Switchable light filter, lighting device and screen

Abstract

The invention relates to a switchable light filter (5) comprising a first optical element (1), which in turn comprises a plurality of light-absorbing transition dipole moments, wherein a relative proportion p of all transition dipole moments is aligned with 0.6≤p≤1.0 with a tolerance of a maximum of 15° parallel to a first preferred direction selectable for the 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 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, wherein the absolute values ​​of the first electric field EF1 and the second electric field EF2 differ by at least 0.1 MV / m,so that the transmission properties of the switchable light filter (5) differ between a first operating mode B1, in which the first electric field EF1 is applied, and a second operating mode B2, in which the second electric field EF2 is applied.

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.Finally, DE 10 2020 006 110 B3 of the applicant discloses a switchable light filter with variable transmission as well as a screen with such a switchable light filter and the use of such a screen. In this document, the direct ratio of the transmission behavior between two operating modes is not defined as a function of a plurality of different angular ranges. The same applies to DE 10 2021 112 947 A1 of the applicant, which describes an optical element with variable transmission and a screen with such an optical element.DESCRIPTION OF THE INVENTIONThe object of the invention is therefore to describe light filters having 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. Such light filters are intended to influence the transmission of light depending on the angle-optionally perpendicularly with respect to a seated or stationary observer-wherein it is possible to switch between at least two operating modes. In this case, the angle-dependent ratio of the transmission behavior between the two operating modes is to be defined in particular.This object is achieved in a first embodiment according to the invention by a switchable light filter comprisinga first optical element, in turn comprising• a plurality of light-absorbing transition dipole moments which are arranged in a layer at least 0.1 μm and at most 40 μm thick, or in a plurality of separate layers having a total thickness of transition dipole moments (adhesive layers which may be present between separate layers of transition dipole moments or substrate layers are not included here) of at most 40 μm,• wherein the transition dipole moments are preferably realized by means of at least two dichroic dyes,• wherein a relative proportion p of all transition dipole moments with 0.6≤p≤1.0, typically p<1, is oriented permanently or at least in a first state with a tolerance of at most 15° parallel to a first preferred direction selectable for the first optical element or varies around the latter, wherein the first preferred direction is arranged at a predetermined angle α, for example with α=0°, α=±2° or magnitude(α)>2°, with respect 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 preferably the said first plane is parallel to an edge of the first optical element, for example the lower edge,• so that light which is incident into the first optical element with an incident direction and a polarization state is transmitted or at least partially absorbed as a function of its incident direction with respect 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 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, wherein the absolute amounts of the first electric field EF1 and the second electric field EF2 differ by at least 0.1 MV / m,a first linear polarization filter X situated in front of the liquid crystal layer in the viewing direction if the liquid crystal layer is arranged in front of the first optical element in the viewing direction,so that the transmission properties of the 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, wherein the respective relative transmission in the two operating modes B 1 and B 2 is described at at least one point (preferably at least 5 points, particularly preferably at a plurality of points, distributed over the entire surface of the switchable light filter) on the switchable light filter, except for a selectable tolerance, in each case by a first transmission T B1( β) for the first operating mode B 1 and a second transmission T B2( β) for the second operating mode B 2, which are dependent on an angle β and are each normalized in this way, that for the values of transmission for the predetermined angle β=α, T B1( α)=1 and T B2( α)=1 apply,so that, when linearly polarized or elliptically polarized light, in which the ratio of large to small half axis is at least 4:1,• At angles (β) with α-5° ≤ β ≤ α +5°, the ratio T B1( β) / T B2( β) is greater than 0.9 + log 10( p) and less than 1.3 - log 10( p),• At angles (β) with α-30° ≤ β ≤ α-20° or α+20° ≤ β ≤ α+30°, the ratio T B1( β) / T B2( β) is greater than 2·(1 + log 10( p)), and• At angles (β) with α-45° ≤ β ≤ α-35° or α+35° ≤ β ≤ α+45°, incident in the switchable light filter, the ratio T B1( β) / T B2( β) is greater than 9·(1+2·log 10( p)).For the optical element, in special applications, a second or even a plurality of further states can be realized, for example, by means of so-called guest host liquid crystal cells.In principle, it should be true here that in angular ranges β≤α-5° or α+5°≤β the first transmission T B1( β) for the first operating mode is generally greater than the second transmission T B2( β) for the second operating mode B 2. This is achieved, among other things, by the selection of the first and the second electric field EF 1 and EF 2, as will be explained in the following. In this case, for the following explanations oBdA, it should be true that linearly polarized or elliptically polarized light, in which the ratio of large to small half axis is at least 4:1, is incident into the switchable light filter, the polarization of which lies predominantly parallel to said first plane, apart from a tolerance of 5°.Depending on the configuration of the liquid crystal layer as TN (twisted nematic), VA (vertical alignment), IPS (in plane switching), FFS (fringe field switching) or possibly another type, if the liquid crystal layer is arranged behind the first optical element in the viewing direction, the first electric field EF 1 is selected such that after transmission through the liquid crystal layer the transmitted light has an electric field component of mind. 40% perpendicular to the first plane, the polarization of which is mostly oriented perpendicular (or at a selectable angle in a range of 45° to 135°) to the first plane, except for a tolerance of 5°, while the second electric field EF2 is chosen such that after transmission through the liquid crystal layer the transmitted light has an electric field component of at least 80% parallel to the first plane. Under the aforementioned precondition that the polarization of the incident light is mostly parallel to said first plane up to a tolerance of 5°, the field strength of the second electric field EF 2 would be 0 V / m here, for example, i.e. there is essentially freedom from fields. The field strength of the first electric field EF 1 can be more than 0.1 MV / m, preferably an alternating field which changes polarity with a frequency in the order of magnitude of at least 0.8 kHz in any rectangular or sinusoidal form, e.g. with at least 0.8 kHz. Frequencies higher than 0.8 kHz are possible. In other words, in the operating mode B 2 with the second electric field EF 2, a polarization state which is as pure as possible is generated, since after transmission through the liquid crystal layer the transmitted light has an electric field component of at least 80% parallel to the first plane. In contrast, in the first operating mode B 1 with the first electric field EF 1, a mixed or significantly rotated electric field component is rather predominant, because after passing through the liquid crystal layer the transmitted light has an electric field component of mind. 40% perpendicular to the first plane. This is implemented, for example, in the case of a liquid crystal layer in the form of an IPS cell, or if a circular polarization is used after the liquid crystal layer.Alternatively, it is also possible for the first electric field EF 1 to represent freedom of field and for the second electric field EF 2 to have a field strength of more than 0.1 MV / m. Naturally, it is also possible that both the first and the second electric field EF 1 and EF 2 are not field-free.If the liquid crystal layer is arranged before the first optical element in the viewing direction, i.e. if a polarization filter is usually also located before the liquid crystal layer in the viewing direction, the above-mentioned values for first and second electric fields EF 1 and EF 2 can likewise be used to generate the operating modes B 1 and B 2. Such a construction is generally used in conjunction with a transparent image display device having a background illumination, the switchable light filter being located between the latter. In this embodiment, the first optical element transmits substantially at angles greater than 20° to at least 90% s-polarized light. Depending on the applied operating mode, p- or s-polarized light is generated therefrom with respect to the first plane (to an extent of at least 95%), which light is then either absorbed or transmitted to great extents at angles greater than 20° depending on the orientation of the polarization filter, which can be part of said image display device.It is essential to the invention in particular that the selection of the relative proportion p of all transition dipole moments in cooperation with the differently selected first and second electric fields EF 1, EF 2 and the associated influencing of the polarization lead to the fulfillment of the above-mentioned inequality. The inequality indicates the angle-dependent contrast of said transmissions. Further, the use of thicker layers of transition dipole moments results in more severe inequality as set forth below:In a preferred embodiment, the optical element comprises a multiplicity of light-absorbing transition dipole moments which are arranged in a layer having a thickness of at least 1 μm and at most 40 μm, or in a plurality of separate layers having a total thickness of transition dipole moments (adhesive layers which may be present between separate layers of transition dipole moments and substrates are not included here) of at least 1 μm and at most 40 μm, as a result of which, if linearly polarized or elliptically polarized light in which the ratio of large to small half axis is at least 4:1,▪ is incident into the switchable light filter at angles β with α-5°≤β≤α+5°, the ratio T B1( β) / T B2( β) being greater than 1.0+log 10( p) and less than 1.3 - log 10( p),• At angles β with α-30° ≤ β ≤ α-20° or α+20° ≤ β ≤ α+30°, the ratio T B1( β) / T B2( β) is greater than 3 · (1 + log 10( p)), and▪ is incident into the switchable light filter at angles β with α-45° ≤ β ≤ α-35° or α+35° ≤ β ≤ α+45°, the ratio T B1( β) / T B2( β) being greater than 16·(1+2·log 10( p)).An important mean-action relationship consists in the following situation: By the changeover between the first operating mode B 1, in which the first electric field EF 1 is present, and the second operating mode B 2, in which the second electric field EF 2 is present, wherein the absolute values of the first electric field EF 1 and the second electric field EF 2 differ by at least 0.1 MV / m, p-polarized light incident on the liquid crystal layer is converted substantially into s-polarized light-depending on the configuration-only in one operating mode, for example the first operating mode B 1, which is then incident on the first optical element, and is transmitted to a significant extent even at angles greater than 20°. In contrast, depending on the configuration, s-polarized light that is incident on the liquid crystal layer is converted essentially into p-polarized light only in the respective other operating mode, e.g. the first operating mode B 2, which is then incident on the first optical element and is absorbed to a significant extent at angles greater than 20°. As a result, the ratios described above for the transmissions T B1( β) / T B2( β) are achieved in the angular ranges indicated.If the absorbing transition dipole moments are furthermore formed by dichroic dyes in which the dye mass density is generally more than 1%, this is additionally helpful in order to satisfy the given inequality for reasonable transmission ratios.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. The latter means that 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 modes is made possible.For normalization of the transmission T B1( α)=1 and T B2( α)=1 the following should be noted: the angles β and α are measured, of course, in the same plane described above. It is also possible in principle for T B1( α) >1 and / or T B2( α) >1 to apply to angle β≠ α. In many cases, T B1( α)<1 and / or T B2( α)<1 again apply to angles β≠α.In the case that the transition dipole moments are variable, for example by their rotation in so-called guest-host liquid crystal cells, such guest-host liquid crystal cells can directly correspond to the aforementioned liquid crystal layer, but need not.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, smaller change in the polarization.For embodiments with TN liquid crystals, 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 20° to 40°.When the liquid crystal layer is disposed behind the first optical element in the viewing direction, it is preferable to include 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 incidence of circularly polarized light.In a further embodiment, the switchable light filter comprises at least two first optical elements, wherein optionally a birefringent retardation plate is arranged between at least two such first optical elements. In addition, the at least two first optical elements could optionally have different thicknesses, but do not have to have layers which each contain the plurality of light-absorbing transition dipole moments.The invention also comprises an illumination device in a first embodiment 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 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 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 to the free viewing mode for a vieweran illumination device as described above,and if no first linear polarization filter is arranged in the switchable light filter of the illumination device, a second linear polarization filter P arranged in front of the backlight in the viewing direction, whereby light which originates from the backlight and penetrates the second linear polarization filter P is restricted in its propagation directions, anda transmissive image display device which is arranged upstream of the 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 P is arranged in or is part of the transmissive image display device.The invention further comprises 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 a viewing angle range restricted with respect to the free viewing mode for a viewera picture display device, wherein basically any type of picture display device is possible here, for example LC panel, OLED, microLED and others,a switchable light filter according to the invention as described 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 switchable light filter is attached to the image reproduction device subsequently by a user and / or reversibly. In this case, a light filter can be marketed as a so-called "after-market product".For some screens described above, it may be advantageous when used in cars if a second 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 second preferred direction selectable for the second optical element or varies around the latter at least in a first state with a tolerance of at most 20° (alternatively 10°), wherein the second preferred direction is arranged at an angle α 1 to the mean perpendicular of the second 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 second plane which contains the said mean perpendicular; the second plane is preferably perpendicular to the first plane of the first optical element,• So that light which is incident into the second optical element is transmitted or at least partially absorbed as a function of its direction of incidence with respect to the second 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 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.It is advantageous that the transition dipole moments of a first optical element (or optionally, if present, also of a second or further optical element) are formed as one or more dichroic dye(s) which are mixed with the liquid crystals in a guest-host arrangement, the dye mass density generally being more than 1% or more than 10%. 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 preferred direction can, for example, each enclose an angle between 0° and 45° to a surface normal of the first optical element. In addition, it is possible for the first preferred direction to vary over the surface of the first optical element. In the sense of the invention, the average, weighted preferred direction then applies.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 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°.The 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 or second 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 the dyes are permanently aligned perpendicular to 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 of one 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 dyes.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 dyes or dye mixtures for different layers within an optical element may be different, but need not be.A 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 of a switchable light filter, FIG. 2 shows exemplary relationships of transmissions for two parameter sets ID 1 and ID 2, FIG. 3 shows exemplary ratios of transmissions for two parameter sets ID 3 and ID 4, FIG. 4 shows exemplary ratios of transmissions for two parameter sets ID 5 and ID 6, FIG. 5 shows a schematic diagram of a further switchable light filter, FIG. 6 shows a schematic diagram for an illumination device with a switchable light filter, FIG. 7 shows a schematic diagram for a first screen with a switchable light filter, and FIG. 8 shows a schematic diagram for a second screen with a switchable light filter.DETAILED DESCRIPTION OF THE DRAWINGSThe drawings are not true to scale and merely represent schematic representations.FIG. 1 shows a schematic diagram of a switchable light filter 5, comprisinga first optical element 1, in turn comprising• a plurality of light-absorbing transition dipole moments which are arranged in a layer at least 0.1 μm and at most 40 μm thick, or in a plurality of separate layers having a total thickness of transition dipole moments (adhesive layers which may be present between separate layers of transition dipole moments or substrate layers are not included here) of at most 40 μm,• wherein said transition dipole moments are realized by means of at least two (different) dichroic dyes,• wherein a relative proportion p of all transition dipole moments with 0.6≤p≤1.0, typically p<1, is oriented permanently or at least in a first state with a tolerance of at most 15° parallel to a first preferred direction that can be selected for the first optical element 1 or varies around the latter, wherein the first preferred direction is arranged at a predetermined angle α, for example with α=0°, α=±2° or magnitude(α)>2°, with respect to the mean perpendicular (see dash-dot line) of the first optical element 1, wherein the angle α is measured in a selectable first plane that contains the said mean perpendicular, and wherein preferably the said first plane is parallel to an edge of the first optical element 1, for example the lower edge,• so that light which is incident into the first optical element 1 with an incident direction and a polarization state is transmitted or at least partially absorbed as a function of its incident direction with respect to the first optical element 1 and its polarization state,means (not shown in the drawing) 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 influences the polarization state of light passing through it in dependence thereon, wherein the absolute amounts of the first electric field EF1 and the second electric field EF2 differ by at least 0.1 MV / m,so that the transmission properties of the 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, wherein the respective relative transmission in the two operating modes B 1 and B 2 is described at at least one point (preferably at least 5 points, particularly preferably at a plurality of points, distributed over the entire surface of the switchable light filter) on the switchable light filter 5 except for a selectable tolerance in each case by a first transmission T B1( β) for the first operating mode B 1 and a second transmission T B2( β) for the second operating mode B 2, which are dependent on an angle β and are each normalized in this way, that for the values of transmission for the predetermined angle β=α, T B1( α)=1 and T B2( α)=1 apply,so that, when linearly polarized or elliptically polarized light, in which the ratio of large to small half axis is at least 4:1,• At angles β with α-5°≤β≤α+5°, the ratio T B1( β) / T B2( β) is greater than 0.9+log 10( p) and less than 1.3 - log 10( p),• At angles β with α-30°≤β≤α-20° or α+20°≤β≤α+30°, the ratio T B1( β) / T B2( β) is greater than 2·(1+log 10( p)), and• At angles β with α-45°≤β≤α-35° or α+35°≤β≤α+45°, the ratio T B1( β) / T B2( β) is greater than 9·(1+2·log 10( p)).In principle, it should be true here that in angular ranges β≤α-5° or α+5°≤β the first transmission T B1( β) for the first operating mode is generally greater than the second transmission T B2( β) for the second operating mode B 2. This is achieved, among other things, by the selection of the first and the second electric field EF 1 and EF 2, as will be explained in the following. In this case, for the following explanations oBdA, it should be true that linearly polarized or elliptically polarized light, in which the ratio of large to small half axis is at least 4:1, is incident into the switchable light filter 5, the polarization of which lies in most cases parallel to said first plane, apart from a tolerance of 5°. For the sake of simplicity, α=0° shall apply to all the following considerations.Depending on the configuration of the liquid crystal layer 3 as TN (twisted nematic), VA (vertical alignment), IPS (in plane switching), FFS (fringe field switching) or possibly another type, if the liquid crystal layer 3 is arranged behind the first optical element 1 in the viewing direction, the first electric field EF 1 is selected such that after transmission through the liquid crystal layer 3 the transmitted light has an electric field component of mind. 40% perpendicular to the first plane, the polarization of which is predominantly oriented perpendicular (or at a selectable angle in a range from 45° to 135°) to the first plane, except for a tolerance of 5°, while the second electric field EF2 is selected such that the transmitted light after passing through the liquid crystal layer 3 has an electric field component of at least 80%, preferably more than 85%, parallel to the first plane. Under the aforementioned precondition that the polarization of the incident light is mostly parallel to said first plane up to a tolerance of 5°, the field strength of the second electric field EF 2 would be 0 V / m here, for example, i.e. there is essentially freedom from field. The field strength of the first electric field EF 1 can be more than 0.1 MV / m, preferably an alternating field which changes polarity with a frequency in the order of magnitude of at least 0.8 kHz in any rectangular or sinusoidal form, e.g. with at least 0.8 kHz. Frequencies higher than 0.8 kHz are possible. In other words, in the operating mode B 2 with the second electric field EF 2, a polarization state which is as pure as possible is generated, since after transmission through the liquid crystal layer 3 the transmitted light has an electric field component of at least 80% parallel to the first plane. In contrast, in the first operating mode B 1 with the first electric field EF 1, a mixed or significantly rotated electric field component is rather predominant, because after passing through the liquid crystal layer 3 the transmitted light has an electric field component of mind. 40% perpendicular to the first plane. This is implemented, for example, in the case of a liquid crystal layer 3 in the form of an IPS cell, or if a circular polarization is used after the liquid crystal layer 3.It is essential to the invention in particular that the selection of the relative proportion p of all transition dipole moments in cooperation with the differently selected first and second electric fields EF 1, EF 2 of the absolute values differ by at least 0.1 MV / m, and the associated influence on polarization leads to the fulfillment of the above-mentioned inequality. The inequality indicates the angle-dependent contrast or the stated ratios of the said transmissions. To this end, FIG. 2 shows exemplary relationships of transmissions for two parameter sets ID 1 and ID 2. These are simulations simulated by measurements. The parameters used here for light of the wavelength of 500 nm and a real part of the refractive index of 1.5 are specified for ID1 the layer thickness of the dichroic dye 6 μm with an imaginary part of the refractive index of 0.15 for p-polarized light and 0.0025 for s-polarized light and, for ID2 6 μm layer thickness with an imaginary part of the refractive index of 0.15 for p-polarized light and 0.005 for s-polarized light. Furthermore, the second electric field EF 2 is field-free, while the first electric field EF 1 has an absolute value of 0.1 MV / m. It can be seen that the ratio T B1( β) / T B2( β) in the case of ID2 unterscheidet differs significantly, in particular from angles β<-40° or β>40°, which is due to the reduced absorption of s-polarized light for ID1relative to ID2. Both parameter sets ID1 and ID2 nevertheless meet the inequality results for the invention. The parameter sets ID1 and ID2 even satisfy the fact that when linearly polarized or elliptically polarized light in which the ratio of large to small half-axis is at least 4:1 is generated,▪ is incident into the switchable light filter ( 5) at angles β with α-5°≤β≤α+5°, the ratio T B1( β) / T B2( β) being greater than 1.0+log 10( p) and less than 1.3 - log 10( p),▪ is incident into the switchable light filter ( 5) at angles β with α-30°≤β≤α-20° or α+20°≤β≤α+30°, the ratio T B1( β) / T B2( β) is greater than 3·(1+log 10( p)), and▪ is incident into the switchable light filter (5) at angles β with α-45°≤β≤α-35° or α+35°≤β≤α+45°, the ratio T B1( β) / T B2( β) being greater than 16·(1+2·log 10( p)).An important mean-action relationship consists in the following situation: By the changeover between the first operating mode B 1, in which the first electric field EF 1 is present, and the second operating mode B 2, in which the second electric field EF 2 is present, wherein the absolute values of the first electric field EF 1 and the second electric field EF 2 differ by at least 0.1 MV / m, p-polarized light incident on the liquid crystal layer is converted substantially into s-polarized light-depending on the configuration-only in one operating mode, for example the first operating mode B 1, which is then incident on the first optical element, and is transmitted to a significant extent even at angles greater than 20°. In contrast, depending on the configuration, s-polarized light that is incident on the liquid crystal layer is converted essentially into p-polarized light only in the respective other operating mode, e.g. the first operating mode B 2, which is then incident on the first optical element and is absorbed to a significant extent at angles greater than 20°. As a result, the ratios described above for the transmissions T B1( β) / T B2( β) are achieved in the angular ranges indicated.If the absorbing transition dipole moments are furthermore formed by at least two dichroic dyes, in which the dye mass density is generally more than 1%, this is additionally helpful in order to satisfy the given inequality for reasonable transmission ratios.For normalization of the transmission T B1( α)=1 and T B2( α)=1 the following should be noted: the angles β and α are measured, of course, in the same plane described above. It is also possible in principle for T B1( α) >1 and / or T B2( α) >1 to apply to angle β≠ α. In many cases, T B1( α)<1 and / or T B2( α)<1 again apply to angles β≠α.Furthermore, FIG. 3 shows exemplary ratios of transmissions for two parameter sets ID 3 and ID 4, and FIG. 4 shows exemplary ratios of transmissions for two parameter sets ID 5 and ID 6. In the case of ID3-ID6, light of the wavelength 500 nm is radiated in and the dye layer has a thickness of 6 μm with the refractive index 1.5+0.15i for p-polarized light and 1.5+0.0025i for p-polarized light. Absorption takes place for positive complex refractive indices. In the case of ID3, the linear polarization of the light differs between B1 and B2 by 45° and for ID4 by 60°. For ID5, the light in state B2 is linearly polarized and circularly polarized in B2. For ID6, the light is linearly polarized in state B2 and elliptically polarized in state B2 when incident on the optical element 1.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, smaller change in the polarization.For embodiments with TN liquid crystals, 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 20° to 40°.When the liquid crystal layer is disposed behind the first optical element in the viewing direction, it is preferable to include 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 incidence of circularly polarized light.Furthermore, FIG. 5 shows a schematic diagram of a further switchable light filter 5. The liquid crystal layer 3 is arranged in front of the first optical element 1 in the viewing direction, i.e. even if a polarization filter X is also usually located in front of the liquid crystal layer 3 in the viewing direction, the values described for FIG. 1 for first and second electric fields EF 1 and EF 2 can likewise be used to generate the operating modes B 1 and B 2. Such a construction is generally used in conjunction with a transparent image display device with a background illumination, with the switchable light filter 5 being located between the latter. In this embodiment, the light transmitted by the first optical element 1 is substantially s-polarized light at angles greater than 20° to at least 75% or even 90%. Depending on the applied operating mode, p- or s-polarized light is generated therefrom with respect to the first plane (to an extent of at least 95%), which light is then either absorbed or transmitted to great extents at angles greater than 20° depending on the orientation of the polarization filter X, which can be part of said image display device.In a further embodiment, the switchable light filter comprises at least two first optical elements, wherein optionally a birefringent retardation plate is arranged between at least two such first optical elements. In addition, the at least two first optical elements could optionally have different thicknesses, but do not have to have layers which each contain the plurality of light-absorbing transition dipole moments.FIG. 6 shows a schematic diagram for an illumination device with a switchable light filter in a first embodiment 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 to a viewer compared to the free viewing mode, comprisinga 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 switchable light filter according to the invention arranged in front of the backlight in the viewing direction as described above.Furthermore, FIG. 7 shows a schematic diagram for a first 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 to a viewer compared to the free viewing mode, in a first embodiment having a switchable light filter. This includesan illumination device as described above,and if no first linear polarization filter is arranged in the switchable light filter of the illumination device, a second linear polarization filter P arranged in front of the backlight in the viewing direction, whereby light which originates from the backlight and penetrates the second linear polarization filter P is restricted in its propagation directions, anda transmissive image display device which is arranged upstream of the 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 P is arranged in or is part of the transmissive image display device.Furthermore, FIG. 8 shows a schematic diagram for a second 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 viewer, having a switchable light filter 5, comprisinga picture display device, wherein basically any type of picture display device is possible here, for example LC panel, OLED, microLED and others,a switchable light filter according to the invention as described 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 switchable light filter is attached to the image reproduction device subsequently by a user and / or reversibly. In this case, a light filter can be marketed as a so-called "after-market product".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 switchable light filter 5 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.The invention achieves the stated object: A light filter having an optical element has been described in which light which is incident into the optical element is transmitted or partially or completely absorbed as a function of its direction of incidence and its polarization properties-not primarily but as a function of its position. Such light filters influence the transmission of light depending on the angle-optionally perpendicularly with respect to a seated or stationary observer-wherein it is possible to switch between at least two operating modes. In this case, in particular the angle-dependent ratio of the transmission behavior between the two operating modes is defined.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 liquid crystal layer 5 switchable light filter 8 backlight 11 image display device P, X polarization filter

Claims

Switchable light filter (5) comprising - a first optical element (1), in turn comprising • a plurality of light-absorbing transition dipole moments which are arranged in a layer having a thickness of at least 0.1 μm and at most 40 μm, or in a plurality of separate layers having a total thickness of transition dipole moments of at most 40 μm, • wherein a relative proportion p of all transition dipole moments is aligned with 0.6≤p≤1.0 permanently or at least in a first state with a tolerance of at most 15° 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 a predetermined angle α to the mean perpendicular of the first optical element (1), wherein the angle α is measured in a selectable first plane which contains said mean perpendicular, • so that light, which is incident into the first optical element (1) with an incident direction and a polarization state, is transmitted or at least partially absorbed in dependence on its incident direction with respect to the first optical element (1) and its polarization state, - means for optionally generating a first electric field or a second electric field, - a liquid crystal layer (3) arranged behind or in front of the first optical element (1) in the viewing direction, on which liquid crystal layer the first electric field or the second electric field acts and which depending thereon influences the polarization state of light passing through it, wherein the absolute amounts of the first electric field and of the second electric field differ by at least 0.1 MV / m, - a first linear polarization filter (X) arranged in front of the liquid crystal layer (3) in the viewing direction, if the liquid crystal layer (3) is arranged in front of the first optical element (1) in the viewing direction, - so that the transmission properties of the switchable light filter (5) differ between a first operating mode B1 in which the first electric field is present and a second operating mode B2 in which the second electric field is present, wherein the respective relative transmission in the two operating modes B1 and B2 is described at at least one point on the switchable light filter (5) by a first transmission T B1( β) for the first operating mode B1 and a second transmission T B2( β) for the second operating mode B2 respectively, except for a selectable tolerance, which transmission is dependent on an angle β and are respectively normalized in this way, wherein, for the values of transmission for the predetermined angle β=α, T B1( α)=1 and T B2( α)=1 apply, - so that when linearly polarized or elliptically polarized light, in which the ratio of large to small half-axis is at least 4:1, • is incident into the switchable light filter (5) at angles β with α-5°≤β≤α+5°, the ratio T B1( β) / T B2( β) is greater than 0.9+log 10( p) and less than 1.3 - log 10( p), • Incident in the switchable light filter (5) at angles β with α-30°≤β≤α-20° or α+20°≤β≤α+30°, the ratio T B1( β) / T B2( β) is greater than 2·(1+log 10( p)), and • incident in the switchable light filter (5) at angles β with α-45°≤β≤α-35° or α+35°≤β≤α+45°, the ratio T B1( β) / T B2( β) is greater than 9·(1+2·log 10( p)).Switchable light filter (5) according to claim 1, characterized in that its first optical element (1) comprises a plurality of light absorbing transition dipole moments, which are arranged in a layer having a thickness of at least 1 μm and at most 40 μm, or in several separate layers having a total thickness of transition dipole moments of at most 40 μm, - so that when linearly polarized or elliptically polarized light, in which the ratio of large to small half axis is at least 4:1, ▪ is incident into the switchable light filter (5) at angles β with α-5° ≤ β ≤ α + 5°, the ratio T B1( β) / T B2( β) is greater than 1.0 + log 10( p) and less than 1.3 - log 10( p), ▪ is incident into the switchable light filter (5) at angles β with α-30°≤β≤α-20° or α+20°≤β≤α+30°, the ratio T B1( β) / T B2( β) is greater than 3·(1+log 10( p)), and ▪ is incident into the switchable light filter (5) at angles β with α-45°≤β≤α-35° or α+35°≤β≤α+45°, the ratio T B1( β) / T B2( β) is greater than 16·(1+2·log 10( p)).Switchable light filter (5) according to claim 1 or 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 a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode, in which light is emitted 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 - a switchable light filter (5) according to one of Claims 1 to 3 arranged in front of the backlight (8) in the viewing direction.Screen which can be operated in at least a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode in which light is emitted in a viewing angle range restricted with respect to the free viewing mode for a viewer, comprising - an illumination device according to Claim 4, - if a first linear polarization filter (X) is not arranged in the switchable light filter (5) of the illumination 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 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 switchable light filter (5) in the viewing direction, wherein the second electric field is present in the second operating mode B 2 and wherein the first electric field is present in the first operating mode B 1.A display screen according to claim 5, characterized in that - if present - the first linear polarization filter (X) and otherwise the second linear polarization filter (P) are arranged in or part of the transmissive image display device (11).A screen which can be operated in at least a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode, in which light is emitted in a viewing angle range restricted to a viewer compared to the free viewing mode, comprising - an image reproduction device (12), - a switchable light filter (5) according to one of claims 1 to 3 in front of the image reproduction device (12) in the viewing direction.Screen according to Claim 7, characterized in that the switchable light filter (5) can be attached to the image display device (12) subsequently and / or reversibly by a user.Screen according to one of Claims 7 to 8, characterized in that, in the viewing direction, a second optical element is arranged upstream of the image reproduction device (12), which second optical element comprises: • a multiplicity of light-absorbing transition dipole moments, • the multiplicity of transition dipole moments being oriented parallel to a second preferred direction which can be selected for the second optical element or fluctuating around the latter at least in a first state with a tolerance of at most 20°, the second preferred direction being arranged at an angle α 1 to the mean perpendicular of the second optical element, the angle α 1 being measured in a selectable second plane which contains the said mean perpendicular, • with the result that light which is incident into the second optical element, in dependence on its direction of incidence with respect to the second optical element and its state of polarization, it is transmitted or at least partially absorbed.

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