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

Abstract

The invention relates to a switchable light filter (5), comprising a first optical element (1), itself comprising a plurality of light-absorbing transition dipole moments, means (SE1, SE2, ...) for selectively generating at least a first electric field (EF1) or a second electric field (EF2), wherein the first or the second electric field (EF1, EF2) can be selected separately for at least two regions on the switchable light filter (5), a liquid crystal layer (3) 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, 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, wherein,if the first electric field (EF1) is selected on the switchable light filter (5) for at least a first area and the second electric field (EF2) is selected for at least a second area, an electric field (EF3) is applied or electric fields (EF3, EF4 ...) are applied for at least one transition area between the at least two areas, so that in this at least one transition area the transmission properties of the switchable light filter (5) differ from the transmission properties of the first operating mode B1 and the second operating mode B2. This avoids unpleasantly perceptible edges between areas with the first operating mode B1 and the second operating mode B2.

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.According to JP 2007-155783 A, special optical surfaces 19 are used which are expensive to calculate and produce and which then deflect light into different narrow or wide ranges depending on the angle of incidence of light. These structures are similar to Fresnel lenses. Furthermore, interference flanks are present which deflect light in undesired directions. It thus remains unclear whether really useful light distributions can be achieved.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.Furthermore, DE 102023 110 269 B3 of the applicant describes a switchable light filter, an illumination device and a screen which can be operated in at least two operating modes for a free and a restricted viewing mode. However, this document does not describe in detail how, in the case of a partial area switching of the operating modes, visible edges between regions of different transmission regions can be avoided.Finally, DE 10 2019 003 383 A1 discloses a private liquid crystal device which enables two viewing modes. This document does not disclose how visible edges between sub-regions of different viewing modes can be avoided. A further disadvantage of this liquid crystal device is that collimated light for a free viewing mode (mode 2 therein) is selectively scattered, which significantly reduces the efficiency.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. Particularly, however, insofar as switching between modes is possible in part, visible edges exist between areas in different modes, which edges are visible with unpleasant feeling.DESCRIPTION OF THE INVENTIONThe object of the invention is therefore to describe light filters in which light which is incident therein is selectively transmitted depending on angle or partially or (almost) completely absorbed at different points on the light filter or on the screen with such a light filter. Visible edges between regions of different transmission regions are to be avoided. In addition, a corresponding screen or an illumination device is to be described, the angle-dependent brightness distribution of which can be varied over a partial area, wherein visible edges between regions located in different modes are likewise to be avoided.The switchable light filter or systems using it should be economically implementable and, in particular, universally usable with various types of screens in order to make it possible to switch between a visual protection, i.e. a restricted viewing mode, which exists at least in the horizontal direction with respect to a stationary or seated observer, and a free viewing mode, wherein the resolution of such a screen should not be reduced, even not slightly.In a first embodiment, this object is achieved according to the invention by a switchable light filter comprisinga first optical element, in turn comprising• a plurality of light absorbing transition dipole moments,• wherein the plurality of transition dipole moments is oriented parallel to a first preferred direction that can be selected for the first optical element or varies around the latter at least in a first state (in particular also permanently) with a tolerance of not more than 20° (preferably not more than 10°),• so that light which is incident into the first optical element is transmitted or at least partially absorbed depending on its state of polarization and on its direction of incidence with respect to the first optical element,means for optionally generating at least one first electric field EF 1 or one second electric field EF 2, wherein the first or the second electric field EF 1, EF 2 can be selected separately for at least two regions on the switchable light filter,a liquid crystal layer disposed in front of or behind the first optical element and 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,such 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,and so that, if the first electric field EF 1 is selected on the switchable light filter for at least one first region and the second electric field EF 2 is selected on at least one second region, an electric field EF 3 is applied for at least one transition region between these at least two regions or electric fields EF 3, EF 4... are applied which are different from the first and the second electric field EF 1, EF 2, so that in this at least one transition region the transmission properties of the switchable light filter differ from the transmission properties of the first operating mode B 1 and the second operating mode B 2.It is thereby advantageously true that, if the first electric field EF 1 is selected for at least one first region and the second electric field EF 2 is selected for at least one second region at the same time, electric fields EF 3, EF 4... are present for each at least one transition region between the two regions, said electric fields representing a stepwise and / or gradual transition from the first electric field EF 1 to the second electric field EF 2, so that at least three, preferably at least five, particularly preferably at least ten different values of the transmission are generated around the at least one transition region between the two regions for a respectively selectable, but always identical angle, which deviates from the perpendicular to the center of the switchable light filter by at least 5° (e.g. by 25°, 35°, 45° or greater than 45°). A gradual transition of the angle-dependent transmission values between the regions is thus made possible for a viewer, which prevents visible edges, since in said transition region there is also a transition from the first electric field EF 1 to the second electric field EF 2 with at least one resulting third electric field EF 3 (but preferably a plurality of electric fields EF 3, EF 4 etc.) present therebetween, which in turn controls the angle-dependent transmission of the switchable light filter at such transition regions differently than in said first or second region where the first or second electric field EF 1, EF 2 is present.It is possible, for example, for the means for the optional generation of at least one first electric field EF 1 or one second electric field EF 2 to comprise at least two mutually opposite flat transparent electrodes (e.g. ITO layers), between which the liquid crystal layer 3 is situated, at least one of these transparent electrodes being interrupted in the vicinity of the boundary between the said two regions. By "proximity" herein is meant a maximum distance of ten percent of the greatest extent of the liquid crystal layer, preferably a maximum of 5 percent. This also applies to all the following cases.It is possible that the at least one interrupted transparent electrode is repeatedly interrupted in the vicinity of the boundary between the two regions.Alternatively, the two transparent electrodes may be interrupted near the boundary between said two regions. Advantageously, the respective interruptions of the transparent electrodes are not congruent during parallel projection perpendicular to the direction of propagation of the first optical element onto the first optical element.By means of the aforementioned embodiments using one or two interrupted transparent electrodes, it is ensured that, if the first electric field EF 1 and the second electric field EF 2 are selected simultaneously for at least one first region on the switchable light filter, an electric field EF 3 is applied for at least one transition region between the at least two regions or electric fields EF 3, EF 4... are applied which are different from the first and the second electric field EF 1, EF 2, which is essential for the implementation of the invention. However, the aforementioned list of configurations is not exhaustive; it is also possible to find further configurations which achieve the aforementioned effect, for example by supplementing a further transparent electrode in the vicinity of the transition region.It is also possible that the first electric field EF 1 acts on the liquid crystal layer in such a way that linearly polarized light passing through it is rotated in its polarization orientation by at least 86°, preferably by at least 88°, while the second electric field EF 2 acts on the liquid crystal layer in such a way that linearly polarized light passing through it is rotated in its polarization orientation by not more than 4°, preferably not at all. It is possible here that, for example, either a first electric field EF 1 or a second electric field EF 2 describes a field-free state for the first region, while the respective other electric field has an absolute field strength greater than zero, e.g. 0.5 MV / m, for the second region. Since the first and the second electric field EF 1, EF 2 differ between the two regions, an electric field EF 3 is present in the transition region between the at least two regions or electric fields EF 3, EF 4... are present which are different from the first and the second electric field EF 1, EF 2, so that in this at least one transition region the transmission properties of the switchable light filter differ from the transmission properties of the first operating mode B 1 and the second operating mode B 2.With an aforementioned configuration, it is generally possible that the angle-dependent 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, in such a way that the transmissions differ between the two operating modes by at least a factor of ten, preferably by at least a factor of 15, for a respectively selectable, but always identical angle, which deviates from the perpendicular to the center of the switchable light filter by at least 45°.The object of the invention is also achieved by a lighting device for a 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 viewera surface-like extended background illumination which emits light, anda switchable light filter 5 arranged in front of the backlighting in the viewing direction, as described above.Finally, the object of the invention is achieved by a screen comprisinga picture reproduction device,a switchable light filter as described above in front of or behind the image display device in the viewing direction.The operating modes of the switchable light filter described above can be easily transmitted to the screen, so that the latter can also be operated in the above-mentioned different operating modes, depending on the configuration of the switchable light filter used.The image display unit can alternatively be an OLED, an LCD, an SED, a field emission display (FED), a microLED panel or a vacuum fluorescent display (VFD), in front of which a switchable light filter is arranged. Since the switchable light filter is effective irrespective of the type of image display unit, any other types of display screen are also possible. Such a screen 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.Said first preferred direction can advantageously each include an angle between 0° and 45° inclusive to a surface normal of the first optical element. Furthermore, in particular configurations, the first preferred direction can vary over the surface of the first optical element. In the sense of the invention, the first preferred direction averaged over the surface then applies.The transition dipole moment-also referred to as transition matrix element-is assigned a quantum-mechanical vector size and a specific transition between a first state-generally the ground state-and a second state-generally an excited state-of a system, i.e. an atom, molecule or solid body, and corresponds macroscopically 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, for example absorbs light of the corresponding polarization direction when transitioning from the ground state to the excited state. The magnitude of the vector corresponds to the strength of the interaction or to the probability of transition.The first preferred direction corresponds to the alignment of the transition dipole moments for a predefined propagation direction of light, in which the absorption is the same for any desired polarization directions of the light.Materials within the meaning of the invention which contain fixed transition dipole moments or transition dipole moments which can be aligned for the second configuration of switchable light filter described below are, for example, dichroic dyes or dye mixtures which are combined with a carrier material which does not impair the properties, for example with liquid crystals or polymers. The optical element may contain only one dye, for example. However, it is also possible for a plurality of dyes, i.e. dye mixtures, to be contained in a single optical element. The materials mentioned relax substantially by nonradiative processes.The extinction, i.e. the absorption, of the light is dependent on the absolute number of transition dipole moments-and therefore inherently also on the layer thickness in which the transition dipole moments are located-and the alignment between transition dipole moment and the polarization of the incident light with respect to one another. Depending on the implementation, the density of said transition dipole moments, their strength or the refractive index in the layers of the optical element can vary. In a passive, non-switchable optical element, the volume density of the transition dipoles can reach 100%.For simplified modeling of the transmission, it is assumed that the transition dipole moments in the optical element are oriented parallel to the plane of incidence of the light on the optical element. The plane of incidence here denotes a plane in which the propagation direction of the light wave lies, wherein the surface of the first optical element and the plane of incidence enclose a right angle. The light wave has as a transverse wave a transverse magnetic oscillation component and a transverse electrical oscillation component, both of which are perpendicular to one another and perpendicular to the propagation direction. Light which strikes the optical element is initially unpolarized in its entirety, i.e. the directions of oscillation of the transverse-electrical and accordingly also of the transverse-magnetic components are statistically distributed. When arriving at the surface of the first optical element with the transition dipole moments oriented in this way perpendicular to the surface of the first optical element, the light interacts with the first optical element or the transition dipole moments of the material contained therein and is polarized. In this case, the oscillation components which lie in the plane of incidence are absorbed. Light which is thus polarized parallel to the plane of incidence, i.e. is transversely magnetic or p-polarized, is absorbed, whereas light which is polarized perpendicular to the plane of incidence, i.e. parallel to the surface of the first optical element, i.e. is transversely electrical or s-polarized, is, on the other hand, completely transmitted or is only absorbed in a significantly reduced manner. Unpolarized light which does not pass through the first optical element parallel to transition dipole moments is therefore at least partially s-polarized when passing through the first optical element with transition dipole moments which are oriented parallel to the plane of incidence of the light.The transition dipole moments can be formed-as described above-as one or more dichroic dye(s), and therefore by such dye molecules, which form liquid crystals in a guest-host arrangement with liquid crystal molecules or are integrated into a liquid crystal layer. The dye molecules in turn are then aligned parallel to the liquid crystal molecules of the liquid crystal layer. The alignment of the liquid crystal molecules in the voltageless electric field is in turn induced by the surfaces surrounding them. One possible material for such surfaces is PMI.In general, one can refer here to liquid crystal polymers doped with dye(s). Various approaches (e.g. thermosettling LCs and epoxy LC) are known in the prior art for this purpose, which therefore need not be discussed in detail. In this context, reference is made by way of example to the following publications:N. Saba, M. Jawaid, O.Y. Alothman, M.T. Paridah, and A. Hassan, "Recent advances in epoxy resin, natural fiber-reinforced epoxy composites and their applications," Journal of Reinforced Plastic sand Composites, vol. 35, no. 6, 2015.C. Carfagna, E. Amendola, and M. Giamberini, "Liquid crystalline epoxy based thermosetting polymers," Progress in Polymer Science, vol. 22, pp. 1607-1647, 1997.V.P. Shibaev and A. Yu. Bobrovsky, "Liquid crystalline polymers: development trends and photocontrollable materials," Russian Chemical Reviews, vol. 86, pp. 1024-1072, 2017.T. Ikeda, J. -i. Mamiiya and Y. Yu, "Photomechanics of liquid-crystalline elastomers and other polymers," Angew. Chem. Int. Ed., vol. 46, pp. 506-528, 2007.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 0.01% to 10% (or optionally also significantly more), preferably of 0.1% to 5%, of the material of the respective layer(s) in the first optical element. 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 in the first optical element can also be formed differently in its different layers, if present.A preferred embodiment is a mixture of liquid crystals with at least one dye, in particular with at least one dichroic dye mixture, for each layer. Examples of suitable dichroic dyes or dye mixtures are azomethine dyes, indigoid and thioindigoid dyes, merocyanines, azulene, quinophtalone dyes, perylene dyes, phthaloperin dyes, dioxazine dyes, triphenodioxazine dyes, quinoxaline dyes, triazine dyes, tartrazine, azo dyes and anthraquinone dyes. The preparation of a liquid crystal dye mixture is described, for example, in U.S. Pat. No. 4,695,131 A. Furthermore, the surfaces closing off the outside of the layers are preferably treated, for example brushed, in order to achieve a homogeneous surface alignment of the transition dipole moments or, if present, of the liquid crystals.In a preferred embodiment, a polarization filter is also present, which can be arranged upstream or downstream of the first optical element as viewed in the direction of incidence. Alternatively, a λ / 4 retardation layer can also be present in order to generate linearly polarized light, if appropriate, from circularly polarized light, depending on the properties of the light originating from a corresponding light source.For switching the optical effect of the switchable light filter according to the invention, in a further embodiment of the invention, the transition dipole moments in each layer can vary in their alignment and / or their magnitude between the first and at least one second state, in order to be able to put the respective layer alternatively into at least two different states. Possible configurations of a switchable optical element or each layer therein are based, for example, on liquid crystals and / or dyes or dye mixtures which can be arranged on the surfaces in said liquid crystal cell with homogeneous alignment and can be rotated therein between at least two states. In this case, the light-absorbing transition dipole moments are also rotated and can therefore assume at least two states of action. It is conceivable in particular in such embodiments that more than two states, for example three or eight states, are achieved with different optical effects in each case. Other configurations of the liquid crystal cells are likewise conceivable. For this purpose, in particular electric fields are used to rotate the liquid crystals. It is possible here that, for example, either a first electric field EF 1 or a second electric field EF 2 describes a field-free state, wherein the respective other electric field has an absolute field strength greater than zero, e.g. 0.5 MV / m.A first such state corresponds to the circumstances described above and at least one second state is different therefrom, i.e. has at least one other preferred direction.In the case of active, i.e. switchable, light filters of this type, volume densities of the transition dipole moments of between 0.1% and 90% based on liquid crystals in the first optical element are conceivable. Alternatively, embodiments of a first optical element or each layer therein are conceivable in which the transition dipole moments are embedded in a liquid which is subjected to an electro-wetting method. In this way, the density of the transition dipole moments can be varied in particular, but not alone.As a rule, the switchable light filter, i.e. here in particular the liquid crystal layer and / or the means for optionally generating at least one first electric field EF 1 or one second electric field EF 2-independently in which of the aforementioned embodiments-is divided into a plurality of separately switchable segments, so that a local changeover between the respective possible operating states is made possible, as a result of which the aforementioned at least two regions can be formed. In cooperation with a picture display unit, this would mean that, for example, only a part of the picture surface can be switched between a private mode for visual protection and a public mode with no visual protection effect, i.e. for free vision, while the part of the picture surface complementary thereto is permanently in a visual protection mode or in no visual protection mode. The invention helps a viewer not perceive a "hard" edge between the regions.It is also possible that the means for optionally generating at least one first electric field EF 1 or one second electric field EF 2 on selectable sections of the switchable light filter can optionally generate further electric fields EF 3, EF 4,... so that third, fourth and optionally further states can be generated for the liquid crystal layer, which then in turn cause third fourth and optionally further absorption or transmission characteristics of the switchable light filter.Moreover, a switchable light filter can be extended by a second optical element which is structurally identical to the first optical element (or also varied with respect to its preferred direction), between which the liquid crystal layer is situated. This makes it possible to selectively switch angular restrictions in the transmission between one and two directions.Ideally, the structure of the first optical element (and, if present, each further optical element of identical construction) or the liquid crystal layer of the switchable light filter is non-periodic. This is inherently achieved if these components are produced as described above and is helpful for avoiding undesired optical effects, for example Moire effects, in cooperation, for example, with bilators.It can furthermore apply to the first optical elements that the respective preferred direction of a transition dipole moment can be selected as a function of its position on the respective optical element.In this context, it may be particularly useful for each first optical element to be divided into different regions (A 1, A 2,... ) along a selectable reference line on the respective first optical element, wherein for each region (A 1, A 2,... ) a separate preferred region direction can be selected, which applies to all transition dipole moments of the corresponding first optical element 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 at most + / -10 degrees. Within an optical element and within each region applicable for this purpose, accordingly, all transition dipole moments are each aligned parallel to the preferred direction applicable there with a tolerance of at most + / -10°.The image display unit advantageously corresponds to an LCD panel, one polarization filter of which corresponds to the polarization filter of the switchable light filter. This may be the front- or rear-side polarizer in the LCD structure. In addition, the switchable light filter can advantageously be arranged between the LCD panel and its backlight in order to switch between a first operating state for a free viewing mode and a second operating state for a restricted viewing mode, because the light of the backlight is focused once in the horizontal direction and is not focused once on account of the switchable light filter-for example when switching over in the horizontal direction. By "focusing" is meant not a focusing in the manner of lenses, but a narrowing of the emission region or transmission region over the angles.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 the schematic diagram of an exemplary switchable light filter in a sectional illustration, FIG. 2 shows a schematic diagram in plan view of the means for generating electric fields in a further exemplary embodiment, FIG. 3 shows an exemplary graph for transmitting a switchable light filter in the prior art, and FIG. 4 shows an exemplary graph for the effect on the transmission of the switchable light filter.DETAILED DESCRIPTION OF THE DRAWINGSThe drawings are not true to scale and merely represent schematic representations.FIG. 1 thus shows the schematic diagram of an exemplary switchable light filter 5 in a sectional illustration. This includesa first optical element 1, in turn comprising• a plurality of light absorbing transition dipole moments,• wherein the plurality of transition dipole moments is aligned parallel to a first preferred direction (e.g. the mean perpendicular to the first optical element 1) or varies around the latter, at least in a first state (in particular also permanently) with a tolerance of not more than 20° (preferably not more than 10°), which can be selected for the first optical element,• so that light which is incident into the first optical element 1 is transmitted or at least partially absorbed depending on its state of polarization and on its direction of incidence with respect to the first optical element 1,means SE1, SE2,... (e.g. transparent electrodes such as ITO layers with respectively connected control electronics) for the selective generation of at least one first electric field EF1 or one second electric field EF2, wherein for at least two regions on the switchable light filter 5 the first or the second electric field EF1, EF2 can be selected separately, in particular also at the same time,a liquid crystal layer 3 arranged (from the top as viewed) behind the first optical element 1, 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 as a function thereof,such 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,and so that, when the first electric field EF 1 is selected on the switchable light filter 5 for at least one first region and the second electric field EF 2 is selected on at least one second region, an electric field EF 3 is applied (or electric fields EF 3, EF 4... are applied) for at least one transition region between these at least two regions, which are different from the first and the second electric field EF 1, EF 2, so that in this at least one transition region the transmission properties of the switchable light filter 5 differ from the transmission properties of the first operating mode B 1 and the second operating mode B 2.The dashed lines in FIG. 1 plot the first and second regions in which the first and second electric fields EF 1 and EF 2 are present. Between these first and second regions there is a transition region in which a third electric field EF 3 is present.The substrates S can consist of polymer or glass and can in principle also be arranged at a different point in the structure.In a preferred embodiment, a polarization filter P, not shown in the drawing, is also present. Alternatively, a λ / 4 retardation layer can also be present in order to generate linearly polarized light, if appropriate, from circularly polarized light, depending on the properties of the light originating from a corresponding light source.Advantageously, in the case of the relationships according to FIG. 1, it holds true that, if at the same time the first electric field EF 1 is selected on the switchable light filter 5 for at least one first region and the second electric field EF 2 is selected for at least one second region, at least one electric field EF 3 is applied for in each case at least one transition region between the two regions, said electric field representing a stepwise and / or gradual transition from the first electric field EF 1 to the second electric field EF 2, such that at least three, preferably at least five, particularly preferably at least ten mutually different values of the transmission for a respectively selectable but always identical angle which is generated by at least 5° (for example by 25°, 35°, in or around the at least one transition region between the first and the second region, 45° or greater than 45°). Thus, a gradual transition of the angle-dependent transmission values between the regions is made possible for a viewer, which avoids visible edges, since in said transition region there is also a transition of the field strength from the first electric field EF 1 to the second electric field EF 2 with the at least one resulting third electric field EF 3 (preferably however a plurality of electric fields EF 3, EF 4 etc.) present, which in turn controls the angle-dependent transmission of the switchable light filter at such transition regions differently than in said first or second region, where the first or second electric field EF 1, EF 2 is present.As shown in FIG. 1, it is possible, for example, for the means SE 1, SE 2,... for the optional generation of at least one first electric field EF 1 or one second electric field EF 2 to comprise at least two mutually opposite planar transparent electrodes (e.g. ITO layers), between which the liquid crystal layer 3 is situated, wherein at least one of these transparent electrodes SE 1, SE 2 is interrupted in the vicinity of the boundary between the said two regions. In this embodiment, even the two transparent electrodes SE1, SE2 are interrupted in the vicinity of the boundary between said two areas (indicated by EF1 and EF2). Advantageously, the respective interruptions of the transparent electrodes SE 1, S 2 are not congruent during parallel projection perpendicular to the direction of propagation of the first optical element 1 onto the first optical element 1, which is the case here. In this way, a gradual transition of the field strength is achieved in the transition region with the third electric field EF 3.By "proximity" herein is meant a maximum distance of ten percent of the greatest extent of the liquid crystal layer 3, preferably a maximum of 5 percent. This also applies to all the following cases.By means of the aforementioned embodiments using one or two interrupted transparent electrodes, it is ensured that, if the first electric field EF 1 and the second electric field EF 2 are selected simultaneously for at least one first region on the switchable light filter 5, an electric field EF 3 is applied (or electric fields EF 3, EF 4... are applied) for at least one transition region between the at least two regions, which are different from the first and the second electric field EF 1, EF 2, which is essential for the implementation of the invention. However, the aforementioned list of configurations is not exhaustive; it is also possible to find further configurations which achieve the aforementioned effect, for example by supplementing a further transparent electrode in the vicinity of the transition region.Furthermore, it is possible that the first electric field EF 1 acts on the liquid crystal layer 3 in such a way that linearly polarized light passing through it is rotated in its polarization orientation by at least 86°, preferably by at least 88°, while the second electric field EF 2 acts on the liquid crystal layer 3 in such a way that linearly polarized light passing through it is rotated in its polarization orientation by not more than 4°, preferably not at all. It is possible here that, for example, either a first electric field EF 1 or a second electric field EF 2 describes a field-free state for the first region, while the respective other electric field has an absolute field strength greater than zero, e.g. 0.5 MV / m, for the second region. Since the first and the second electric field EF 1, EF 2 differ between the two regions, an electric field EF 3 is present in the transition region between the at least two regions or electric fields EF 3, EF 4... are present which are different from the first and the second electric field EF 1, EF 2, so that in this at least one transition region the transmission properties of the switchable light filter 5 differ from the transmission properties of the first operating mode B 1 and the second operating mode B 2. The liquid crystal layer 3 is, for example, a TN cell or a vertical alignment LC cell, each with a corresponding drive. Other configurations are possible.With an aforementioned configuration, it is generally possible that the angle-dependent 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, in such a way that the transmissions differ between the two operating modes by at least a factor of ten, preferably by at least a factor of 15, for a respectively selectable, but always identical angle, which deviates from the perpendicular to the center of the switchable light filter 5 by at least 45°.Said first preferred direction can advantageously each include an angle between 0° and 45° inclusive to a surface normal of the first optical element. Furthermore, in particular configurations, the first preferred direction can vary over the surface of the first optical element. In the sense of the invention, the first preferred direction averaged over the surface then applies.Furthermore, FIG. 2 shows a schematic diagram in plan view of the means for generating electric fields in a further exemplary embodiment. Here, the interruption of a transparent electrode is zigzag-shaped in order to generate a third electric field EF 3 deviating therefrom in the transition region between the first and the second electric field EF 1, EF 2.FIG. 3 shows an exemplary graph for transmitting a switchable light filter in the prior art. Relative locations on the light filter are indicated on the abscissa; a second electric field EF2 acts in the left-hand part, a first electric field EF1 acts in the right-hand part. The ordinate indicates a normalized transmission of a switchable light filter in the prior art, namely from an angle of, for example, 5° deviating from the first preferred direction (here: the mean perpendicular to the filter) to the corresponding light filter. It can be seen that, in the case of the jump from the first electric field EF 1 to the second electric field EF 2, the normalized transmission also changes abruptly, for example when a viewer's eye (or a camera) moves along the spatial direction. This in turn means that the transition of the regions with the first and second electric fields EF 1, EF 2 is connected to the perception of a prominent edge for a viewer as soon as he does not look exactly from the first preferred direction (here: the mean perpendicular to the filter).This problem is solved by the invention: FIG. 4 illustrates an exemplary graph for the effect on the transmission of the switchable light filter 5. Relative locations on the light filter 5 are also indicated here on the abscissa; a second electric field EF 2 acts in the left-hand part, a first electric field EF 1 acts in the right-hand part, but a third electric field EF 3 acts therebetween, which in this case causes a gradual transition of the field strengths from the first to the second electric field EF 1 or EF 2. The ordinate indicates a normalized transmission of a switchable light filter 5, namely from an angle of, for example, 5° deviating from the first preferred direction (here: the perpendicular to the center of the filter) to the light filter 5. This in turn means that the transition of the first and second regions with the first and second electric fields EF 1, EF 2 is not associated for a viewer with the perception of a prominent edge as soon as he does not look exactly from the first preferred direction (here: the mean perpendicular to the filter).Materials within the meaning of the invention which contain fixed or directable transition dipole moments are, for example, dichroic dyes or dye mixtures which are combined with a carrier material which does not impair the properties, for example with liquid crystals or polymers. The optical element may contain only one dye, for example. However, it is also possible for a plurality of dyes, i.e. dye mixtures, to be contained in a single optical element. The materials mentioned relax substantially by nonradiative processes.In the case of active, i.e. switchable, light filters of this type, volume densities of the transition dipole moments of between 0.1% and 90% based on liquid crystals in the first optical element are conceivable. Alternatively, embodiments of a first optical element or each layer therein are conceivable in which the transition dipole moments are embedded in a liquid which is subjected to an electro-wetting method. In this way, the density of the transition dipole moments can be varied in particular, but not alone.The invention achieves the stated object: switchable light filters or implementations thereof have been described, in which light which is incident therein is selectively transmitted or partially or (almost) completely absorbed at different points on the light filter or the screen with such a light filter. Visible edges between regions of different transmission regions are thereby avoided. In addition, a corresponding screen or an illumination device has been described, the angle-dependent brightness distribution of which can be varied over a partial area, wherein visible edges between regions located in different modes are likewise avoided.Furthermore, the switchable light filter or systems using it can be implemented inexpensively and can be used universally, in particular with various types of screens, in order to make it possible to switch between a visual protection, i.e. a restricted viewing mode, which exists at least in the horizontal direction with respect to a stationary or seated viewer, and a free viewing mode, wherein the resolution of such a screen is not reduced.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. The invention can also be used in a passenger car in order to optionally reserve disruptive image contents to the driver or passenger.

Claims

Switchable light filter (5) comprising - a first optical element (1), in turn comprising • a plurality of light-absorbing transition dipole moments, • wherein the plurality of transition dipole moments is oriented parallel to a first preferred direction selectable for the first optical element (1) with a tolerance of at most 20° or varies around the latter, • so that light which is incident into the first optical element (1) is transmitted or at least partially absorbed with respect to the first optical element (1) as a function of its polarization state and of its direction of incidence, - means (SE1, SE2,...) for the optional generation of at least one first electric field (EF1) or one second electric field (EF2), wherein the first or the second electric field (EF1, respectively, is transmitted for at least two regions on the switchable light filter (5), ef2) can be selected separately, - a liquid crystal layer (3) arranged in front of or behind the first optical element (1), on which the first electric field (EF1) or the second electric field (EF2) acts and which, as a function thereof, influences the polarization state of light passing through it, 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 present, and a second operating mode B2, in which the second electric field (EF2) is present, - characterized in that, if the first electric field (EF1) is selected on the switchable light filter (5) for at least one first region and the second electric field (EF2) is selected on the switchable light filter (5), For at least one transition region between the at least two regions, an electric field (EF 3) is present or electric fields (EF 3, EF 4... ) are present which are different from the first and the second electric field (EF 1, EF 2), so that in this at least one transition region the transmission properties of the switchable light filter (5) differ from the transmission properties of the first operating mode B 1 and the second operating mode B 2.Switchable light filter (5) according to claim 1, characterised in that, if the first electric field (EF1) is selected on the switchable light filter (5) for at least one first region and the second electric field (EF2) is selected for at least one second region, electric fields (EF3, EF4...) are present for each at least one transition region between the two regions, said electric fields representing a stepwise and / or gradual transition from the first electric field (EF1) to the second electric field (EF2), so that at least three, preferably at least five, particularly preferably at least ten different values of transmission are generated around the at least one transition region between the two regions for a respectively selectable, but always identical angle which deviates from the perpendicular to the mean point on the switchable light filter (5) by at least 5°.Switchable light filter (5) according to claim 1 or 2, characterised in that the means (SE1, SE2,...) for selectively generating at least a first electric field (EF1) or a second electric field (EF2) comprise at least two mutually opposite flat transparent electrodes, between which the liquid crystal layer (3) is located, at least one of these transparent electrodes being interrupted in the vicinity of the boundary between said two regions.Switchable light filter (5) according to claim 3, characterized in that the at least one interrupted transparent electrode is repeatedly interrupted in the vicinity of the boundary between said two areas.Switchable light filter (5) according to claim 3, characterized in that the two transparent electrodes are interrupted near the boundary between said two areas.Switchable light filter (5) according to claim 5, characterised in that the respective interruptions of the transparent electrodes are not congruent in parallel projection perpendicular to the direction of propagation of the first optical element (1) onto the first optical element (1).Switchable light filter (5) according to one of the preceding claims, characterized in that the first electric field (EF1) acts on the liquid crystal layer (3) in such a way that linearly polarized light passing through it is rotated in its polarization orientation by at least 86°, while the second electric field (EF2) acts on the liquid crystal layer (3) in such a way that linearly polarized light passing through it is rotated in its polarization orientation by not more than 4°, preferably not at all.Switchable light filter (5) according to claim 7, characterised in that the transmission properties of the switchable light filter (5) are angle-dependent and differ between a first operating mode B1, in which the first electric field (EF1) is present, and a second operating mode B2, in which the second electric field (EF2) is present, in such a way that the transmissions differ between the two operating modes by at least a factor of ten, preferably by at least a factor of 15, for a respectively selectable, but always identical angle, which deviates from the perpendicular to the centre of the switchable light filter (5) by at least 45°.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 with respect to the free viewing mode for a viewer, comprising - a surface-like extended backlight which emits light, and - a switchable light filter (5) according to one of Claims 1 to 8 arranged in front of the backlight in the viewing direction.A display screen, comprising - a display device, - a switchable light filter (5) according to one of claims 1 to 8 in front of or behind the display device in the viewing direction.

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