Optical element, switchable light filters, lighting device and screen

Abstract

The invention relates to a first optical element (1) comprising a plurality of light-absorbing transition dipole moments, wherein the plurality of the transition dipole moments are permanently or at least in a first state aligned with a tolerance ≤ 20° parallel to or varied around a first preferred direction selectable for the first optical element (1), and wherein the transition dipole moments with a tolerance ≤ 20° are oriented parallel to or varied around a first preferred direction selectable for the first optical element (1).The transition dipole moments aligned parallel to the first preferred direction have a density N1 and the remaining transition dipole moments have a density of N2, such that light incident on the first optical element (1) is transmitted or partially absorbed depending on its direction of incidence relative to the first optical element (1) and its polarization state, wherein the inequality N1 / N2 > 10−3 ⋅ log10[(20°−&tgr;)1°] is satisfied permanently or at least in the said first state for the ratio of the respective densities N1 and N2 on the optical element (1), so that higher tolerance values ​​&tgr;The resulting increase in the ratio of the densities N1 and N2 of the transition dipole moments at least partially compensates for the absorption of p-polarized light incident on the optical element (1) at angles greater than 45°. The invention further discloses first and second switchable light filters (5, 5a) as well as lighting devices and screens.

Description

Technical Field of the InventionIn recent years, great advances have been made in widening the visual angle of LCDs. However, there are often situations where this very wide range of vision of a screen may be disadvantageous. Increasingly, information is also becoming available on mobile devices such as notebooks and tablet PCs, such as bank data or other personal information, and sensitive data. Accordingly, people need control over who can see these sensitive data; they must be able to choose between a wide viewing angle - a public mode - to share information on their display with others, e.g. when viewing vacation photos or also for advertising purposes. On the other hand, they require a small viewing angle-in a private mode-if they want to handle the image information confidentially.A similar problem arises in vehicle construction: there, when the engine is switched on, the driver must not be distracted by image contents, such as digital entertainment programs, while the passenger would like to consume them while driving. Thus, a screen is needed that can switch between the respective display modes.Additional films based on micro-lamellae have already been used for mobile displays in order to achieve their visual data protection. However, these films were not switchable or switchable, they had to be placed first by hand and then removed again. It must also be transported separately to the display if it is not required. A substantial disadvantage of the use of such laminated films is also associated with the associated light losses.U.S. Pat. No. 6,765,550 B2 describes such a visual protection by micro-lamellae. The greatest disadvantage here is the mechanical removal or mechanical attachment of the filter and the loss of light in the protected mode.U.S. Pat. No. 5,993,940 A describes the use of a film which has small strip-shaped prisms arranged uniformly on its surface in order to achieve a private mode, i.e. a restricted viewing mode with a small viewing angle range. Development and production are very complicated from a technical point of view.In WO 2012 / 033583 A1, the switching between free and restricted view is produced by means of the control of liquid crystals between so-called "chromonic" layers. This results in a loss of light and the technical complexity is very high.US 2012 / 0235891 A1 describes a very complex backlight-a backlight-in a screen. According to FIGS. 1 and 15, not only a plurality of light guides are used there, but also further complex optical elements such as microlens elements 40 and prism structures 50, which convert the light from the rear illumination on the way to the front illumination. This is expensive and technically complicated to implement and also associated with light loss. According to the variant according to FIG. 17 in US 2012 / 0235891 A1, both light sources 4R and 18 produce light with a narrow illumination angle, wherein the light from the rear light source 18 is only converted into light with a large illumination angle in a complicated manner. This complex conversion is, as already mentioned above, greatly reducing brightness.US 2013 / 0308185 A1 describes a special light guide formed with steps, which emits light on a large surface in different directions, depending on the direction from which it is illuminated from a narrow side. In cooperation with a transmissive image display device, e.g. an LC display, a screen switchable between free and restricted viewing modes can thus be generated. One disadvantage here is that the restricted visual effect can be generated simultaneously either only for left / right or for top / bottom, but not for left / right / top / bottom, as is necessary for particular payment processes. In addition, in the restricted viewing mode, residual light is still visible from blocked viewing angles.WO 2015 / 121398 A1 by the applicant describes a screen with two operating modes in which particles of scattering are present in the volume of the corresponding light guide for the switching of the operating modes. However, the scattering particles selected there from a polymer generally have the disadvantage that light is coupled out from both large surfaces, as a result of which approximately half of the useful light can be emitted in the wrong direction, namely toward the backlighting, and cannot be recycled to a sufficient extent there on account of the structure. Moreover, the scattering particles of polymer distributed in the volume of the light guide can under certain circumstances, in particular at a higher concentration, lead to scattering effects which reduce the visual protection effect in the protected operating mode.The approach of the technology of "electrical birefringence (EDB)" is based on the concept of using the switchable liquid crystals of an additionally applied LC panel for "filtering" all light beams which do not emerge from the imaging layer at a specific emission angle. Disadvantages of this technology are high additional energy and cost and the ±40° sweet spot, which is difficult to change, i.e. the best possible viewing position. The absorbance of the LC structures is also insufficient, since the attenuation of the light intensity for viewing angles greater than the sweet spot increases again, so that the light intensity for viewing angles greater than ±40° is up to 3% of the maximum light intensity.The aforementioned methods and arrangements are generally associated with the disadvantage that they significantly reduce the brightness of the base screen and / or require a complicated and expensive optical element for mode switching and / or reduce the resolution in the freely viewable, public mode and / or have visual artifacts in very high-resolution displays.DESCRIPTION OF THE INVENTIONThe object of the invention is therefore to describe an optical element in which light which is incident into the optical element is transmitted or partially or completely absorbed depending on its direction of incidence and its polarization properties-not primarily but depending on its position. Production-related alignment tolerances are to be counteracted by the optical effect. Switchable light filters which use the optical element are intended to influence the transmission of light depending on the angle-optionally with respect to a seated or standing observer-wherein it is possible to switch between at least two operating states. In this case, the transmission behavior is to be switchable for specific directions. Furthermore, the intention is to disclose lighting devices and screens using switchable light filters.This object is achieved according to the invention by a first optical element comprising• a plurality of light-absorbing transition dipole moments, wherein the corresponding absorption cross sections are denoted by σ abs and optionally apply to one wavelength, to a plurality of selected wavelengths or, on average, to a plurality of wavelengths in the light spectrum visible to humans,• wherein the plurality of transition dipole moments is aligned permanently or at least in a first state with a tolerance τ with τ≤20°, preferably with τ≤18°, parallel to a first preferred direction selectable for the first optical element or varies around the latter (the tolerance τ applies in both directions, that is to say for the value of a respective angle plus ±τ), wherein the first preferred direction is arranged at an angle α to the mean perpendicular of the first optical element, wherein the angle α is measured in a selectable first plane which contains the said mean perpendicular, and wherein the transition dipole moments aligned parallel to the first preferred direction with a tolerance τ have a density N 1 and the remaining transition dipole moments (i.e. lying outside the described tolerance range) have a density of N 2,• so that light which is incident into the first optical element is transmitted or at least partially absorbed as a function of its direction of incidence with respect to the first optical element and its state of polarization,wherein the inequality is satisfied permanently or at least in said first state for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, on the optical element, so that higher tolerance values τ are at least partially compensated on account of the then correspondingly increased relationship of the densities N 1 and N 2 of the transition dipole moments, whereby the absorption of p-polarized light which is incident on the optical element at angles of greater than 45°, measured in the first plane, is enhanced.The plurality of light absorbing transition dipole moments are arranged in a layer at least 0.2 microns thick. It is also possible for a plurality of layers of transition dipole moments to be present, which layers are separated, for example, by OCA (optically clear adhesive) and / or substrates. The layer thicknesses of the transition dipole moments can be, for example, from 0.2 μm to 50 μm, preferably 0.2 μm to 20 μm, particularly preferably 1 μm to 10 μm. In a preferred embodiment, two layers of transition dipole moments are present in the first optical element 1, each of which layers is between 1 μm and 10 μm thick, wherein both have a substrate (e.g. consisting of TAC or another material with little or no birefringence and a thickness preferably less than 100 μm), wherein the two substrates are additionally bonded to one another with the respective layers of transition dipole moments by means of an OCA (optically clear adhesive).The invention thus shows a way in which manufacturing-related limitations in the alignment of the transition dipole moments on the optical element must be counteracted by using only optical elements in which the inequality is satisfied permanently or at least in the said first state for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, so that manufacturing-related higher tolerance values τ are at least partially compensated on account of the then correspondingly increased relation of the densities N 1 and N 2 of the transition dipole moments.It is preferably the case that the inequality is fulfilled permanently or at least in the said first state for the ratio of the respective densities N 1 and N 2 of the transition dipole moments.The greater the relative proportion of the remaining transition dipole moments (i.e. lying outside the described tolerance range) with the density of N 2 the poorer the general transmission of the optical element in general in all directions.The relationship of the densities N 1 and N 2 can be measured indirectly, for example, by means of DLS (dynamic light scattering) methods, which are known in the prior art and therefore are not explained here in any more detail. While observing the inequality according to the invention, first optical elements suitable for the task can then be evaluated and selected accordingly after their production.A further object of the invention is achieved according to the invention by a first switchable light filter comprisinga first optical element as described above,means for optionally generating a first electric field EF1 or a second electric field EF2, e.g. ITO layers, with a connected signal generator,a liquid crystal layer disposed behind or in front of the first optical element in the viewing direction, on which the first electric field EF1 or the second electric field EF2 acts and which influences the polarization state of light passing through it in dependence thereon,when the liquid crystal layer is arranged before the first optical element in the viewing direction, a first linear polarization filter arranged before the liquid crystal layer in the viewing direction,such that the transmission properties of the first switchable light filter differ between a first operating mode B 1, in which the first electric field EF 1 is present, and a second operating mode B 2, in which the second electric field EF 2 is present.In the case that the transition dipole moments can be changed, for example via what are known as guest-host liquid crystal cells, such guest-host liquid crystal cells can directly correspond to the aforementioned liquid crystal layer, but do not have to.In a preferred embodiment, the light penetrating the liquid crystal layer is transmitted essentially unchanged when the second electric field EF 2 is present, while the incident light is circularly or elliptically polarized or the polarization of the light is rotated through 90° when the first electric field EF 1 is present. In this context, essentially means that the orientation of the liquid crystal molecules is determined at the boundary surfaces by electric fields and surface-induced forces, so that the liquid crystal molecules are not ideally aligned, which leads to an undesired, small change in the polarization.For embodiments with TN liquid crystal cells, the alignment of the liquid crystal molecules typically differs by 90° at the large areas delimiting the liquid crystal layer. Such alignment is aided by PMI or PVA and additionally mechanical or optical processing of the surface. Furthermore, for TN liquid crystal layers, it is generally the case that, when switching between the electric fields EF 1 and EF 2, the plurality of liquid crystals in the liquid crystal layer are rotated 75 to 90 degrees out of the plane. In the case of IPS and FFS liquid crystal layers, the rotations of the LC molecules are less than 45°, typically about 25° to 40° in-plane.The first electric field EF 1 can have, for example, a field strength not equal to 0 V / μm, for example of the order of magnitude of 1 V / μm, for example as a square wave having 1 kHz, while the second electric field EF 2 has, for example, a field strength of 0 V / μm. However, the configuration can also be exactly the reverse, or also in such a way that both fields EF 1 and EF 2 are not field-free.When the liquid crystal layer is disposed behind the first optical element in the viewing direction, the liquid crystal layer is preferably incident with linearly polarized light or elliptically polarized light in which the ratio of the major to minor half axes is at least 4:1 (preferably at least 5:1 or more). This can be achieved, for example, by linear polarization filters in the light path, or also by λ / 4 layers in the case of the use of circularly polarized light.Advantageously, the first optical element (and each further such optical element, if present) and / or the liquid crystal layer is divided into a plurality of separately switchable segments, so that a local changeover between the respective possible operating states is made possible.In a further embodiment, the switchable light filter comprises at least two first optical elements, wherein optionally a birefringent layer is arranged between at least two such first optical elements. In addition, the at least two first optical elements could optionally have different thicknesses of the layers respectively containing the plurality of light-absorbing transition dipole moments, but do not have to.A further object of the invention is furthermore achieved by a second switchable light filter, comprisinga second optical element, in turn comprising• a plurality of light-absorbing transition dipole moments, wherein the corresponding absorption cross sections are denoted by σ abs and optionally apply to one wavelength, to a plurality of selected wavelengths or, on average, to a plurality of wavelengths in the light spectrum visible to humans,• wherein the transition dipole moments are formed by one or more dichroic dyes and are each contained in a guest-host arrangement in a second liquid crystal layer,• wherein, furthermore, the plurality of transition dipole moments is oriented parallel to a second preferred direction selectable for the second optical element or varies around the latter at least in a first state with a tolerance τ with τ≤20°, preferably with τ≤18° (here too the tolerance τ applies in both directions, that is to say ±τ), wherein the second preferred direction is arranged at an angle α to the mean perpendicular of the second optical element, wherein the angle α is measured in a selectable second plane which contains the said mean perpendicular, and wherein the transition dipole moments oriented parallel to the second preferred direction with a tolerance τ have a density N 1 and the remaining transition dipole moments have a density of N 2,• so that light which is incident into the second optical element is transmitted or at least partially absorbed at the location of incidence of light, depending on its direction of incidence with respect to the second optical element, its state of polarization and the state of the second liquid crystal layer,means for selectively generating at least one first electric field EF1 or one second electric field EF2, wherein the respective electric field acts on the second liquid crystal layer, so that the transmission properties of the second switchable light filter differ between a first operating mode B1, in which the first electric field EF1 is applied and the second liquid crystal layer is in said first state, and a second operating mode B2, in which the second electric field EF2 is applied and the second liquid crystal layer is in a second state,wherein the inequality is fulfilled on the second optical element at least in said first state for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, so that higher tolerance values τ are at least partially compensated due to the then correspondingly increased relationship of the densities N 1 and N 2 of the transition dipole moments, whereby the absorption of p-polarized light which is incident on the second optical element at angles of greater than 45°, measured in the first plane, is amplified.The means for optionally generating at least one first electric field EF 1 or one second electric field EF 2 can also advantageously be designed such that different electric fields EF 1, EF 2 (or optionally electric fields EF 3, EF 4, etc.) are present at different positions on the liquid layer at the same time. A position-dependent partial changeover of the operating modes of the second switchable light filter can thus be achieved.For the second switchable light filter, it can optionally be true that the inequality is fulfilled on its second optical element at least in said second state for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, whereby the absorption of p-polarized light incident on the second optical element at angles of greater than 45° measured in the first plane is attenuated. In this way, a higher transmission is achieved along the first plane in said second state.Advantageously, both in the case of the first switchable light filter and in the case of the second switchable light filter, the potential differences between the electrodes for generating the first electric field EF 1 and the second electric field EF 2 differ by at least 2 volts. Additional operating modes B 3, B 4, etc. with electric fields EF 3, EF 4, etc. deviating from the electric fields EF 1 and EF 2 can be explicitly provided. In addition, the operating modes B 1, B 2, etc. can also differ locally on the switchable light filter, as already indicated above.The invention also comprises a first illumination device in a first configuration for a screen which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in a viewing angle range restricted with respect to the free viewing mode for a viewera two-dimensionally extended backlight which emits light and which is optionally constructed so as to be directly illuminated (for example with an LED matrix), anda first or second switchable light filter according to the invention arranged in front of the backlight in the viewing direction as described above.This additionally also comprises a first screen in a first embodiment, which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in a viewing angle range restricted to the free viewing mode for a viewera first illumination device as described above,and if no first linear polarization filter is arranged in the first or second switchable light filter of the first lighting device, a second linear polarization filter arranged before the backlight in the viewing direction, whereby light which originates from the backlight and passes through the second linear polarization filter is restricted in its propagation directions, anda transmissive image display device which is arranged upstream of the first or second switchable light filter in the viewing direction,wherein the second electric field EF 2 is present in the operating mode B 2 and wherein the first electric field EF 1 is present in the operating mode B 1.It is preferred that the first or second linear polarization filter is arranged in the transmissive image display device or is a part of it.The invention further comprises a second screen which can be operated in at least two modes of operation B 1 for a free viewing mode and B 2 for a restricted viewing mode in which light is emitted in a viewing angle range restricted with respect to the free viewing mode for a viewer, comprisinga picture display device, wherein basically any type of picture display device is possible here, for example LC panel, OLED, microLED and others,a first or second switchable light filter according to the invention as described further above,wherein the second electric field EF 2 is present in the operating mode B 2 and wherein the first electric field EF 1 is present in the operating mode B 1.In this case, it can optionally be the case that the first or second switchable light filter is attached to the image reproduction device subsequently and / or reversibly by a user. In this case, a light filter can be marketed as a so-called "after-market product".For some first and second screens described above, it may be advantageous when used in cars if a third optical element is arranged in front of the transmissive image display device in the viewing direction, which comprises:• A plurality of light absorbing transition dipole moments; here the dye mass density is greater than 1% or even greater than 10%.• wherein the plurality of transition dipole moments is oriented parallel to a third preferred direction selectable for the third optical element or fluctuates around the latter at least in a first state with a tolerance of at most 20° (alternatively 10°), wherein the third preferred direction is arranged at an angle α 1 to the mean perpendicular of the third optical element (here, for example, α 1= 0 °, α 1= ±2° or magnitude (α 1) > 2°) can apply, wherein the angle α 1 is measured in a selectable third plane which contains the said mean perpendicular; the third plane is preferably perpendicular to the first plane of the first optical element,• So that light which is incident into the third optical element is transmitted or at least partially absorbed as a function of its direction of incidence with respect to the third optical element and its state of polarization.This latter embodiment advantageously ensures reduced transmission in the vertical direction and can thus reduce or completely avoid reflections of image contents displayed on the screen on the windshield in the vehicle.The invention further comprises a second illumination device for a screen in a second configuration, which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in an angle range restricted with respect to the free viewing mode, comprisinga two-dimensionally extended backlight which emits light in a restricted angular range and which is optionally constructed so as to be directly luminous, anda plate-shaped light guide which is situated in front of the background illumination in the viewing direction and has decoupling elements on at least one of the large surfaces and / or within its volume,lighting means arranged laterally on at least one narrow side of the light guide, andoptionally a linear polarization filter,a first or second switchable light filter according to the invention, as described above, arranged in front of the background illumination in the viewing direction (this also includes a position in front of a screen with which the illumination device is used),wherein the backlighting is switched on and the lighting means are switched off in the operating mode B 2, and wherein at least the lighting means are switched on in the operating mode B 1, andwherein the second electric field EF 2 is present in the operating mode B 2 and wherein the first electric field EF 1 is present in the operating mode B 1.The term "restricted angular range" is understood to mean the scope of the invention, and in particular with regard to backlighting, that the corresponding luminance is concentrated to at least 80% or 90% in a defined angular range, while outside the defined restricted angular range there may certainly also be residual light, which is generally technically conditional. Ideally, such residual light is minimal and decreases toward larger angles. In order to achieve a particularly strong minimization, a corresponding light filter is used in addition to the background illumination emitting light in a restricted angular range. This also applies to the variant described below with a light guide which predominantly emits or couples light into a restricted angular range. In contrast to this embodiment of the invention, luminance curves of backlights over in particular horizontal (optionally also vertical) angle ranges are typically bell-shaped, wherein, however, no true concentration of the luminance around a smaller angle range needs to be present.The invention furthermore comprises a third lighting device for a screen which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in an angle range restricted with respect to the free viewing mode,a two-dimensionally extended background illumination which emits light in an unlimited angle range and which is optionally constructed so as to be directly illuminated (for example by means of a locally dimmable LED matrix illumination unit), anda plate-shaped light guide which is situated in front of the backlighting in the viewing direction and has output elements on at least one of the large surfaces and / or within its volume, wherein the said output elements output light coupled laterally into at least one narrow side of the light guide predominantly (i.e. to over half, preferably to over 80% or 90%) into a restricted angular range,lighting means arranged laterally on at least one narrow side of the light guide, andoptionally a linear polarization filter,a first or second switchable light filter according to the invention, as described above, arranged in the viewing direction before the background illumination, preferably before the light guide,wherein in operating mode B 2 the backlighting is turned off and the lighting means are turned on, and wherein in operating mode B 1 at least the backlighting is turned on (in addition, the lighting means can optionally also be turned on), andwherein the second electric field EF 2 is present in the operating mode B 2 and wherein the first electric field EF 1 is present in the operating mode B 1.The invention further comprises a fourth lighting device for a screen which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode in which light is emitted in an angle range restricted with respect to the free viewing mode, comprisinga two-dimensionally extended backlight which emits light into a restricted angular range and contains a first optical element as described above for restricting the angular range of said light, and which is optionally constructed so as to be directly luminous, anda plate-shaped light guide which is situated in front of the background illumination in the viewing direction and has decoupling elements on at least one of the large surfaces and / or within its volume,lighting means arranged laterally on at least one narrow side of the light guide, andwherein in operating mode B 2 the backlighting is switched on and the lighting means are switched off, and wherein in operating mode B 1 at least the lighting means are switched on.The first, second, third and / or fourth illumination devices are advantageously 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 B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in an angle range restricted with respect to the free viewing mode.It is advantageous that the transition dipole moments of a first optical element (or optionally, if present, also of a further first or second or third optical element) are formed as at least one, at least two, at least three or more dichroic dye(s) which are mixed with the liquid crystals in a guest-host arrangement. For permanent transition dipole moments, the liquid crystals can preferably be fixed via a curing process.In contrast, the transition dipole moments can also be embedded in a liquid crystal layer in a non-fixed manner as a guest-host arrangement, with the result that the alignment and / or amount of the transition dipole moments can be varied between the first and at least one second state depending on the influence of this liquid crystal layer.The dichroic dye molecules are generally aligned parallel to the liquid crystal molecules.Alternatively, the first optical element may be formed as a laminate of layers of polymeric film polarizers.By means of the aforementioned embodiments, it is also ensured that the first optical element is non-periodically constructed in its structure. This is very advantageous because there is then no risk for artifacts such as Moire effects in the interaction with pixel structures of screens.The first (or second) preferred direction can, for example, each enclose an angle between 0° and 45° to a surface normal of the first (or second) optical element. In addition, it is possible for the first or second preferred direction to vary over the surface of the first (or second) optical element. In the sense of the invention, the average, weighted preferred direction then applies. The first and second preferred directions can also be identical or differ in their alignment only by a few degrees (maximum 10°) and both are in particular perpendicular to the relevant optical element. This is a preferred case. Depending on the application, however, it is also possible for the first and second preferred directions to differ from one another by more than 10°.Furthermore, it is possible that at least two such preferred directions differ by more than 10° in a selectable plane and / or that the respective preferred direction of a transition dipole moment is selectable depending on its position in the first or second optical element.It can also be advantageous if the first or second optical element is divided into different regions (A 1, A 2,... ) along a selectable reference line, wherein a separate preferred region direction can be selected for each region (A 1, A 2,... ), which applies to all transition dipole moments lying within a region (A 1, A 2,... ), wherein all preferred region directions are different in pairs and point toward a viewer apart from a tolerance of a maximum of ±10%. This arrangement has the advantage that the observer perceives a screen with a first or second switchable light filter as homogeneously illuminated in the restricted viewing mode.The transition dipole moment-also referred to as transition matrix element-is associated with a quantum mechanical vector size and a specific transition between an initial state-generally the ground state-and a final state-generally an excited state-of a system, i.e. an atom, molecule or solid body, and corresponds to the electrical dipole moment which is connected to this transition. The direction of the vector defines the polarization of the transition, which in turn determines how the system interacts with an electromagnetic wave having a predefined polarization; in the transition from the ground state to the excited state, light of the corresponding polarization is absorbed, for example. The magnitude of the vector corresponds to the strength of the interaction or to the probability of transition.The first (second) preferred direction corresponds to the alignment of the transition dipole moments of the first (second) optical element for a predefined propagation direction of light, in which the absorption is the same for any polarizations of the light.The first or second light filter can furthermore comprise a polarization filter which is arranged upstream or downstream of the first or the second optical element as viewed in the direction of incidence. Alternatively or additionally, a λ / 4 layer is also conceivable, for example when circularly polarized light is incident, which is converted into (substantially) linearly polarized light on account of this layer.A first exemplary production variant for a first optical element using the guest-host principle is based on mixtures of dichroic dyes or dichroic dye mixtures with liquid crystal mixtures or compounds, and comprises the following steps for production (with reference to U.S. Pat. No. 9,481,658 B2 or WO2021 / 177308A1 paragraph 37 ff)A low or non-birefringent substrate is coated with a film which determines the orientation of the molecules relative to the surface, usually parallel or perpendicular to the surface. Polymers, preferably polyvinyl alcohol or polyimides, are used for this purpose.Optional: Optical or mechanical treatment of the surfaces to improve the later quality of molecular alignment.applying the mixture of dichroic dye and thermotropic liquid crystalline compounds or polymers.The side chains are locally condensed by irradiation of light, so that they provide birefringence along the surface.An alternative, second production variant uses thermotropic, liquid-crystalline dichroic dyes (reference to JP2011-237513A) and comprises the following steps:Preparation of the corresponding dyes and addition of a polar group.applying the dye mixture and photoalignment and curing of the dye mixture by means of polarized light.The following materials are suitable for different production variants, for example, wherein this list makes no claim to completeness:as polymer substrate having little or no birefringence: preferably TAC,Dichroic substances or mixtures which are used are: dichroic dyes (preferably azo dyes) or dichroic metal nanoparticles (preferably gold, silver, copper and aluminum); these are generally individual dyes or mixtures of typically up to three different dyes in order to enable absorption over the entire spectrum,for surface treatment by alignment of dyes or liquid-crystalline substances: polymers, preferably polyvinyl alcohol or polyimides,For thermotropic liquid crystalline compounds or polymers, reference is made by way of example to JP 2011-237513A.as chemical groups for crosslinking which are bonded to the thermotropic liquid crystalline compounds or polymers (cross linking): metaacryloyl groups, epoxy groups, an oxetanyl groups, and styrene groups, preferably methacryloyl groups. Alternatively, these may be polymerizable liquid crystal compounds described in, for example, JP 6268730B2.Polymerizable liquid-crystalline dichroic dyes, for example azo dyesThe at least one dye consists of dye molecules, wherein advantageously a transition dipole or transition dipole moment is associated with each dye molecule, i.e. each dye molecule corresponds to a transition dipole or transition dipole moment. Typically, a dye has a mass fraction of at least 0.01 %, preferably from 1% to 15%, based on the material of the respective layers of the relevant optical element. In particular cases, the concentration in the case of liquid crystalline dichroic dyes can even reach 95%. The thickness of the layers is preferably in the range from 0.2 μm to 50 μm, preferably in the range from 0.5 μm to 20 μm, all boundary values in each case included. The dyes or dye mixtures for different layers within an optical element may be different, but need not be.In all embodiments of the invention, it can be true that the tolerance τ is 5°≤τ≤20° or 10°≤τ≤20°, for example τ=5° or τ=10° or τ=15° or τ=19,9°.It is furthermore advantageous if, in the case of the described image display devices (of the screens) and / or illumination devices, in each case a luminance curve is implemented which is emitted or transmitted for example at angles from 25° less than 80%, preferably less than 60%, of the luminance emitted or transmitted to the maximum in (any) direction (in particular measured horizontally from the viewer's view or in the first plane). In this way, the visibility protection is enhanced at such angle ranges because the luminance curve is multiplied by the respective transmission curve of a switchable light filter used with a described screen and / or an illumination device. In this context, it is also advantageous if, for example, such a luminance curve has approximately 50% of the maximum luminance at an angle of 45°.For certain applications in which viewers with (usually vertically) linearly polarizing sun glasses look at a screen having a first or second switchable light filter described above, it can be advantageous to arrange a λ / 4 layer in the light path in front of the screen in the viewing direction for the conversion of linearly polarized light into circularly polarized light. Depending on the state of the first liquid crystal layer in the first switchable light filter or depending on the state of the second liquid crystal layer in the second switchable light filter, circularly polarized light rotating to the left or to the right would then be generated, which is visible through said sunglasses with a reduced brightness. In the case of crossed linear polarizations of a pair of sunglasses and the light emitted from the screen, if no such λ / 4 layer is present, the situation could otherwise occur, depending on the state, that a viewer wearing such pair of sunglasses sees essentially no image.A switchable light filter, a lighting device or such a screen described above is advantageously used in a mobile device, a motor vehicle, air vehicle or watercraft, in a payment terminal or in an access system. In this case, it is possible to switch over between the aforementioned operating modes in order to protect sensitive data, i.e. to represent it in a perceptible manner for only one observer, or alternatively to represent image contents simultaneously for a plurality of observers.In principle, the performance of the invention is maintained if the above-described parameters are varied within certain limits.It is understood that the features mentioned above and those still to be explained below can be used not only in the combinations indicated, but also in other combinations or alone, without departing from the scope of the present invention.Brief Description of the DrawingsThe invention is explained in more detail below on the basis of exemplary embodiments with reference to the appended drawings, which likewise disclose features essential to the invention. These exemplary embodiments are intended merely for illustrative purposes and should not be interpreted as restrictive. For example, a description of an embodiment having a plurality of elements or components should not be construed as requiring all of these elements or components for implementation. Rather, other embodiments may also include alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of various embodiments may be combined with each other unless otherwise indicated. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary embodiments. To avoid repetitions, identical or mutually corresponding elements are denoted by identical reference symbols in different figures and are not explained multiple times. The following are shown: FIG. 1 shows a schematic diagram for the construction of a first optical element, FIGS. 2 to 5 show exemplary normalized transmission curves for the incidence of light of two different polarizations (s, p) into the first optical element, plotted against a viewing angle onto the first optical element for the respective values of N 1 / N 2, FIG. 6 ashows a comparison of exemplary normalized transmission curves for the incidence of light of two different polarizations into a first optical element, plotted on a linear scale over a viewing angle onto the first optical element for two different values of τ and with the same ratio N 1 / N 2, FIG. 6 bshows the comparison from FIG. 6 a, here on the ordinate in a logarithmic scale, FIG. 7 ashows a comparison of two further exemplary normalized transmission curves, each for the incidence of light of two different polarizations into a first optical element, plotted on a linear scale over a viewing angle onto the first optical element for two different values of the ratio N 1 / N 2 and for the same value for τ, FIG. 7 bshows the comparison from FIG. 7 a, here on the ordinate in a logarithmic scale, FIGS. 8 and 9 show further exemplary normalized transmission curves for the incidence of light of two different polarizations into the first optical element, plotted against a viewing angle onto the first optical element, wherein the inequality according to the invention is not complied with in each case, FIG. 10 shows the schematic diagram of an exemplary design of a first switchable light filter, FIG. 11 shows the schematic diagram of an exemplary design of a second switchable light filter, FIG. 12 shows the schematic diagram of an exemplary design of an illumination device in a first embodiment having a first or second switchable light filter, FIG. 13 shows the schematic diagram of an exemplary structure of a screen in a first embodiment, which can be operated in at least two operating modes B 1 and B 2, having a first or second switchable light filter, FIG. 14 shows the schematic diagram of an exemplary structure of a screen in a second embodiment, which can be operated in at least two operating modes B 1 and B 2, having a first or second switchable light filter, FIG. 15 shows the schematic diagram of an exemplary design of an illumination device in a second embodiment, having a first or second switchable light filter, FIG. 16 shows the schematic diagram of an exemplary design of an illumination device in a third embodiment, having a first or second switchable light filter, and FIG. 17 shows the schematic diagram of an exemplary design of an illumination device in a fourth embodiment, having a first optical element.DETAILED DESCRIPTION OF THE DRAWINGSThe drawings are not true to scale and merely represent schematic representations. In some cases, selected beams are displayed while a plurality of light beams are present in reality. The same applies to the representation of selected dipole moments in FIG. 1. viewing directions should, where necessary, always be assumed from above in the plane of the leaf.FIG. 1 shows a schematic diagram for constructing a first optical element 1 in a permanent (or optionally first) state. The first optical element 1 comprises• a plurality of light absorbing transition dipole moments (represented by the arrows), wherein the corresponding absorption cross sections are denoted by σ abs and optionally apply to one wavelength, to a plurality of selected wavelengths or, on average, to a plurality of wavelengths in the light spectrum visible to humans,• wherein the plurality of transition dipole moments is aligned permanently or at least in a first state with a (selectable) tolerance τ with τ ≤ 20° parallel to a first preferred direction (see dashed line) selectable for the first optical element 1 or varies around the latter (the tolerance τ applies in both directions, that is to say for the value of a respective angle plus ±τ), wherein the first preferred direction is arranged at an angle α to the mean perpendicular (see dashed-dotted line) of the first optical element 1, wherein the angle α is measured in a selectable first plane which contains the said mean perpendicular, and wherein the transition dipole moments aligned parallel to the first preferred direction with a tolerance τ have a density N 1 and the remaining transition dipole moments (i.e. lying outside the described tolerance range) have a density of N 2 ; FIG. 1 shows for a few transition dipole moments selected by way of example, with respect to which density N 1 or. N 2 each belonging to it,• so that light which is incident into the first optical element 1 is transmitted or at least partially absorbed as a function of its direction of incidence with respect to the first optical element 1 and its state of polarization,wherein the inequality is fulfilled permanently or at least in said first state for the ratio of the respective densities N 1 and N 2 of the transition dipole moments on the optical element 1, so that higher tolerance values τ are at least partially compensated on account of the then correspondingly increased relationship of the densities N 1 and N 2 of the transition dipole moments, whereby the absorption of p-polarized light which is incident on the optical element 1 at angles of greater than 45°, measured in the first plane, is enhanced.The plurality of light absorbing transition dipole moments are arranged in a layer at least 0.2 microns thick. It is also possible for a plurality of layers of transition dipole moments to be present, separated for example by OCA (optically clear adhesive) and / or substrates. The layer thicknesses of the transition dipole moments can be, for example, in the sum of all layers present from 0.2 μm to 50 μm, preferably 0.2 μm to 20 μm, particularly preferably 1 μm to 10 μm. In particular configurations, at least two layers of transition dipole moments separated by at least one substrate are present.The invention shows a way in which manufacturing-related limitations in the alignment of the transition dipole moments on the optical element 1 are to be counteracted by using only optical elements 1 in which the inequality is satisfied permanently or at least in the said first state for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, so that manufacturing-related higher tolerance values τ are at least partially compensated on account of the then correspondingly increased relation of the densities N 1 and N 2 of the transition dipole moments. As a result, not only is the absorption of p-polarized light which is incident on the optical element 1 at angles of greater than 45°, measured in the first plane, amplified, but the normalized transmission in the preferred direction or in an angle range of a few degrees is also increased beyond this.The greater the proportion of the remaining transition dipole moments (i.e. lying outside the described tolerance range) with the density of N 2 the poorer the general transmission of the optical element 1 in general, i.e. in all directions.Approximate quantitative simulations of the normalized transmission of the first optical element 1 are shown for various parameters in FIGS. 2 to 9. The transmissions were calculated independently for s- and p-polarized light incident on an optical element 1. In each of these drawings, the normalized transmission is shown with a dashed line for incident s-polarized light and with a solid line for p-polarized light. The starting point for the calculation of the transmission through an absorbing layer is the Labert-Beer law. In addition to the thickness of the layer, it also contains the product of extinction coefficient and concentration of the absorbing species. In the present case, the absorbance is approximately equalized with the absorbance because the absorbance is produced by very small molecules which scatter light only weakly. These absorption coefficients are denoted by α s and α p respectively. These coefficients correspondingly describe the absorption for s- and p-polarized light, respectively. For the ideal alignment of the molecules parallel to the first preferred direction, α s reaches the numerical value 0. This allows the propagation of s-polarized light to be described as propagating in the plane which includes the surface normal and the first preferred direction. In order to be able to describe the propagation of p-polarized light as well, α p must be modified with the aid of Malus's law. The following is assumed for the ratios in the simulation:First, FIG. 2 shows a first exemplary normalized transmission curve for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted against a viewing angle onto a first optical element 1 for a first value of N 1 / N 2= 6,66. This ratio of N 1 / N 2 satisfies the inequality mentioned, for example, for τ=1° (here, precisely speaking, N 1 / N 2 >6.16 would already suffice). It can be seen that even a relatively poor ratio of the densities of the transition dipole moments for p-polarized light, which ratio is therefore rather simple to achieve for the production of a first optical element 1, produces an absorption of more than 90% at 45°.Furthermore, FIG. 3 illustrates a second exemplary normalized transmission curve for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted against a viewing angle on the first optical element 1, for a second value of N 1 / N 2= 7 in each case. This corresponds to the limit value of said inequality for τ=10°. It can also be seen here that, therefore, even a relatively poor ratio of the densities of the transition dipole moments for p-polarized light, which ratio is therefore rather simple to achieve for the production of a first optical element 1, produces an absorption of more than 90%, for example at 45°.Furthermore, FIG. 4 shows a third exemplary normalized transmission curve for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted against a viewing angle on the first optical element for a third value of N 1 / N 2= 10 in each case. This corresponds to the limit value of said inequality for τ=19°. The tolerance τ=19°, which is significantly greater than the circumstances according to FIGS. 2 and 3, is (over)compensated here too by the density N 1 of the transition dipole moments, which density is increased in the relation. The absorption desired for p-polarized light at about 45° is improved.Moreover, FIG. 5 shows a fourth exemplary normalized transmission curve for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted against a viewing angle on the first optical element for a fourth value of N 1 / N 2=16. This corresponds to the limit value of said inequality for τ=19,99°. The remarks given above with reference to FIG. 4 apply accordingly.In FIG. 6 a, a comparison of exemplary normalized transmission curves can be seen in each case for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted on a linear scale over a viewing angle on the first optical element 1 for two different values of τ (τ=0° with thin lines and τ=10° with thick lines) and, at the same ratio in each case, N 1 / N 2= 16,7. Qualitatively, the curves for s- and p-polarized light respectively look comparable. It is evident here that the transmission for τ=10° decreases markedly compared to τ=0°, which is caused by the greater tolerance τ=10° of the alignment of the transition dipole moments with respect to the first preferred direction: poorly aligned transition dipole moments, that is to say those which have a greater tolerance, cause a greater α s.FIG. 6 bshows the comparison from FIG. 6 a, but on the ordinate on a logarithmic scale. It can be seen here that, starting from absolute values of the angle of view of more than 25°, both parameter sets have approximately the same transmission or absorption for p-polarized light, but the total transmission for τ=10° in the first preferred direction for p-polarized light and in many directions for s-polarized light is reduced by a factor of approximately 1.2, which is undesirable.In contrast, FIG. 7 ashows a comparison of exemplary normalized transmission curves for the incidence of light of two different polarizations (s, p) into a first optical element 1, plotted on a linear scale over a viewing angle onto the first optical element 1 for two different values of the ratio N 1 / N 2(16,7 with thick lines and 29.2 with thin lines) and for the same value for τ=10° in each case. It is evident here that the transmission decreases significantly for the lower ratio N 1 / N 2= 16,7 compared to N 1 / N 2= 29,2.FIG. 7 bshows the comparison from FIG. 7 a, but on the ordinate on a logarithmic scale. It can be seen here that, starting from absolute values of the angle of view of more than approximately 30°, both parameter sets have approximately the same transmission or absorption for p-polarized light, but the total transmission for N 1 / N 2= 16,7 in the first preferred direction for p-polarized light and in many directions for s-polarized light is reduced by a factor of approximately 1.2, which is undesirable. The comparison of the drawings FIGS. 6 band 7 btherefore allows the conclusion that the absorption for p-polarized light can be achieved even at angles with absolute amounts of less than approximately 30° by a lower tolerance τ rather than by a higher ratio of the densities N 1 / N 2.Finally, FIGS. 8 and 9 represent further exemplary normalized transmission curves in each case for the incidence of light of two different polarizations (p, s) into a first optical element 1, plotted against a viewing angle on the first optical element 1, wherein the inequality according to the invention is in each case not complied with: the absorption for p-polarized light is in no way capable here of generating a transmission of less than 10% at angles of, for example, ±45°. In FIG. 8, the ratio N is 1 / N is 2=3 and in FIG. 9, N is 1 / N is 2=1.5.Furthermore, FIG. 10 shows the schematic diagram of an exemplary design of a first switchable light filter 5. This includesa first optical element 1 as described above,means (not shown in the drawing) for selectively generating a first electric field EF1 or a second electric field EF2, for example ITO layers with a connected signal generator,a liquid crystal layer 3 arranged in front of the first optical element 1 in the viewing direction (here), on which the first electric field EF 1 or the second electric field EF 2 acts and which influences the polarization state of light passing through it as a function thereof,since the liquid crystal layer 3 is arranged upstream of the first optical element 1 in the viewing direction, a first linear polarization filter X is arranged upstream of the liquid crystal layer 3 in the viewing direction,such that the transmission properties of the first switchable light filter 5 differ between a first operating mode B 1, in which the first electric field EF 1 is present, and a second operating mode B 2, in which the second electric field EF 2 is present.In the case that the transition dipole moments can be changed, for example via what are known as guest-host liquid crystal cells, such guest-host liquid crystal cells can directly correspond to the aforementioned liquid crystal layer, but do not have to.In a preferred embodiment, the light penetrating the liquid crystal layer is transmitted essentially unchanged when the first electric field EF 1 is present, while the incident light is circularly or elliptically polarized or the polarization of the light is rotated by (essentially) 90° when the second electric field EF 2 is present.When the liquid crystal layer 3 is disposed behind the first optical element 1 in the viewing direction, the liquid crystal layer 3 is preferably incident with linearly polarized light or elliptically polarized light in which the ratio of the major to minor half axes is at least 4:1 (preferably at least 5:1 or more). This can be achieved, for example, by linear polarization filters in the light path, or also by λ / 4 layers in the case of the use of circularly polarized light.Furthermore, FIG. 11 shows the schematic diagram of an exemplary design of a second switchable light filter 5 a, comprisinga second optical element 2, in turn comprising• a plurality of light-absorbing transition dipole moments, wherein the corresponding absorption cross sections are denoted by σ abs and optionally apply to one wavelength, to a plurality of selected wavelengths or, on average, to a plurality of wavelengths in the light spectrum visible to humans,• wherein the transition dipole moments are formed by one or more dichroic dyes and are each contained in a guest-host arrangement in a second liquid crystal layer 3 a,• wherein, furthermore, the plurality of transition dipole moments is oriented parallel to or varies around a first preferred direction that can be selected for the second optical element 2 at least in a first state with a tolerance τ with τ≤20° (here too the tolerance τ applies in both directions, i.e. ±τ), wherein the second preferred direction is arranged at an angle α to the mean perpendicular of the second optical element 2, wherein the angle α is measured in a selectable second plane that contains the said mean perpendicular, and wherein the transition dipole moments oriented parallel to the second preferred direction with a tolerance τ have a density N 1 and the remaining transition dipole moments have a density of N 2 (FIG. 1 can be used for explaining the geometric relationships in the transferred sense; The optical element 1 would correspond to the second liquid crystal layer 3a),• so that light which is incident into the second optical element 2 is transmitted or at least partially absorbed at the location of incidence, depending on its direction of incidence with respect to the second optical element 2, its state of polarization and the state of the second liquid crystal layer 3 a,means (not shown in the drawing) for selectively generating at least one first electric field EF1 or one second electric field EF2, wherein the respective electric field acts on the second liquid crystal layer 3a, so that the transmission properties of the second switchable light filter 5a differ between a first operating mode B1, in which the first electric field EF1 is applied and the second liquid crystal layer 3a is in said first state, and a second operating mode B2, in which the second electric field EF2 is applied and the second liquid crystal layer 3a is in a second state,wherein the inequality is fulfilled on the second optical element 2 at least in said first state for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, so that higher tolerance values τ are at least partially compensated due to the then correspondingly increased relationship of the densities N 1 and N 2 of the transition dipole moments, whereby the absorption of p-polarized light which is incident on the second optical element 2 at angles of greater than 45°, measured in the first plane, is enhanced.The means for optionally generating at least one first electric field EF 1 or one second electric field EF 2 can also advantageously be designed such that different electric fields EF 1, EF 2 (or optionally electric fields EF 3, EF 4, etc.) are present at different positions on the liquid layer 3 aat the same time. A position-dependent partial changeover of the operating modes of the second switchable light filter 5 acan thus be achieved.For the second switchable light filter 5 a, it can optionally be applicable that the inequality is fulfilled on its second optical element 2, at least in said second state, for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, whereby the absorption of p-polarized light which is incident on the second optical element 2 at angles of greater than 45°, measured in the first plane, is attenuated. In this way, a higher transmission is achieved along the first plane in said second state.Advantageously, both in the case of the first switchable light filter 5 and in the case of the second switchable light filter 5 a, the potential differences between the electrodes for generating the first electric field EF 1 and the second electric field EF 2 differ by at least 2 volts. Additional operating modes B 3, B 4, etc. with electric fields EF 3, EF 4, etc. deviating from the electric fields EF 1 and EF 2 can be explicitly provided. In addition, the operating modes B 1, B 2, etc. can also differ locally on the switchable light filter, as already indicated above.FIG. 12 shows the schematic diagram of an exemplary design of an illumination device for a screen which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in a viewing angle range restricted with respect to the free viewing mode for a viewer, in a first configuration having a first or second switchable light filter, comprisinga two-dimensionally extended backlight 8 which emits light and which is optionally constructed so as to be directly luminous (for example with an LED matrix), anda first or second switchable light filter 5, 5 aarranged in front of the backlight 8 in the viewing direction as described above.Furthermore, FIG. 13 shows the schematic diagram of an exemplary construction of a screen in a first embodiment, which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted into a viewing angle range restricted with respect to the free viewing mode for a viewer, having a first or second switchable light filter 5, 5 a, which comprisesa first illumination device as described above,and if no first linear polarization filter is arranged in the first or second switchable light filter 5, 5 aof the first lighting device, a second linear polarization filter P arranged in front of the backlight 8 in the viewing direction, whereby light which originates from the backlight 8 and passes through the second linear polarization filter P is restricted in its propagation directions, anda transmissive image display device 11 which is arranged upstream of the first or second switchable light filter 5, 5a in the viewing direction,wherein the second electric field EF 2 is present in the operating mode B 2 and wherein the first electric field EF 1 is present in the operating mode B 1.It is preferred that the first or second linear polarization filter P is arranged in or is part of the transmissive image display device 11.Furthermore, FIG. 14 shows the schematic diagram of an exemplary structure of a screen in a second embodiment, which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in a viewing angle range restricted with respect to the free viewing mode for a viewera picture display device 12, wherein basically any type of picture display device 12 is possible here, for example LC panel, OLED, microLED and others,a first or second switchable light filter 5, 5a according to the invention as described further above, upstream of the image display device 12 in the viewing direction,wherein the second electric field EF 2 is present in the operating mode B 2 and wherein the first electric field EF 1 is present in the operating mode B 1.Furthermore, FIG. 15 shows the schematic diagram of an exemplary design of an illumination device for a screen in a second embodiment, which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in an angle range restricted with respect to the free viewing mode, in a second embodiment, comprising a first or second switchable light filter 5, 5 aa two-dimensionally extended backlight 8b which emits light in a restricted angular range (indicated by the 2 nearly parallel exemplary beams) and which is optionally constructed so as to be directly luminous, anda plate-shaped light guide 9 which is situated in front of the backlight 8 bin the viewing direction and has decoupling elements on at least one of the large surfaces and / or within its volume,lighting means 10 arranged laterally on at least one narrow side of the light guide, andoptionally a linear polarization filter (not shown in the drawing),a first or second switchable light filter 5, 5a according to the invention, as described above, arranged in front of the backlight 8b in the viewing direction (this also includes a position in front of a screen with which the illumination device is used),wherein in operating mode B 2 the backlight 8 bis switched on and the lighting means 10 are switched off, and wherein in operating mode B 1 at least the lighting means 10 are switched on, so that the light guide 9 emits light in a non-restricted angular range, which is indicated in FIG. 15 with the exemplary three beams which are not approximately parallel, andwherein the second electric field EF 2 is present in the operating mode B 2 and wherein the first electric field EF 1 is present in the operating mode B 1.The term "restricted angular range" is understood to mean within the scope of the invention, and in particular with regard to the backlighting or the screen, that the corresponding luminance is concentrated to at least 80% or 90% in a defined angular range, while outside the defined restricted angular range there may certainly also be residual light, which is generally technically conditional. Ideally, such residual light is minimal and decreases toward larger angles. In order to achieve a particularly strong minimization, a corresponding light filter is used in addition to the background illumination emitting light in a restricted angular range. This also applies to the variant described below with a light guide which predominantly emits or couples light into a restricted angular range. In contrast to this embodiment of the invention, luminance curves of backlights over in particular horizontal (optionally also vertical) angle ranges are typically bell-shaped, wherein, however, no true concentration of the luminance around a smaller angle range needs to be present.FIG. 16 shows the schematic diagram of an exemplary design of an illumination device for a screen, which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in an angle range restricted compared to the free viewing mode, in a third embodiment, comprising a first or second switchable light filter 5, 5 aa two-dimensionally extended backlight 8c which emits light in a non-restricted angular range (which is indicated in FIG. 16 with the exemplary three beams which are not approximately parallel), and which is optionally constructed to be directly illuminated (e.g. by means of a locally dimmable LED matrix lighting unit), anda plate-shaped light guide 9 clocated in front of the backlight 8 cin the viewing direction, which light guide has output elements on at least one of the large surfaces and / or within its volume, wherein the said output elements output light input laterally into at least one narrow side of the light guide 9 cin a restricted angular range predominantly (i.e. to over half, preferably to over 80% or 90%) (see two nearly parallel beams),lighting means 10 arranged laterally on at least one narrow side of the light guide 9c, anda first or second switchable light filter 5, 5 aaccording to the invention, which is arranged in front of the backlight 8 cin the viewing direction, preferably in front of the light guide 9 c, as described above,wherein in operating mode B 2 the backlighting 9 cis switched off and the lighting means 10 are switched on, and wherein in operating mode B 1 at least the backlighting 8 cis switched on (in addition optionally the lighting means 10 can also be switched on), andwherein the second electric field EF 2 is present in the operating mode B 2 and wherein the first electric field EF 1 is present in the operating mode B 1.Finally, FIG. 17 gives the schematic diagram of an exemplary design of an illumination device for a screen which can be operated in at least two operating modes B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in an angle range restricted with respect to the free viewing mode, in a fourth configuration, comprising a first optical element 1a two-dimensionally extended backlight 8b which emits light in a restricted angular range (see two nearly parallel beams) and which contains a first optical element 1 as described above for restricting the angular range and which is optionally constructed so as to be directly luminous, anda plate-shaped light guide 9 which is situated in front of the backlight 8 bin the viewing direction and has decoupling elements on at least one of the large surfaces and / or within its volume,lighting means 10 arranged laterally on at least one narrow side of the light guide 9, andwherein in operating mode B 2 the backlight 8 bis switched on and the lighting means 10 are switched off, and wherein in operating mode B 1 at least the lighting means 10 are switched on, so that the light guide 9 emits light in a non-restricted angular range, which is indicated in FIG. 17 with the exemplary three beams which are not approximately parallel.The first, second, third and / or fourth illumination devices are advantageously 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 B 1 for a free viewing mode and B 2 for a restricted viewing mode, in which light is emitted in an angle range restricted with respect to the free viewing mode.It is advantageous that the transition dipole moments of a first optical element 1 (or optionally, if present, also of a further first or second optical element 1, 2) are formed as one or more dichroic dye(s) which are mixed with the liquid crystals in a guest-host arrangement. For permanent transition dipole moments, the liquid crystals can preferably be fixed via a curing process.In contrast, the transition dipole moments can also be embedded in a liquid crystal layer in a non-fixed manner as a guest-host arrangement, with the result that the alignment and / or amount of the transition dipole moments can be varied between the first and at least one second state depending on the influence of this liquid crystal layer.The dichroic dye molecules are generally aligned parallel to the liquid crystal molecules.By means of the aforementioned embodiments, it is also ensured that the first optical element is non-periodically constructed in its structure. This is very advantageous because there is then no risk for artifacts such as Moire effects in the interaction with pixel structures of screens.The first (or second) preferred direction can, for example, each enclose an angle between 0° and 45° to a surface normal of the first (or second) optical element. In addition, it is possible for the first preferred direction to vary over the surface of the first (or second) optical element 1, 2. In the sense of the invention, the average, weighted preferred direction then applies.The first or second switchable light filter 5, 5 acan furthermore comprise a polarization filter which is arranged upstream or downstream of the first or the second optical element 1, 2 as seen in the direction of incidence. Alternatively or additionally, a λ / 4 layer is also conceivable, for example when circularly polarized light is incident, which is converted into (substantially) linearly polarized light on account of this layer.The at least one dye consists of dye molecules, wherein advantageously a transition dipole or transition dipole moment is associated with each dye molecule, i.e. each dye molecule corresponds to a transition dipole or transition dipole moment. Typically, a dye has a mass fraction of at least 0.01 %, preferably from 1% to 15%, based on the material of the respective layers of the relevant optical element. In particular cases, the concentration in the case of liquid crystalline dichroic dyes can even reach 95%. The thickness of the layers is preferably in the range from 0.2 μm to 50 μm, preferably in the range from 0.5 μm to 20 μm, all boundary values in each case included. The dyes or dye mixtures for different layers within an optical element may be different, but need not be.The invention achieves the stated object: An optical element has been described in which light which is incident into the optical element is transmitted or partially or completely absorbed depending on its direction of incidence and its polarization properties-not primarily but depending on its position. Production-related alignment tolerances are hereby counteracted by the optical effect. Switchable light filters which use the optical element according to the invention influence the transmission of light depending on the angle-optionally with respect to a seated or standing observer-wherein it is possible to switch between at least two operating states. In this case, in particular, the transmission behavior can be switched over for specific directions. Further disclosed are lighting devices and screens using switchable light filters.The invention described above can be used in cooperation with an image reproduction device advantageously wherever confidential data is displayed and / or input, such as for example in the case of PIN input or for data display to automated teller machines or payment terminals or for password input or for reading e-mails on mobile devices. As described above, the invention can also be used in a passenger car in order to optionally reserve disruptive image contents to the driver or passenger.List of reference characters1 first optical element 2 second optical element 3 first liquid crystal layer 3 asecond liquid crystal layer 5 first switchable light filter 5 asecond switchable light filter 8 backlight 8 b backlight 8 c backlight 9 light guide 10 illuminant 11, 12 image display device P, X polarization filterReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedU.S. Pat. No. 6,765,550 B2

[0004] U.S. Pat. No. 5,993,940 A

[0005] WO 2012 / 033583 A1

[0006] US 2012 / 0235891 A1

[0007] US 2013 / 0308185 A1

[0008] WO 2015 / 121398 A1

[0009] U.S. Pat. No. 9,481,658 B2

[0054] WO 2021 / 177308A1

[0054] JP 2011-237513A [0055, 0056]JP 6268730B2

[0056]

Claims

First optical element (1) comprising • a plurality of light-absorbing transition dipole moments, wherein the corresponding absorption cross sections are denoted by σ abs and optionally apply to a wavelength, to a plurality of selected wavelengths or, on average, to a plurality of wavelengths in the light spectrum visible to humans, • wherein the plurality of transition dipole moments is aligned permanently or at least in a first state with a tolerance τ with τ ≤ 220° parallel to a first preferred direction which can be selected for the first optical element (1) or varies around the latter, wherein the first preferred direction is arranged at an angle α to the perpendicular bisector of the first optical element (1), wherein the angle α is measured in a selectable first plane which contains the said perpendicular bisector, and wherein the transition dipole moments oriented parallel to the first preferred direction with a tolerance τ have a density N 1 and the remaining transition dipole moments have a density of N 2 • so that light which is incident into the first optical element (1) is transmitted or at least partially absorbed as a function of its direction of incidence with respect to the first optical element (1) and its state of polarization, characterized in that, that the inequality N 1 / N 2 >10-3 ⋅ log 10 [20°-τ 1°] is fulfilled on the optical element (1) permanently or at least in said first state for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, so that higher tolerance values τ are at least partially compensated on account of the then correspondingly increased relationship of the densities N 1 and N 2 of the transition dipole moments, whereby the absorption of p-polarized light which is incident on the optical element (1) at angles of greater than 45°, measured in the first plane, is increased.First switchable light filter (5) comprising - a first optical element (1) according to claim 1, - means for selectively generating a first electric field (EF1) or a second electric field (EF2), - a liquid crystal layer (3) arranged behind or in front of the first optical element (1) in the viewing direction, on which the first electric field (EF1) or the second electric field (EF2) acts and which depending thereon influences the polarization state of light passing through it, - when the liquid crystal layer (3) is arranged in front of the first optical element (1) in the viewing direction, a first linear polarization filter (X) arranged in front of the liquid crystal layer (3) in the viewing direction, - so that the transmission properties of the first switchable light filter (5) between a first operating mode B1, in which the first electric field (EF 1) is applied and a second operating mode B 2 in which the second electric field (EF 2) is applied are different.Second switchable light filter (5a) comprising - a second optical element (2), in turn comprising • a plurality of light-absorbing transition dipole moments, wherein the corresponding absorption cross sections are denoted by σ abs and optionally apply to a wavelength, to a plurality of selected wavelengths or, on average, to a plurality of wavelengths in the light spectrum visible to humans, • wherein the transition dipole moments are formed by one or more dichroic dyes and are each contained in a second liquid crystal layer (3a) in a guest-host arrangement, • wherein furthermore the plurality of transition dipole moments is oriented parallel to a second preferred direction selectable for the second optical element (2) or varies around it at least in a first state with a tolerance τ where τ≤20°, wherein the second preferred direction is arranged at an angle α to the perpendicular bisector of the second optical element (2), wherein the angle α is measured in a selectable second plane which contains said perpendicular bisector, and wherein the transition dipole moments aligned parallel to the second preferred direction with a tolerance τ have a density N 1 and the remaining transition dipole moments have a density of N 2 • so that light which is incident into the second optical element (2) is transmitted or at least partially absorbed as a function of its direction of incidence with respect to the second optical element (2), its state of polarization and the state of the second liquid crystal layer (3a), - means for the selective generation of at least one first electric field (EF1) or one second electric field (EF2), wherein the respective electric field acts on the second liquid crystal layer (3a) such that the transmission properties of the second switchable light filter (5a) differ between a first operating mode B1 in which the first electric field (EF1) is present and the second liquid crystal layer (3a) is in said first state and a second operating mode B2 in which the second electric field (EF2) is present and the second liquid crystal layer (3a) is in a second state, characterized in that, that on the second optical element (2), at least in said first state, for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, the inequality N 1 / N 2 >10-3 ⋅ log 10 [(20° - τ) 1°] is fulfilled, so that higher tolerance values τ are at least partially compensated due to the then correspondingly increased relationship of the densities N 1 and N 2 of the transition dipole moments, whereby the absorption of p-polarized light incident on the second optical element (2) at angles of greater than 45°, measured in the first plane, is enhanced.Second switchable light filter (5a) according to claim 3, characterized in that on the second optical element (2), at least in said second state, for the ratio of the respective densities N 1 and N 2 of the transition dipole moments, the inequality N 1 / N 2 ≤ 10 - 3 ⋅ log 10 [( 20° - τ ) 1° ] is fulfilled, thereby attenuating the absorption of p-polarized light incident on the second optical element (2) at angles greater than 45° measured in the first plane.First switchable light filter (5) according to claim 2 or second switchable light filter (5a) according to claim 3 or 4, characterised in that the potential differences between the electrodes for generating the first electric field (EF1) and the second electric field (EF2) differ by at least 2 volts.First switchable light filter (5) according to claim 2, characterised in that the first optical element (1) and / or the liquid crystal layer (3) is divided into several, separately switchable segments, so that a local switching capability between the respective possible operating states is made possible.Lighting device for a screen which can be operated in at least two operating modes B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is emitted in a viewing angle range restricted to a viewer compared to the free viewing mode, comprising - a two-dimensionally extended backlight (8) which emits light and which is optionally constructed so as to be directly luminous, and - a first switchable light filter (5) according to Claim 2 arranged in front of the backlight (8) in the viewing direction or a second switchable light filter (5a) according to Claim 3 or 4.Screen which can be operated in at least two modes of operation B1 for a free viewing mode and B2 for a restricted viewing mode in which light is emitted in a viewing angle range restricted with respect to the free viewing mode for a viewer, comprising - an illumination device according to Claim 7, - if a first linear polarization filter (X) is not arranged in the first or second switchable light filter (5, 5a) of the illumination device, a second linear polarization filter (P) arranged in front of the backlight (8) in the viewing direction, as a result of which light which originates from the backlight and passes through the second linear polarization filter (P) is restricted in its propagation directions, and - a transmissive image reproduction device (11) which is arranged in front of the first or second switchable light filter (5, 5a) in the viewing direction, wherein the second electric field (EF2) is present in the operating mode B2 and wherein the first electric field (EF1) is present in the operating mode B1.A display screen according to claim 7, characterized in that the first or second linear polarization filter (P, X) is arranged in or is part of the transmissive image display device (11).A screen which can be operated in at least two operating modes B1 for a free viewing mode and B2 for a restricted viewing mode in which light is emitted in a viewing angle range restricted with respect to the free viewing mode for a viewer, comprising - an image reproduction device (12), - a switchable first light filter (5) according to claim 2 or a second light filter (5a) according to claim 3 or 4 in the viewing direction upstream of the image reproduction device (12), - wherein the second electric field (EF2) is present in the operating mode B2 and wherein the first electric field (EF1) is present in the operating mode B1.Lighting device for a screen which can be operated in at least two operating modes B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is emitted in an angle range restricted with respect to the free viewing mode, comprising - a surface-like extended background lighting (8b) which emits light in a restricted angle range and which is optionally constructed so as to be directly luminous, and - a plate-shaped light guide (9) which is situated in front of the background lighting (8b) in the viewing direction and has output coupling elements on at least one of the large surfaces and / or within its volume, - lighting means (10) arranged laterally on at least one narrow side of the light guide (9), and - optionally a linear polarization filter (P), - a first switchable light filter (5) according to claim 2 arranged in front of the backlight (8b) in the viewing direction or a second switchable light filter (5a) according to claim 3 or 4, - wherein the backlight (8b) is switched on and the lighting means (10) are switched off in the operating mode B2, and wherein at least the lighting means (10) are switched on in the operating mode B1, and - wherein the second electric field (EF2) is applied in the operating mode B2 and wherein the first electric field (EF1) is applied in the operating mode B1.Lighting device for a screen which can be operated in at least two operating modes B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is emitted in an angle range restricted with respect to the free viewing mode, comprising - a surface-like extended background lighting (8c) which emits light in an unrestricted angle range and which is optionally constructed so as to be directly luminous, and - a plate-shaped light guide (9c) which is situated in front of the background lighting (8c) in the viewing direction and has output elements on at least one of the large surfaces and / or within its volume, wherein the said output elements output light input laterally into at least one narrow side of the light guide (9c) predominantly into a restricted angle range, - lighting means (10) arranged laterally on at least one narrow side of the light guide (9c), and - optionally a linear polarization filter (P), - a first switchable light filter (5) according to claim 2 arranged in front of the backlight (8c) in the viewing direction or a second switchable light filter (5a) according to claim 3 or 4, - wherein the backlight (8c) is switched off and the lighting means (10) are switched on in the operating mode B2, and wherein at least the backlight (8c) is switched on in the operating mode B1, and - wherein the second electric field (EF2) is applied in the operating mode B2 and wherein the first electric field (EF1) is applied in the operating mode B1.Lighting device for a screen which can be operated in at least two operating modes B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is emitted in an angle range restricted with respect to the free viewing mode, comprising - a surface-like extended backlighting (8b) which emits light in a restricted angle range and contains a first optical element (1) according to Claim 1, and which is optionally constructed so as to be directly luminous, and - a plate-shaped light guide (9) which is situated in front of the backlighting (8b) in the viewing direction and has decoupling elements on at least one of the large surfaces and / or within its volume, - lighting means (10) arranged laterally on at least one narrow side of the light guide (9), and - wherein, in the operating mode B2, the backlighting (8b) is switched on and the lighting means (10) are switched off, and wherein at least the lighting means (10) are switched on in operating mode B 1.

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