wherein in the operating mode B1 at least the background lighting device 8c is switched on (additionally, the lighting device 10 can optionally also be switched on), and - wherein in the operating mode B2 a second electric field EF2 is applied, and wherein in the operating mode B1 a first electric field EF1 is applied.

[0116] The lighting devices of the first, second and third designs are advantageously combined with a transmissive image reproduction device 11 (see also for example Figure 12 and Figure 13 ), for example an LC panel, in order to produce a screen which can be operated in at least two operating modes B1 (for a free field mode) and B2 (for a restricted field mode), in which restricted field mode light is radiated into an angular range which is restricted compared to the free field mode.

[0117] The invention solves the task set: An optical filter with optical elements is described, in which light incident into the optical elements is transmitted or partially or completely absorbed depending on its direction of incidence and its polarization properties, not primarily but depending on its position. By using the optical filter with optical elements, the transmission of light is influenced in an angle-dependent manner, optionally vertically with respect to a sitting or standing observer, between which at least two operating states can be switched. Here, in particular the transmission behavior for specific directions can be switched.

[0118] The above invention can advantageously be applied in the display and / or input of confidential data interactively with image reproduction means, for example at the time of PIN entry or for data display at an automatic cash machine or payment terminal, or for password entry, or when reading an e-mail on a mobile device. The invention can also be applied in a passenger vehicle, as further described above, in order to selectively conceal the disturbing image content from the driver or fellow passengers.

[0119] List of reference signs 1 first optical element 2 second optical element 3 liquid crystal layer 5 switchable optical filter 8 background illumination means 8a background illumination means 8b background illumination means 8c background illumination means 8x background illumination means 9, 9c light guide 10 light emitting means 11 image reproduction means P, X polarization filter

Claims

1. A switchable filter (5), comprising: - A first optical element (1), the first optical element comprising on its side: • Multiple light absorption transition dipole moments, said transition dipole moments being arranged in a layer at least 0.2 micrometers thick; •The majority of the transition dipole moments in the first state are oriented parallel to a first preferred direction selectable for the first optical element (1) with a tolerance of at least 20°, or fluctuate around the first preferred direction, wherein the first preferred direction is arranged at a predetermined angle α relative to the perpendicular bisector of the first optical element (1), wherein the angle α is measured in a selectable first plane including the perpendicular bisector. • Light incident on the first optical element (1) in the incident direction and polarization state is transmitted or at least partially absorbed according to its incident direction relative to the first optical element (1) and its polarization state. - A device for selectively generating a first electric field (EF1) or a second electric field (EF2); - A liquid crystal layer (3) is arranged after or before the first optical element (1) along the observation direction, the first electric field (EF1) or the second electric field (EF2) acts on the liquid crystal layer (3), and the liquid crystal layer (3) thereby affects the polarization state of light passing through the liquid crystal layer; - If the liquid crystal layer (3) is arranged in front of the first optical element (1) along the observation direction, then the first linear polarization filter (X) is located in front of the liquid crystal layer (3) along the observation direction. - This causes the transmission characteristics of the switchable optical filter (5) to differ between a first operating mode B1 where the first electric field (EF1) is applied and a second operating mode B2 where the second electric field (EF2) is applied, wherein at at least one point on the first optical element (1), the corresponding relative transmittance in both operating modes B1 and B2, apart from selectable tolerances, is obtained by passing the first transmittance T for the first operating mode B1. B1 (β) or the second transmittance T for the second operating mode B2 B2 (β) is used to describe this, which is related to the angle β, and is normalized to make T B1 (α) = 1 and T B2 (α)=1 applies to the value of transmittance for a pre-given angle α. -The way is that if light • It is incident on the first optical element (1) at an angle (β), wherein or Under the first operating mode B1 where the first electric field (EF1) is applied, the s-polarization component of the light has a transmittance value of at least the first normalized value T. B1 (β) ≥0.25 is transmitted; as well as • It is incident on the first optical element (1) at an angle (β), wherein or Then, under the second operating mode B2 where the second electric field (EF2) is applied, the p-polarization component of the light is at most the second normalized transmittance value T. B2 (β) ≤ 0.2 is transmitted.

2. The switchable filter (5) according to claim 1, characterized in that, The first optical element (1) and / or the liquid crystal layer (3) are divided into multiple individually switchable segments, enabling local switchability between different possible operating states.

3. The switchable filter (5) according to any one of the preceding claims, characterized in that, The switchable filter includes at least two first optical elements (1, 1'), wherein a delayer is optionally arranged between the at least two such first optical elements (1, 1').

4. The switchable filter (5) according to any one of the preceding claims, characterized in that, There exists at least one first specific angle (β1) for which the transmittance of the p-polarized component of the light is not the same as the transmittance of the s-polarized component of the light.

5. The switchable filter (5) according to any one of claims 1 or 2, characterized in that, In both operating modes B1 and B2, there exists at least one second specific angle (β2) where the transmittance of the s-polarized component of the light is greater than the transmittance of the p-polarized component of the light.

6. A lighting device for a screen capable of operating in at least one first operating mode B1 for a free-view mode and a second operating mode B2 for a limited-view mode, wherein light is radiated into a viewing angle range limited for the observer compared to the free-view mode, the lighting device comprising: - A planar extended background lighting device (8), the background lighting device radiating light; as well as - A switchable filter (5) arranged in front of the background lighting device (8) along the observation direction according to any one of claims 1 to 5.

7. A screen capable of operating in at least one first operating mode B1 for a free-view mode and a second operating mode B2 for a limited-view mode, wherein in the limited-view mode, light is radiated into a viewing angle range that is limited for the observer compared to the free-view mode, the screen comprising: - The lighting device according to claim 6, the lighting device having a switchable filter (5), the switchable filter including a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged after or before the first optical element (1) along the observation direction, and if the liquid crystal layer (3) is arranged before the first optical element (1) along the observation direction, including a first linear polarization filter (X) located before the liquid crystal layer (3) along the observation direction. - If the first linear polarization filter (X) is not arranged in the switchable filter (5) of the lighting device, then a second linear polarization filter (P) is arranged in front of the background lighting device (8) along the viewing direction, thereby restricting the light originating from the background lighting device and passing through the second linear polarization filter (P) in its propagation direction, and - Transmissive image reproduction device (11), which is arranged in front of the switchable filter (5) along the observation direction. - wherein the second electric field (EF2) is applied in the second operating mode B2, and wherein the first electric field (EF1) is applied in the first operating mode B1.

8. The screen according to claim 7, characterized in that, If present, the first linear polarization filter (X) and otherwise the second linear polarization filter (P) are arranged in or as part of the transmissive image reproduction device (11).

9. A screen capable of operating in at least one first operating mode B1 for a free-view mode and a second operating mode B2 for a limited-view mode, wherein in the limited-view mode, light is radiated into a viewing angle range that is limited for the observer compared to the free-view mode, the screen comprising: -Image reproduction device (12); -A switchable filter (5) according to any one of claims 1 to 5, located before the image reproduction device (12) along the observation direction, the switchable filter comprising a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged after or before the first optical element (1) along the observation direction, and, if the liquid crystal layer (3) is arranged before the first optical element (1) along the observation direction, a first linear polarization filter (X) located before the liquid crystal layer (3) along the observation direction. - wherein the second electric field (EF2) is applied in the second operating mode B2, and wherein the first electric field (EF1) is applied in the first operating mode B1.

10. The screen according to claim 8, characterized in that, The switchable filter (5) can be subsequently and / or reversibly installed by the user at the image reproduction device (12).

11. A screen capable of operating in at least one first operating mode B1 for a free-view mode and a second operating mode B2 for a limited-view mode, wherein in the limited-view mode, light is radiated into a viewing angle range that is limited for the observer compared to the free-view mode, the screen comprising: - Transmission-type image reproduction device (11); - A background lighting device (8a) is arranged downstream of the transmissive image reproduction device (11) along the observation direction, wherein the background lighting device (8a) has a light density distribution and its peak brightness is radiated in a direction forming an angle of at least 3° with the first preferred direction; -A switchable filter (5) according to any one of claims 1 to 5, located before or after the image reproduction device (11) along the observation direction, comprising a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged before or after the first optical element (1) along the observation direction, and, if the liquid crystal layer (3) is arranged before the first optical element (1) along the observation direction, a first linear polarization filter (X) located before the liquid crystal layer (3) along the observation direction, wherein the transmission characteristics of the switchable optical filter (5) differ between a first operating mode B1 and a second operating mode B2, wherein at at least one point on the first optical element (1), the corresponding relative transmittances in the two operating modes B1 and B2, apart from selectable tolerances, are respectively determined by a first transmittance T for the first operating mode B1. B1 (β) or the second transmittance T for the second operating mode B2 B2 (β) is used to describe this, which is related to the angle β, and is normalized to make T B1 (α) = 1 and T B2 (α)=1 applies to the value of transmittance for a pre-given angle α; - wherein the second electric field (EF2) is applied in the second operating mode B2, and wherein the first electric field (EF1) is applied in the first operating mode B1.

12. The screen according to claim 11, characterized in that, In the second operating mode B2, for at least The angular range, applicable to all angles β contained within said angular range, is the second transmittance T. B2 (β), the angle-dependent light density L of the background lighting device (8a) at the angle β. V (β) and the angle-dependent transmittance T of the image reproduction device (11) at the angle β. BW The product of (β) deviates from the value of the product for the angle α=β by at most + / -10%.

13. The screen according to claim 11, characterized in that, In the second operating mode B2, for at least The angular range, applicable to all angles β contained within said angular range, is the second transmittance T. B2 (β) and the angle-dependent light density L of the background lighting device (8a) at the angle β. V The product of (β) deviates from the value of the product for the angle α=β by at most + / -10%.

14. A screen capable of operating in at least one first operating mode B1 for a free-view mode and a second operating mode B2 for a limited-view mode, wherein in the limited-view mode, light is radiated into a viewing angle range that is limited for the observer compared to the free-view mode, the screen comprising: - Transmissive image reproduction device (11), preferably LC panel; - A background illumination device (8x) arranged downstream of the transmissive image reproduction device (11) along the observation direction, wherein the background illumination device (8x) has an asymmetrical light density distribution, wherein the asymmetry preferably exists with respect to the horizontal direction as viewed from the observer's perspective; -A switchable filter (5) according to any one of claims 1 to 5, which is located before or after the image reproduction device (11) along the observation direction, includes a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged after or before the first optical element (1) along the observation direction, and if the liquid crystal layer (3) is arranged before the first optical element (1) along the observation direction, it includes a first linear polarization filter (X) located before the liquid crystal layer (3) along the observation direction. - wherein the second electric field (EF2) is applied in the second operating mode B2, and wherein the first electric field (EF1) is applied in the first operating mode B1.

15. The screen according to any one of claims 7 to 14, characterized in that, A second optical element (2) is arranged in front of the transmissive image reproduction device (11) along the observation direction. The second optical element includes: • Multiple optical absorption transition dipole moments • The majority of the transition dipole moments, at least in the first state, are oriented parallel to or fluctuate around a second preferred direction selectable for the second optical element (2) with a tolerance of up to 20°, wherein the second preferred direction is arranged at an angle α1 relative to the perpendicular bisector of the second optical element (1), wherein the angle α1 is measured in a selectable second plane including the perpendicular bisector. • This causes light incident on the second optical element (2) to be transmitted or at least partially absorbed according to its incident direction relative to the second optical element (2) and its polarization state.

16. A lighting device for a screen, the lighting device being operable in at least one first operating mode B1 for a free field of view mode and a second operating mode B2 for a restricted field of view mode, wherein in the restricted field of view mode light is radiated into an angular range limited compared to the free field of view mode, the lighting device comprising: - A planar extended background lighting device (8b), which radiates light within a limited angular range and is optionally constructed to emit light directly; and - A plate-shaped light conductor (9) is placed in front of the background lighting device (8b) along the observation direction, the light conductor having an output coupling element on at least one of its large areas and / or within its volume; - A light-emitting device (10) arranged laterally on at least one narrow side of the light conductor (9); and - A switchable filter (5) according to any one of claims 1 to 5, arranged in front of the background illumination device (8b) along the observation direction, the switchable filter comprising a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged in front of or behind the first optical element (1) along the observation direction, and if the liquid crystal layer (3) is arranged in front of the first optical element (1) along the observation direction, a first linear polarization filter (X) located in front of the liquid crystal layer (3) along the observation direction. -In the second operating mode B2, the background lighting device (8b) is turned on and the light-emitting device (10) is turned off, and in the first operating mode B1, at least the light-emitting device (10) is turned on; as well as The second electric field (EF2) is applied in the second operating mode B2, and the first electric field (EF1) is applied in the first operating mode B1.

17. A lighting device for a screen, the lighting device being operable in at least one first operating mode B1 for a free field of view mode and a second operating mode B2 for a restricted field of view mode, wherein in the restricted field of view mode light is radiated into an angular range limited compared to the free field of view mode, the lighting device comprising... - A planar extended background lighting device (8c), which radiates light over an unrestricted angular range and is optionally constructed to emit light directly. - A plate-shaped light conductor (9c) is placed in front of the background lighting device (8c) along the observation direction. The light conductor has an output coupling element on at least one of its large areas and / or within its volume, wherein the output coupling element couples most of the light input laterally into at least one narrow side of the light conductor (9c) into a limited angular range. - A light-emitting device (10) arranged laterally on at least one narrow side of the light conductor (9c); and - A switchable filter (5) according to any one of claims 1 to 5, arranged in front of the background illumination device (8c) along the observation direction, the switchable filter comprising a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged in front of or after the first optical element (1) along the observation direction, and if the liquid crystal layer (3) is arranged in front of the first optical element (1) along the observation direction, a first linear polarization filter (X) located in front of the liquid crystal layer (3) along the observation direction. -In the second operating mode B2, the background lighting device (8c) is turned off and the light-emitting device (10) is turned on, and in the first operating mode B1, at least the background lighting device (8c) is turned on, and - wherein the second electric field EF2 is applied in the second operating mode B2, and wherein the first electric field EF1 is applied in the first operating mode B1.

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