Switchable light filter, lighting device, and screen

Abstract

The invention relates to a switchable light filter (5) comprising: a first optical element (1), which itself comprises a plurality of light-absorbing transition dipole moments, wherein the transition dipole moments are formed by at least two dichroic dyes, the absorption maxima of which are each at different wavelengths; means for selectively generating a first electrical field (EF1) or a second electrical field (EF2); a liquid crystal layer (3), which is arranged behind or in front of the first optical element (1) in the viewing direction and on which the first electrical field (EF1) or the second electrical field (EF2) acts and which, depending thereon, influences the polarisation state of light passing through the liquid crystal layer, wherein the polarisation direction of linearly polarised light, which passes through the liquid crystal layer (3), is, by comparison, rotated about an angle τ between the operating modes B1 and B2 such that the transmission properties of the switchable light filter (5) for light of a plurality of wavelengths or wavelength ranges differ between a first operating mode B1 and a second operating mode B2, wherein the transmission values of the switchable light filter (5) for said plurality of wavelengths or wavelength ranges for at least one propagation direction, plotted in a polar coordinate system, between the two operating modes B1 and B2 are rotated about an angle φ which corresponds, with a tolerance of at most + / -5°, to the angle τ. The invention also relates to a lighting device and to a screen having such a switchable light filter (5).

Description

title

[0001] Switchable light filter, lighting device and screen Technical field of the invention

[0002] Significant progress has been made in recent years in widening the viewing angle of LCDs. However, there are often situations where this very wide viewing area can be a disadvantage. Information such as banking details, personal information, and sensitive data is increasingly accessible on mobile devices like laptops and tablets. Consequently, people need control over who can see this sensitive data; they must be able to choose between a wide viewing angle—a public mode—to share information on their display with others, for example, when viewing vacation photos or for advertising purposes. Conversely, they need a narrow viewing angle—a private mode—when they want to keep the image information confidential.

[0003] A similar problem arises in vehicle manufacturing: The driver must not be distracted by visual content, such as digital entertainment programs, when the engine is running, while the passenger wants to consume these programs even while the vehicle is in motion. Therefore, a screen is needed that can switch between the corresponding display modes.

[0004] Additional films based on microlouvers have already been used for mobile displays to achieve visual privacy. However, these films were neither switchable nor reversible; they always had to be manually applied and then removed. They also had to be transported separately from the display when not in use. Furthermore, a significant disadvantage of using such lamellar films is the associated loss of light transmission.

[0005] US 6,765,550 B2 describes such a privacy screen using micro-louvers. The biggest disadvantages are the mechanical removal and installation of the filter, as well as the light loss in the protected mode.

[0006] US Patent 5,993,940 A describes the use of a film with small, strip-shaped prisms evenly spaced across its surface to achieve a private mode, i.e., a restricted viewing mode with a small viewing angle. Development and manufacturing are technically quite complex.

[0007] In WO 2012 / 033583 A1, the switching between clear and restricted vision is achieved by controlling liquid crystals between so-called "chromonic" layers. This results in a loss of light and the technical effort is quite high.

[0008] US 2012 / 0235891 A1 describes a very complex backlight in a screen. According to Figures 1 and 15, not only are multiple light guides used, but also other complex optical elements such as microlens elements 40 and prism structures 50, which reshape the light from the rear light source on its way to the front light source. This is expensive and technically complex to implement and also results in light loss. According to the variant shown in Figure 17 of US 2012 / 0235891 A1, both light sources 4R and 18 produce light with a narrow illumination angle, whereby the light from the rear light source 18 is first converted into light with a wide illumination angle in a complex process. This complex conversion—as already mentioned above—significantly reduces brightness.

[0009] US patent 2013 / 0308185 A1 describes a special, stepped light guide that emits light in different directions over a large area, depending on the direction from which it is illuminated from one of its narrow sides. In combination with a transmissive image display device, such as an LCD, this allows for the creation of a screen that can be switched between free and restricted viewing modes. A disadvantage of this design is that the restricted viewing effect can only be generated for either left / right or top / bottom simultaneously, but not for left / right / top / bottom simultaneously, as is possible with other technologies. Certain payment transactions are necessary. Furthermore, even in restricted viewing mode, some residual light is still visible from blocked viewing angles.

[0010] WO 2015 / 121398 A1, filed by the applicant, describes a screen with two operating modes, in which scattering particles are present in the volume of the corresponding light guide for switching between the operating modes. However, the scattering particles chosen, made of a polymer, generally have the disadvantage that light is coupled out from both large areas, causing approximately half of the useful light to be emitted in the wrong direction, namely towards the backlight, where it cannot be sufficiently recycled due to the design. Furthermore, the scattering polymer particles distributed in the volume of the light guide can, under certain circumstances, especially at higher concentrations, lead to scattering effects that reduce the visual privacy effect in the protected operating mode.

[0011] The approach of "electrical birefringence (EDB)" technology is based on the idea of ​​using the switchable liquid crystals of an additionally applied LC panel to "filter" all light rays that do not exit the imaging layer at a specific beam angle. Disadvantages of this technology include high additional energy and cost consumption and the difficulty in adjusting the ±40° sweet spot, i.e., the optimal viewing position. The absorption coefficient of the LC structures is also insufficient, as the attenuation of light intensity increases again for viewing angles larger than the sweet spot, resulting in a light intensity of up to 3% of the maximum light intensity for viewing angles greater than ±40°.

[0012] The aforementioned methods and arrangements generally share the disadvantage that they significantly reduce the brightness of the base screen and / or require a complex and expensive optical element for mode switching and / or reduce the resolution in the freely viewable, public mode and / or exhibit visual artifacts on very high-resolution displays. Furthermore, most of the aforementioned methods are applicable to self-illuminating panels such as OLED or microLED in the prior art. Description of the invention

[0013] The object of the invention is therefore to describe light filters with an optical element in which light incident on 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 perpendicular to a seated or standing observer – and allow switching between at least two operating modes, also for self-illuminating panels such as OLEDs or microLEDs.

[0014] In a first embodiment according to the invention, this problem is solved by a switchable light filter comprising a first optical element, which in turn comprises • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.1 pm thick, wherein the transition dipole moments are formed by at least two dichroic dyes (preferably at least three dichroic dyes) whose absorption maxima are each at different wavelengths, • wherein the majority of all transition dipole moments are permanently or at least in a first state with a tolerance of at most 20°, preferably at most 10°, particularly preferably at most 5°, aligned parallel to or varying around a first preferred direction selectable for the first optical element, wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first optical element, wherein the angle α is measured in a selectable first plane which contains said perpendicular bisector, • so that light comprising several wavelengths or wavelength ranges and incident on the first optical element with a direction of incidence and a polarization state is transmitted or at least partially absorbed depending on its direction of incidence relative to the first optical element and its polarization state, Means for selectively generating (at least) a first electric field EF1 or a second electric field EF2, a liquid crystal layer arranged in the viewing direction behind or in front of the first optical element, on which the first electric field EF1 or the second electric field EF2 acts and which, depending on this, 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, and wherein the polarization direction of linearly polarized light passing through the liquid crystal layer is rotated by an angle T between operating modes B1 and B2, optionally a first linear polarization filter located in the viewing direction in front of the liquid crystal layer if the liquid crystal layer is arranged in the viewing direction in front of the first optical element, optionally a retardation layer,such that the transmission properties of the switchable light filter for light of several (at least two, preferably three or more than three) wavelengths or wavelength ranges 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 the transmission values ​​of the switchable light filter for said several wavelengths or wavelength ranges for at least one direction of propagation (given by angle of incidence 0' and plane of incidence (|)'), plotted in a polar coordinate system, are rotated by an angle between the two operating modes B1 and B2, wherein the angle corresponds to the angle T with a tolerance of at most + / -5° (which may be necessary due to a possible curve distortion of the transmission values ​​between the operating modes B1 and B2).

[0015] The object of the invention is further achieved in a second embodiment according to the invention by a switchable light filter comprising a first optical element, which in turn comprises • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.1 pm thick, wherein the transition dipole moments are formed by at least two dichroic dyes (preferably at least three dichroic dyes) whose absorption maxima are each at different wavelengths, • wherein the majority of all transition dipole moments are permanently or at least in a first state with a tolerance of at most 20°, preferably at most 10°, particularly preferably at most 5°, aligned parallel to or varying around a first preferred direction selectable for the first optical element, wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first optical element, wherein the angle α is measured in a selectable first plane which contains said perpendicular bisector, • so that light comprising several wavelengths or wavelength ranges and incident on the first optical element with a direction of incidence and a polarization state is transmitted or at least partially absorbed depending on its direction of incidence relative to the first optical element and its polarization state, Means for selectively generating (at least) a first electric field EF1 or a second electric field EF2, a liquid crystal layer arranged in the viewing direction behind or in front of the first optical element, 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 magnitudes of the first electric field EF1 and the second electric field EF2 differ by at least 0.1 MV / m, and wherein the polarization direction of linearly polarized light passing through the liquid crystal layer is rotated by an angle T between operating modes B1 and B2, optionally a first linear polarization filter located 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.optionally a delay layer, so that the transmission properties of the switchable light filter for light of several (at least two, preferably three or more than three) wavelengths or wavelength ranges 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 the line of maximum transmission, defined as dT / dv = 0, is rotated by an angle, where the angle corresponds to angle T with a tolerance of at most + / -5°, and where v is a direction vector perpendicular to the perpendicular bisector.

[0016] In other words, the rotated profile of the transmission values ​​of the switchable light filter in operating mode B1 essentially corresponds to the profile in operating mode B2, whereby (with a few exceptions) every point of the profile of operating mode B2 can be converted to the rotated profile of operating mode B1 with a tolerance of + / -5°.

[0017] The switchable light filters according to the invention can further be designed such that the axis of rotation with respect to the respective angle ^ is parallel to the first preferred direction up to a tolerance of 10°, which in turn preferably corresponds to the perpendicular to the center of the switchable light filter.

[0018] Typically, a first optical element of a switchable light filter contains a plurality of light-absorbing transition dipole moments arranged in a layer at least 1 pm and at most 40 pm thick, or in several separate layers with a total thickness of transition dipole moments of at most 40 pm, not including the thickness of any substrates that may be present.

[0019] For the first optical element, a second or even several further states can be realized in special applications - for example, through so-called guest-host liquid crystal cells.

[0020] 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 EF1 is selected such that, after transmission through the liquid crystal layer, the transmitted light has an electric field component of at least 40% perpendicular to the first plane, with its polarization being predominantly perpendicular (or at a selectable angle in a range of 45° to 135°) to the first plane, within a tolerance of 5°. Meanwhile, the second electric field EF2 is selected 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. This is based on the aforementioned condition that the polarization of the incident light... Since the second electric field EF2 lies predominantly parallel to the aforementioned first plane, with a tolerance of 5°, its field strength could be 0 V / m, meaning there is essentially no field. The field strength of the first electric field EF1 can be greater than 0.1 MV / m; preferably, it is an alternating field that changes polarity at a frequency on the order of, for example, 0.8 kHz, in a rectangular or sinusoidal waveform. However, lower and higher frequencies than 0.8 kHz are also possible, for example, 100 Hz, 1 kHz, or 10 kHz. In other words, in operating mode B2 with the second electric field EF2, a polarization state that 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 B1 with the first electric field EF1, a mixed or distinctly rotated electric field component predominates because, after passing through the liquid crystal layer, the transmitted light has an electric field component of at least 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 when circular polarization is used after the liquid crystal layer.

[0021] Alternatively, it is also possible that the first electric field EF1 is field-free and the second electric field EF2 has a field strength greater than 0.1 MV / m. Naturally, it is also possible that neither the first nor the second electric field EF1 and EF2 are field-free.

[0022] If the liquid crystal layer is positioned in front of the first optical element in the viewing direction, i.e., if a polarizing filter is typically located in front of the liquid crystal layer in the viewing direction, the aforementioned values ​​for first and second electric fields EF1 and EF2 can also be used to generate operating modes B1 and B2. Such a setup is generally used in conjunction with a transparent image display device with backlighting, with the switchable light filter located between them. In this configuration, the first optical element transmits at least 80% s-polarized light, essentially at angles greater than 20°. Depending on the selected operating mode, this is converted into p- or s-polarized light (at least 95%) with respect to the first plane, which is then further polarized depending on the orientation of the Polarization filters, which may be part of the said image display device, either absorb or transmit to a large extent at angles greater than 20°.

[0023] An important mean-effect relationship lies in the following fact: Since the multitude of light-absorbing transition dipole moments, arranged in a layer at least 0.1 pm thick, are formed by at least two (or at least three) dichroic dyes whose absorption maxima lie at different wavelengths, the rotation of the polarization by the liquid crystal layer through the angle T causes the absorption direction (and thus also the transmission direction) of the first optical element to also be rotated for at least two (or at least three) wavelengths or wavelength ranges, specifically by the angle 0, which corresponds to the angle T with a tolerance of + / -5°. This tolerance range is necessary because the rotation of the absorption direction can distort the transmission or absorption curve.

[0024] By switching between the first operating mode B1, in which the first electric field EF1 is present, and the second operating mode B2, in which the second electric field EF2 is present, where the absolute values ​​of the first electric field EF1 and the second electric field EF2 differ by at least 0.1 MV / m, p-polarized light incident on the liquid crystal layer is—depending on the configuration—converted substantially into s-polarized light only in one operating mode, e.g., the first operating mode B1. This s-polarized light 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, only in the other operating mode, e.g.,In the first operating mode B2, s-polarized light incident on the liquid crystal layer is essentially converted into p-polarized light, which then incidents on the first optical element and is absorbed to a significant extent at angles greater than 20°.

[0025] The means for selectively generating a first electric field EF1 or a second electric field EF2 can explicitly generate further electric fields EF3, EF4, ... which in turn generate further operating modes B3, B4, ..., each of which is assigned different corresponding rotation angles 03, 04, ...

[0026] Furthermore, the operating modes B1, B2, etc. can also differ locally on the switchable light filter. The latter means that the first optical element (and any further such optical element, if present) and / or the liquid crystal layer and / or the means for selectively generating a first electric field EF1 or a second electric field EF2 is / are divided into several separately switchable segments, thus enabling local switching between the respective possible operating modes.

[0027] In the case that the transition dipole moments are variable, e.g. 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 do not have to.

[0028] In a preferred embodiment, light penetrating the liquid crystal layer is transmitted essentially unchanged when the second electric field EF2 is applied, whereas when the first electric field EF1 is applied, the incident light is circularly or elliptically polarized, or its polarization is rotated by 90°. "Essentially" in this context means that at the interfaces, the orientation of the liquid crystal molecules is determined by electric fields and surface-induced forces. Therefore, the liquid crystal molecules are not ideally aligned, resulting in an undesirable, smaller change in polarization.

[0029] For TN liquid crystal structures, the following applies: The orientation of the liquid crystal molecules typically differs by 90° at the large surfaces that define the liquid crystal layer. This orientation is enhanced by PMI or PVA and additionally by mechanical or optical surface treatment. Furthermore, for TN liquid crystal layers, switching between the electric fields EF1 and EF2 generally results in the majority of the liquid crystals in the layer being rotated 75 to 90 degrees out of plane. In the case of IPS and FFS liquid crystal layers, the rotations of the LC molecules are less than 45°, typically around 20° to 40°.

[0030] If the liquid crystal layer is positioned behind the first optical element in the viewing direction, linearly polarized light or elliptically polarized light is preferentially incident, in which the ratio of the major to the minor semi-axis is The polarization ratio should be at least 4:1 (preferably at least 5:1 or higher). This can be achieved, for example, by using linear polarizing filters in the light path, or by using α / 4 layers in the case of circularly polarized light.

[0031] In a further embodiment, the switchable light filter comprises at least two first optical elements, with an optional birefringent retardation plate arranged between at least two such first optical elements. Furthermore, the at least two first optical elements could, but need not, have different thicknesses of layers containing the multitude of light-absorbing transition dipole moments.

[0032] The invention also includes a lighting device in a first embodiment 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 into a viewing angle area that is more restricted for a viewer compared to the free viewing mode, comprising - an extensive, surface-like backlight that emits light and is optionally designed to be directly luminous (e.g., with an LED matrix), as well as - a switchable light filter according to the invention arranged in the direction of viewing in front of the backlight as described above.

[0033] In addition, the invention also comprises a screen in a first embodiment 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 into a viewing angle area that is more restricted for a viewer compared to the free viewing mode, comprising - a lighting device as described above, - and, if no first linear polarizing filter is arranged in the switchable light filter of the lighting device, a second linear polarizing filter arranged in the viewing direction in front of the backlight, whereby light emanating from the backlight and passing through the second linear polarizing filter is restricted in its directions of propagation, and - a transmissive image reproduction device, which is arranged in front of the switchable light filter in the viewing direction, - wherein in operating mode B2 the second electric field EF2 is present and wherein in operating mode B1 the first electric field EF1 is present.

[0034] Preferably, the first or second linear polarization filter, if present, is arranged in or is part of the transmissive image display device.

[0035] Furthermore, the invention comprises a screen in a second embodiment 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 into a viewing angle area that is more restricted for a viewer compared to the free viewing mode, comprising - an image display device, whereby basically any type of image display device is eligible, for example LC panel, OLED, microLED and others, - in the viewing direction in front of the image display device, a switchable light filter according to the invention as described above, - wherein in operating mode B2 the second electric field EF2 is present and wherein in operating mode B1 the first electric field EF1 is present.

[0036] Optionally, the switchable light filter can be retrofitted by a user and / or reversibly attached to the display device. In this case, a light filter can be sold as an "after-market product".

[0037] For some of the screens described above, it may be advantageous when used in cars if a second optical element is arranged in the viewing direction in front of the transmissive image display device, which includes: • a large number of light-absorbing transition dipole moments; in this case, the dye mass density is greater than 1% or even greater than 10%. • wherein the majority of the transition dipole moments are aligned parallel to a second preferred direction selectable for the second optical element or vary around it with a tolerance of at most 20° (alternatively 10°) in at least a first state, wherein the second preferred direction is arranged at an angle oci to the perpendicular bisector of the second optical element (where, for example, ai = 0°, ai = ±2° or magnitude(oci) > 2°) the angle oci is measured in a selectable second plane which contains the said perpendicular bisector; the second plane is preferably perpendicular to the first plane of the first optical element, • so that light entering the second optical element is transmitted or at least partially absorbed, depending on its direction of incidence relative to the second optical element and its polarization state.

[0038] This latter design advantageously reduces transmission in the vertical direction and can thus reduce or completely avoid reflections of image content displayed on the screen onto the windshield in the vehicle.

[0039] Within the context of the invention, and particularly with regard to the backlighting, the term "restricted angular range" refers to a situation where the corresponding luminance is concentrated at least 80% or 90% within a defined angular range, while some residual light may still be present outside this limited angular range, which is generally due to technical limitations. Ideally, this residual light is minimal and decreases with increasing angle. To achieve particularly strong minimization, a suitable light filter is used in addition to the backlighting that emits light within a limited angular range.

[0040] Advantageously, the transition dipole moments of a first optical element (or, if present, all further optical elements) are formed as at least two (preferably at least three) different dichroic dyes, which are mixed with the liquid crystals in a guest-host arrangement, with the dye mass density typically being more than 1% or more than 10%. For permanent transition dipole moments, the liquid crystals can preferably be fixed via a curing process.

[0041] In contrast, the transition dipole moments can also be movably embedded in a liquid crystal layer as a guest-host arrangement, so that the transition dipole moments can be varied in their orientation and / or their magnitude between the first and at least a second state, depending on the influence of this liquid crystal layer.

[0042] The dichroic dye molecules generally align themselves parallel to the liquid crystal molecules.

[0043] Alternatively, the first optical element can be designed as a laminate of layers of polymer film polarizers.

[0044] The aforementioned designs also ensure that the first optical element has a non-periodic structure. This is highly advantageous because it eliminates the risk of artifacts such as moiré effects when used with pixel structures in screens.

[0045] The first preferred direction can, for example, include an angle between 0° and 45° to a surface normal of the first optical element. Furthermore, it is possible for the first preferred direction to vary across the surface of the first optical element. For the purposes of the invention, the average, weighted preferred direction then applies.

[0046] The transition dipole moment – ​​also known as the transition matrix element – ​​is a quantum mechanical vector quantity associated with a specific transition between an initial state – usually the ground state – and a final state – usually an excited state – of a system, i.e., an atom, molecule, or solid. It corresponds to the electric dipole moment associated with 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 of a given polarization. For example, during the transition from the ground state to the excited state, light of the corresponding polarization is absorbed. The magnitude of the vector corresponds to the strength of the interaction or the transition probability.

[0047] The first (second) preferred direction corresponds to the orientation of the transition dipole moments of the first (second) optical element for a given direction of propagation of light, where the absorption is the same for any polarization of the light.

[0048] The first and second preferred directions can also be identical or differ in their orientation by only a few degrees (maximum 10°) and both, in particular, be perpendicular to the optical element in question. This is a preferred case. However, depending on the application, it is also possible that the first and second preferred directions differ from each other by more than 10°.

[0049] The switchable light filter can also include a polarizing filter, which is positioned upstream or downstream of the first or second optical element in the direction of incidence. Alternatively or additionally, an X / 4 layer is also conceivable, for example, when circularly polarized light is incident, which is converted into (essentially) linearly polarized light by this layer.

[0050] A first exemplary manufacturing 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 its manufacture (with reference to US 9,481,658 B2 or WO 2021 / 177308 A1 paragraph 37 ff.). A substrate with low or no birefringence is coated with a film that 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 subsequent quality of the molecular alignment. - Application of the mixture of dichroic dye and thermotropic liquid crystalline compounds or polymers. When exposed to light, the side chains condense locally, so that the dyes are permanently oriented perpendicular to the surface.

[0051] An alternative, second manufacturing variant uses thermotropic, liquid-crystalline dichroic dyes (reference to JP 201 1 -237513 A), and comprises the following steps: Production of the corresponding dyes and addition of a polar group. - Application of the dye mixture as well as photo-alignment and curing of the dye mixture using polarized light.

[0052] The following materials are suitable for various manufacturing methods, for example, although this list is not exhaustive: - As a polymer substrate with low or no birefringence: preferably cellulose triacetate (TAC), - As dichroic substances or mixtures: dichroic dyes (preferably azo dyes) or dichroic metal nanoparticles (preferably gold, silver, copper, and aluminum); these are usually single dyes of one or mixtures of typically up to three different dyes to enable absorption across 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 to JP 201 1 -237513 A as an example. - Chemical groups for crosslinking, bonded to the thermotropic liquid crystal compounds or polymers: metaacryloylic groups, epoxy groups, an oxetanyl group, and styrene groups, preferably methacryloylic groups. Alternatively, polymerizable liquid crystal compounds, such as those described in JP 6268730 B2, may be used. Polymerizable liquid crystalline dichroic dyes, for example azo dyes.

[0053] Each of the at least two dyes consists of dye molecules, advantageously with each dye molecule being associated with a transition dipole or transition dipole moment; that is, 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 1% to 15%, of the material of the respective layers of the optical element in question. In special cases, the concentration can even reach 95% in the case of liquid-crystalline dichroic dyes. The dyes or dye mixtures for different layers within an optical element can be different, but need not be.

[0054] A previously described light filter, lighting device, or screen is advantageously used in a mobile device, a motor vehicle, aircraft, or watercraft, in a payment terminal, or in an access control system. It is possible to switch between the aforementioned operating modes to protect sensitive data, i.e., to display it perceptibly to only one viewer, or alternatively, to display image content simultaneously to multiple viewers.

[0055] In principle, the performance of the invention is maintained if the parameters described above are varied within certain limits.

[0056] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations given, but also in other combinations or on their own, without leaving the scope of the present invention. Brief description of the drawings

[0057] The invention is explained in more detail below with reference to exemplary embodiments and the accompanying drawings, which also disclose essential features of the invention. These exemplary embodiments serve only for illustration and are not to be interpreted as limiting. For example, a description of an exemplary embodiment with a plurality of elements or components is not to be interpreted as meaning that all of these elements or components are necessary for implementation. Rather, other exemplary embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components from different exemplary embodiments may be combined with one another unless otherwise specified. Modifications and variations described for one of the exemplary embodiments may also be applicable to other exemplary embodiments.To avoid repetition, identical or corresponding elements in different figures are designated with the same reference symbols and are not explained multiple times. They show: Fig. 1 shows the schematic diagram of an exemplary first optical element in sectional view. Fig. 2 shows a schematic diagram of a switchable light filter in sectional view. Fig. 3 shows a schematic diagram of a lighting device with an exemplary switchable light filter. Fig. 4 shows a schematic diagram of a screen in a first embodiment with a lighting device comprising an exemplary switchable light filter, Fig. 5 shows a schematic diagram of a screen in a second embodiment with an exemplary switchable light filter. Fig. 6 shows an exemplary graph of the effect on the transmission of the switchable light filter, here with an angle T=0°, Fig. 7 shows an exemplary graph of the effect on the transmission of the switchable light filter, here with an angle T=30°, Fig. 8 shows an exemplary graph of the effect on the transmission of the switchable light filter, here with an angle T=45°, and Fig. 9 shows an exemplary graph of the effect on the transmission of the switchable light filter, here with an angle T=90°. Detailed description of the drawings

[0058] The drawings are not to scale and only represent schematic diagrams.

[0059] Figure 1 shows a schematic diagram of an exemplary first optical element 1 in sectional view. The optical element 1 comprises a plurality of light-absorbing transition dipole moments, the majority of which are permanently or at least in a first state with a tolerance of at most + / -20. 0The first optical element 1 is aligned parallel to, or varies around, the selectable first preferred direction, such that light incident on the first optical element 1 is transmitted or at least partially absorbed, depending on its direction of incidence relative to the first optical element 1 and its polarization state. Generally, s-polarized light is transmitted by such an optical element 1 in almost all directions, with losses, while p-polarized light with angles of incidence greater than approximately 20° from the first preferred direction is partially or completely absorbed, with absorption increasing with the magnitude of the deviation. Figure 1 shows a schematic diagram of a first optical element 1, where the dashed line indicates the perpendicular to the center of the first optical element 1 and the dashed line indicates the first preferred direction. the angle α is spanned. The solid arrows indicate aligned transition dipole moments. In reality, there are significantly more than just the few transition dipole moments shown here. In Figure 1, the angle α is shown with a value greater than 0° for clarity. For the exemplary considerations below, however, the angle α = 0°, although other values ​​for the angle α are, of course, possible. The transition dipole moments are formed by at least two, preferably at least three, dichroic dyes whose absorption maxima are each at different wavelengths, e.g., at 465 nm, 532 nm, and / or 630 nm (or deviating from these by ±15 nm; for blue, green, and red light, respectively).In particular, it should also apply that for each group of transition dipole moments, which each contains transition dipole moments which together have essentially the same absorption maximum at a certain wavelength, the majority of the transition dipole moments of each such group are permanently or at least in a first state with a tolerance of at most + / -20. 0 is aligned parallel to the selectable first preferred direction or varies around it. In other words: The condition that the majority of transition dipole moments are permanent or at least in a first state with a tolerance of at most + / -20 0The condition that the first optical element 1 is aligned parallel to or varies around the selectable first preferred direction also applies, in groups, to transition dipole moments, each with the same absorption maximum. Naturally, all such groups of transition dipole moments are preferably distributed over substantially the entire area of ​​the first optical element 1. If these aforementioned conditions were not met, the invention might fail because the desired rotation of the transmission curve would then no longer be adequately achieved for all affected wavelengths.

[0060] The multitude of light-absorbing transition dipole moments of the first optical element 1 are typically arranged in a layer at least 0.1 micrometers thick. Multiple layers of transition dipole moments, separated, for example, by OCA (optically clear adhesive) and / or substrates, may also be present. The layer thicknesses of the transition dipole moments can range, for example, from 0.2 pm to 50 pm, preferably from 0.2 pm to 20 pm, and particularly preferably from 1 pm to 10 pm. In a preferred embodiment, the first optical element 1 contains two layers of transition dipole moments, each with a thickness of 1 pm. are up to 10 pm thick, each having a substrate (e.g. consisting of cellulose triacetate (TAC) or another non- or slightly birefringent material with a thickness preferably less than 100 pm), wherein the two substrates are furthermore bonded or glued together at transition dipole moments by means of an OCA (Optically Clear Adhesive), LOCA (Liquid Optically Clear Adhesive), PSA (Pressure Sensitive Adhesive) or by means of another means.

[0061] Furthermore, Fig. 2 shows a schematic diagram of an exemplary switchable light filter 5 in a first embodiment in sectional view. This comprises a first optical element 1, which in turn comprises • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.1 pm thick, wherein the transition dipole moments are formed by at least two dichroic dyes whose absorption maxima are each at different wavelengths, • wherein the majority of all transition dipole moments are permanently or at least in a first state with a tolerance of at most 20° aligned parallel to or varying around a first preferred direction selectable for the first optical element 1, wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first optical element 1, wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, • so that light comprising several wavelengths or wavelength ranges and incident on the first optical element 1 with an incident direction and a polarization state is transmitted or at least partially absorbed depending on its incident direction relative to the first optical element 1 and its polarization state, (Not shown in the drawing) means for selectively generating (at least) a first electric field EF1 or a second electric field EF2, for example two transparent electrodes (e.g. ITO layers or electrodes on a glass substrate) which are controlled by electronics acting as a signal generator, a liquid crystal layer 3 arranged behind or in front of the first optical element 1 in the direction of view, onto which the first electric field EF1 is generated for a first operating mode B1 or the second electric field EF2 for a second operating mode B2. an electric field EF2 acts and, depending on this, 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, and wherein the polarization direction of linearly polarized light passing through the liquid crystal layer 3 is rotated by an angle T between operating modes B1 and B2, optionally a first linear polarization filter X located in the viewing direction in front of the liquid crystal layer, if the liquid crystal layer 3 is arranged in the viewing direction in front of the first optical element 1, which is the case in Fig. 2, optionally a uniaxial or biaxial retardation layer, e.g.a B-plate, which is not shown here, such that the transmission properties of the switchable light filter 5 for light of several (at least two, preferably three or more than three) wavelengths or wavelength ranges 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 the transmission values ​​of the switchable light filter for the said several wavelengths or wavelength ranges for at least one direction of propagation (given by angle of incidence 0' and plane of incidence <)>'), plotted in a polar coordinate system, are rotated by an angle between the two operating modes B1 and B2, wherein the angle corresponds to the angle T with a tolerance of a maximum of + / -5° (where the tolerance may be necessary due to a possible curve distortion of the transmission values ​​between the operating modes B1 and B2).

[0062] Figure 2 can also be used to illustrate a switchable light filter 5 in a second embodiment. This comprises a first optical element 1, which in turn comprises • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.1 pm thick, wherein the transition dipole moments are formed by at least two dichroic dyes whose absorption maxima are each at different wavelengths, • wherein the majority of all transition dipole moments are permanently or at least in a first state with a tolerance of at most 20° aligned parallel to or varying around a first preferred direction selectable for the first optical element 1, wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first optical element 1, wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, • so that light comprising several wavelengths or wavelength ranges and incident on the first optical element 1 with an incident direction and a polarization state is transmitted or at least partially absorbed depending on its incident direction relative to the first optical element 1 and its polarization state, (Not shown in the drawing) means for selectively generating (at least) a first electric field EF1 or a second electric field EF2, for example two transparent electrodes (e.g. ITO layers or electrodes on a glass substrate) which are controlled by electronics acting as a signal generator, 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 acts for a first operating mode B1 or the second electric field EF2 acts for a second operating mode B2 and which, depending on this, 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, and wherein the polarization direction of linearly polarized light passing through the liquid crystal layer 3,in comparison between operating modes B1 and B2, rotated by an angle T, optionally a first linear polarization filter X located in the viewing direction in front of the liquid crystal layer 3, if the liquid crystal layer 3 is arranged in the viewing direction in front of the first optical element 1, (the viewing direction is assumed here to be from above), optionally a delay layer, e.g. a B-plate, so that the transmission properties of the switchable light filter 5 for light of several (at least two, preferably three or more than three) wavelengths or wavelength ranges between a first operating mode B1, in, to which the first electric field EF1 is applied, and to a second operating mode B2, in which the second electric field EF2 is applied, wherein for the switchable light filter 5 the line of maximum transmission, defined as dT / dv = 0, is rotated by an angle, wherein the angle corresponds to the angle T with a tolerance of a maximum of + / -5°, and wherein v is a direction vector perpendicular to the midpoint.

[0063] In other words, the rotated profile of the transmission values ​​of the switchable light filter 5 in operating mode B1 essentially corresponds to the profile in operating mode B2, whereby (with a few exceptions) every point of the profile of operating mode B2 can be converted to the rotated profile of operating mode B1 with a tolerance of + / -5°, as will be explained further below.

[0064] An important mean-effect relationship for all embodiments of the switchable light filter 5 is as follows: Since the multitude of light-absorbing transition dipole moments arranged in a layer at least 0.1 pm thick are formed by at least two dichroic dyes whose absorption maxima lie at different wavelengths, the rotation of the polarization by the liquid crystal layer 3 by the angle T causes the absorption direction (and thus also the transmission direction) of the first optical element 1 to also be rotated for at least two wavelengths or wavelength ranges, specifically by the angle , which corresponds to the angle T with a tolerance of + / -5°. This tolerance range is necessary because a distortion of the transmission or absorption curve can occur when the absorption direction is rotated. Such a distortion of the transmission or absorption curveThe absorption curve can be more pronounced the more birefringent effects are present in the structure. Furthermore, such a distortion of the transmission or absorption curve can be caused, among other things, by the use of uniaxially or biaxially birefringent retarder layers to improve the privacy effect (such as one or more A, B, and / or C plates), each of which is optimized for a selected polarization state of the light passing through it, but not for others.

[0065] The switchable light filters 5 according to the invention can further be designed such that the axis of rotation with respect to the respective angle is parallel to the first preferred direction up to a tolerance of 10°, which in turn is advantageously The perpendicular bisector of the switchable light filter 5 corresponds to this. For the following considerations, the axis of rotation with respect to the respective angle ^ is conceptually parallel to the first preferred direction, which in turn corresponds to the perpendicular bisector of the switchable light filter 5.

[0066] The mode of operation of the invention is explained in more detail with reference to the drawings Fig. 6 to Fig. 9, first for switchable light filters 5 of the first embodiment.

[0067] Figure 6 shows an exemplary, simulated graph of the effect on the transmission of the switchable light filter 5, here with an angle T=0°, while Figure 7 shows an exemplary, simulated graph of the effect on the transmission of the switchable light filter, here with an angle T=30°, Figure 8 with an angle T=45° and Figure 9 with an angle T=90°. These transmissions apply to the aforementioned at least two wavelengths (or wavelength ranges), e.g., for red, green and / or blue light.

[0068] The simulations were based on the assumption that at T = 0° (see Fig. 6) a first electric field EF1 is applied, which is essentially field-free (i.e., the field strength is 0 V / m), while for the conditions shown in Figs. 7 to 9 with angles of T > 0°, a second, third, and fourth electric field EF2, EF3, and EF4, respectively, are applied. These latter three electric fields are not field-free and are distinct from each other (e.g., 0.1 MV / m, 0.2 MV / m, and 0.3 MV / m, respectively). This means that when the first electric field EF1 is applied, the liquid crystal layer 3 (e.g., a VALC cell or an IPS-LC cell) does not substantially change the polarization direction of the incident light.

[0069] When a non-field-free electric field EF2, EF3, EF4, etc., is applied to the liquid crystal layer 3, its director is rotated by different angles (here, for example, 15°, 22.5°, 45°). This typically corresponds to half the desired rotation angle T of the polarization direction.

[0070] The drawings Fig. 6 to Fig. 9 thus graphically illustrate the mode of operation of the invention, namely that the transmission properties of the switchable light filter 5 for light of several (at least two, preferably three or more than three) wavelengths or wavelength ranges can be adjusted between a first operating mode B1, in which the first electric field EF1 is applied (see Fig. 6), and a second Operating mode B2, in which the second electric field EF2 is applied (see Fig. 7), is distinguished by rotating the transmission values ​​of the switchable light filter 5 for the aforementioned multiple wavelengths or wavelength ranges for at least one propagation direction (given by angle of incidence 0' and plane of incidence), plotted in a polar coordinate system, by an angle between the two operating modes B1 and B2. This angle corresponds to the angle T with a tolerance of a maximum of + / -5° (the tolerance may be necessary due to a possible distortion of the transmission values ​​curve between operating modes B1 and B2). Since these are simulations, T = T here, without requiring the tolerance. This description applies analogously to third and fourth (and also further) electric fields EF3, EF4, etc.

[0071] A comparison between, for example, Fig. 6 and Fig. 7 reveals that the profile of the transmission values ​​of the switchable light filter 5, rotated by the angle, in operating mode B1 (see Fig. 6) essentially corresponds to the profile in operating mode B2 (see Fig. 7), whereby (with a few exceptions) every point of the profile in operating mode B2 can be transformed to the rotated profile in operating mode B1 with a tolerance of + / -5°. This fact also applies analogously to third and fourth (and also further) electric fields EF3, EF4, etc.

[0072] A deformation of the transmission or absorption curve (already described above) when rotated by the angle can be stronger the more birefringent effects are present in the setup.

[0073] Figures 6 to 9 can also be used to illustrate the mode of operation of the invention, now for switchable light filters 5 of the second embodiment. The explanations given above apply analogously, and it is also evident from the aforementioned drawings that the transmission properties of the switchable light filter 5 for light of several (at least two, preferably three or more than three) wavelengths or wavelength ranges 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. For the switchable light filter 5, the line of maximum transmission, defined as dT / dv = 0, is rotated by an angle, where the angle corresponds to the angle T with a tolerance of at most + / - 5°, and where v is a direction vector perpendicular to the perpendicular bisector.

[0074] For the first optical element 1, a second or even several further states can be realized in special applications - for example, by so-called guest-host liquid crystal cells.

[0075] 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 other type, for example, if the liquid crystal layer 3 is arranged behind the first optical element 1 in the viewing direction, the first electric field EF1 is selected such that, after transmission through the liquid crystal layer 3, the transmitted light has an electric field component of at least 40% perpendicular to the first plane, its polarization being predominantly perpendicular (or at a selectable angle in a range of 45° to 135°) to the first plane, while the second electric field EF2 is selected such that, 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.Under the aforementioned condition that the polarization of the incident light is predominantly parallel to the first plane, with a tolerance of 5°, the field strength of the second electric field EF2 could, for example, be 0 V / m, meaning that there is essentially no field. The field strength of the first electric field EF1 can be greater than 0.1 MV / m; preferably, it is an alternating field that changes polarity at a frequency on the order of at least 40 Hz in a rectangular or sinusoidal waveform. Higher frequencies than 40 Hz are possible, e.g., 1 kHz or 10 kHz, or even varying frequencies. In other words, in operating mode B2 with the second electric field EF2, a polarization state that 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 B1 with the first electric field EF1, a mixed or distinctly rotated electric field component predominates because, after passing through the liquid crystal layer, the transmitted light has an electric field component of at least 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 when circular polarization is used after the liquid crystal layer.

[0076] Alternatively, it is also possible that the first electric field EF1 is field-free and the second electric field EF2 has a field strength greater than 0.1 MV / m. Naturally, it is also possible that neither the first nor the second electric field EF1 and EF2 are field-free.

[0077] The means for selectively generating a first electric field EF1 or a second electric field EF2 can explicitly generate further electric fields EF3, EF4, ... which in turn generate further operating modes B3, B4, ..., each of which is assigned different corresponding rotation angles 03, 04, ...

[0078] Furthermore, the operating modes B1, B2, etc. can also differ locally on the switchable light filter 5. The latter means that the first optical element 1 (and each further such optical element 1', if present) and / or the liquid crystal layer 3 and / or the means for selectively generating a first electric field EF1 or a second electric field EF2 is / are divided into several separately switchable segments, so that local switching between the respective possible operating modes B1, B2, etc. is enabled.

[0079] In the case that the transition dipole moments are variable, e.g. 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 3, but do not have to.

[0080] In a preferred embodiment, light penetrating the liquid crystal layer 3 is transmitted essentially unchanged when the second electric field EF2 is applied, whereas when the first electric field EF1 is applied, the incident light is circularly or elliptically polarized, or its polarization is rotated by 90°. "Essentially" in this context means that at the interfaces, the orientation of the liquid crystal molecules is determined by electric fields and surface-induced forces; thus, the liquid crystal molecules are not ideally aligned, which leads to an undesirable, smaller change in polarization.

[0081] For designs using TN liquid crystals, the following applies: The orientation of the liquid crystal molecules typically differs by 90° at the large surfaces that define liquid crystal layer 3. Such an orientation is supported by PMI or PVA and additionally by mechanical or optical surface treatment. For TN liquid crystal layers, it is generally true that when switching between the electric fields EF1 and EF2, the majority of the liquid crystals in liquid crystal layer 3 are rotated out of plane by 75 to 90 degrees. 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°.

[0082] If the liquid crystal layer 3 is positioned behind the first optical element in the viewing direction, linearly polarized or elliptically polarized light is preferentially incident, with a ratio of major to minor semi-axis of at least 4:1 (preferably at least 5:1 or more). This can be achieved, for example, by linear polarizing filters in the light path, or by α / 4 layers in the case of circularly polarized light.

[0083] In a further embodiment, the switchable light filter 5 comprises at least two first optical elements 1 and 1', wherein a birefringent retardation plate (e.g., an A, B, or C plate) is optionally arranged between at least two such first optical elements 1, 1'. Furthermore, the at least two first optical elements 1, 1' could, but need not, have different thicknesses of layers containing the multitude of light-absorbing transition dipole moments.

[0084] The invention, as shown in the schematic diagram in Fig. 3, also comprises a lighting device in a first embodiment 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 into a viewing angle area that is restricted for a viewer compared to the free viewing mode, comprising - an area-wide backlight 8 that emits light and is optionally designed to be directly luminous (e.g. with an LED matrix), as well as - a switchable light filter 5 according to the invention arranged in front of the backlight 8 in the direction of viewing, as described above.

[0085] In addition, as shown in the schematic diagram in Fig. 4, the invention also comprises a screen with a lighting device in a first embodiment, which has at least two operating modes: B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is projected onto a screen opposite the screen. Free viewing mode for a viewer with a limited viewing angle range, can be operated comprehensively - a lighting device as described above, - and, if no first linear polarizing filter is arranged in the switchable light filter 5 of the lighting device, a second linear polarizing filter P arranged in the viewing direction in front of the backlight, whereby light emanating from the backlight 8 and passing through the second linear polarizing filter P is restricted in its directions of propagation, and - a transmissive image display device 1 1 , which is arranged in the viewing direction in front of the switchable light filter 5, - wherein in operating mode B2 the second electric field EF2 is present and wherein in operating mode B1 the first electric field EF1 is present.

[0086] Preferably, the first or second linear polarization filter X, P, if each is present, is arranged in or is part of the transmissive image display device 11.

[0087] If, as shown in Fig. 4, the liquid crystal layer 3 is arranged in the viewing direction in front of the first optical element 1, i.e., if a polarizing filter X is generally located in the viewing direction in front of the liquid crystal layer 3, the aforementioned values ​​for first and second electric fields EF1 and EF2 can likewise be used to generate operating modes B1 and B2. Such a setup is generally used in conjunction with a transparent image display device 1 1 with a backlight 8, with the switchable light filter 5 located between them. In this configuration, the first optical element 1 transmits essentially s-polarized light at angles greater than 20° with at least 80% s-polarization.Depending on the operating mode, p- or s-polarized light is generated with respect to the first plane (at least 95%), which is then either largely absorbed or transmitted at angles greater than 20° depending on the orientation of the polarizing filter P, which can be part of the aforementioned image display device.

[0088] Furthermore, as shown in Fig. 5, the invention comprises a screen in a second embodiment which has at least two operating modes: B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is projected into a compared to the free viewing mode, which emits a limited viewing angle range for a viewer, can be operated comprehensively - an image display device 12, whereby basically any type of image display device 12 is eligible, for example LC panel, OLED, microLED and others, - in the viewing direction in front of the image display device 12 a switchable light filter 5 according to the invention as described above, - wherein in operating mode B2 the second electric field EF2 is present and wherein in operating mode B1 the first electric field EF1 is present.

[0089] Optionally, the switchable light filter 5 can be subsequently attached to the image display unit 12 by a user and / or reversibly. In this case, a light filter 5 can be sold as a so-called "after-market product".

[0090] Within the scope of the invention, and particularly with regard to the backlighting, the term "restricted angular range" means that the corresponding luminance is concentrated at least 80% or 90% within a defined angular range, while some residual light may still be present outside this restricted angular range, which is generally due to technical limitations. Ideally, this residual light is minimal and decreases with increasing angle. To achieve particularly strong minimization, a suitable light filter may be used in addition to any backlighting 8 that emits light within a restricted angular range.

[0091] Advantageously, the transition dipole moments of a first optical element 1 (or, if present, also of a second or further optical element) are formed as at least two dichroic dye(s) mixed with the liquid crystals in a guest-host arrangement, the dye mass density typically being more than 1% or more than 10%. For permanent transition dipole moments, the liquid crystals can preferably be fixed via a curing process.

[0092] The aforementioned configurations also ensure that the first optical element 1 has a non-periodic structure. This is very advantageous because it allows for better performance in conjunction with pixel structures of screens or... Image display devices 12 no risk of artifacts such as moiré effects exists.

[0093] The first preferred direction can, for example, include an angle between 0° and 45° to a surface normal of the first optical element 1. Furthermore, it is possible for the first preferred direction to vary across the surface of the first optical element 1. In this case, the average, weighted preferred direction applies within the meaning of the invention.

[0094] The switchable light filter 5 can, as described, further comprise a polarizing filter X, P, which is positioned upstream or downstream of the first optical element 1 in the direction of incidence. Alternatively or additionally, an X / 4 layer is also conceivable, for example, in the case of circularly polarized light, which is converted into (essentially) linearly polarized light by this layer.

[0095] The invention solves the stated problem: A light filter with an optical element is described in which light incident on 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 influence the transmission of light depending on the angle – optionally perpendicular to a seated or standing observer – and allow switching between at least two operating modes, also for self-illuminating panel types such as OLED or microLED.

[0096] The invention described above can be advantageously used in conjunction with an image display device wherever confidential data is displayed and / or entered, such as during PIN entry or data display at ATMs or payment terminals, password entry, or when reading emails on mobile devices. As described above, the invention can also be used in passenger cars to selectively shield the driver or passenger from distracting image content. A switchable light filter as described above can be used, in particular, in a passenger car equipped with a head-up display (HUD), i.e., an image generator projects image information onto a screen, especially the windshield. This screen then displays the image information for the driver and / or passenger, e.g., via a partially transparent and partially reflective holographic grating. Preferably, the switchable light filter is positioned between the image sensor and the lens or within the image sensor itself, ensuring that – depending on the operating mode – either only one person (e.g., the driver) or several people can see the displayed image information of the HLID. The same application is, of course, also applicable to the described Screens with switchable light filters are conceivable.

Claims

Patent claims 1. Switchable light filter (5), comprising a first optical element (1), in turn comprising • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.1 pm thick, wherein the transition dipole moments are formed by at least two dichroic dyes whose absorption maxima are each at different wavelengths, • wherein the majority of all transition dipole moments are permanently or at least in a first state with a tolerance of at most 20° aligned parallel to or varying around a first preferred direction selectable for the first optical element (1), wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first optical element (1), wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, • so that light comprising several wavelengths or wavelength ranges and incident on the first optical element (1) with an incident direction and a polarization state is transmitted or at least partially absorbed depending on its incident direction relative to the first optical element (1) and its polarization state, 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) acts for a first operating mode B1 or the second electric field (EF2) acts for a second operating mode B2 and which, depending on this, influences the polarization state of light passing through it, wherein the absolute magnitudes of the first electric field (EF1) and the second electric field (EF2) differ by at least 0.1 MV / m, and wherein the polarization direction of linearly polarized light passing through the liquid crystal layer (3) is rotated by an angle T between operating modes B1 and B2. optionally a first linear polarization filter (X) located in the viewing direction in front of the liquid crystal layer (3), if the liquid crystal layer (3) is arranged in the viewing direction in front of the first optical element (1), optionally a delay layer, so that the transmission properties of the switchable light filter (5) for light of several wavelengths or wavelength ranges 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 the transmission values ​​of the switchable light filter (5) for said several wavelengths or wavelength ranges for at least one direction of propagation, plotted in a polar coordinate system, are rotated between the two operating modes B1 and B2 by an angle which corresponds to the angle T with a tolerance of at most + / -5°.

2. Switchable light filter (5), comprising a first optical element (1), itself comprising • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.1 pm thick, wherein the transition dipole moments are formed by at least two dichroic dyes whose absorption maxima are each at different wavelengths, • wherein the majority of all transition dipole moments are permanently or at least in a first state with a tolerance of at most 20° aligned parallel to or varying around a first preferred direction selectable for the first optical element (1), wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first optical element (1), wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, • so that light comprising several wavelengths or wavelength ranges and incident on the first optical element (1) with an incident direction and a polarization state is transmitted or at least partially absorbed depending on its incident direction relative to the first optical element (1) and its polarization state, Means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged in the viewing direction behind or in front of the first optical element (1), on which the first electric field (EF1) or the second electric field (EF2) acts and which, depending on this, 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, and wherein the polarization direction of linearly polarized light passing through the liquid crystal layer (3) is rotated by an angle T between operating modes B1 and B2, optionally a first linear polarization filter (X) located in the viewing direction in front of the liquid crystal layer (3), if the liquid crystal layer (3) is arranged in the viewing direction in front of the first optical element (1), optionally a retardation layer,such that the transmission properties of the switchable light filter (5) for light of several wavelengths or wavelength ranges 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 the line of maximum transmission, defined as dT / dv = 0, is rotated by an angle 0, which corresponds to the angle T with a tolerance of at most + / -5°, where v is a direction vector perpendicular to the midpoint.

3. Switchable light filter (5) according to one of claims 1 or 2, characterized in that the axis of rotation with respect to the respective angle 0 is parallel to the first preferred direction up to a tolerance of 10°, which in turn preferably corresponds to the perpendicular to the center of the switchable light filter (5).

4. Switchable light filter (5) according to one of the preceding claims, characterized in that the means for selectively generating a first electric field (EF1 ) or a second electric field (EF2) can generate further electric fields (EF3, EF4, ...) which generate further operating modes B3, B4, ..., wherein these are each assigned different corresponding rotation angles 03, 04, ...

5. Switchable light filter (5) according to one of the preceding claims, characterized in that the first optical element (1) and / or the The liquid crystal layer (3) and / or the means for generating a first electric field (EF1) or a second electric field (EF2) is / are divided into several separately switchable segments, so that local switching between the respective possible operating modes B1, B2, ... is enabled.

6. Switchable light filter (5) according to one of the preceding claims, characterized in that for each group of transition dipole moments, which each contains transition dipole moments which together have the same absorption maximum at a certain wavelength, the majority of the transition dipole moments of each such group are also permanently or at least in a first state with a tolerance of at most + / -20 0 is aligned parallel to the selectable first preferred direction or varies around it.

7. Lighting device for a screen which can be operated in at least one 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 into a viewing angle area that is more restricted for a viewer compared to the free viewing mode, comprising - an area-wide backlight (8) that emits light as well as - a switchable light filter (5) arranged in the direction of viewing in front of the backlight (8) according to one of claims 1 to 6.

8. Screen that can be operated in at least one 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 into a viewing angle area that is more limited for a viewer than in the free viewing mode, comprising - a lighting device according to claim 7, - if no first linear polarizing filter (X) is arranged in the switchable light filter (5) of the lighting device, a second linear polarizing filter (P) is arranged in the viewing direction in front of the backlight, whereby light emanating from the backlight (8) and the second linear polarization filter (P), is restricted in its propagation directions, and - a transmissive image display device (1 1 ) which is arranged in the viewing direction in front of the switchable light filter (5), - wherein in the second operating mode B2 the second electric field (EF2) is present and wherein in the first operating mode B1 the first electric field (EF1) is present.

9. Screen according to claim 8, characterized in that - if present - the first linear polarization filter (X) and otherwise the second linear polarization filter (P) is arranged in or is a part of the transmissive image display device (11).

10. Screen that can be operated in at least one 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 into a viewing angle area that is more restricted for a viewer than in the free viewing mode, comprising - an image display device (12), - in the viewing direction in front of the image display device (12) a switchable light filter (5) according to one of claims 1 to 6, - wherein in the second operating mode B2 the second electric field (EF2) is present and wherein in the first operating mode B1 the first electric field (EF1) is present.

11. Screen according to one of claims 8 to 10, characterized in that a second optical element is arranged in the viewing direction in front of the image display device (1 1 , 12), which comprises: • a multitude of light-absorbing transition dipole moments, • wherein the majority of the transition dipole moments are aligned parallel to or fluctuate around a second preferred direction selectable for the second optical element with a tolerance of at most 20° in at least a first state, wherein the second preferred direction is arranged at an angle oci to the perpendicular bisector of the second optical element, wherein the angle oci is measured in a selectable second plane which contains said perpendicular bisector, • so that light entering the second optical element is reflected depending on its direction of incidence relative to the second optical element and its polarization state is transmitted or at least partially absorbed.

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