Switchable light filter and screen

Abstract

The invention relates to a switchable light filter (5) comprising a first optical element (1) which transmits light or at least partially absorbs it depending on its polarization state and / or its direction of incidence, means (7) for limiting the angular spectrum of light, means for selectively generating a first electric field (EF1) or a second electric field (EF2), an anamorphic diffuser (D), a linear polarization filter (P), 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 penetrating it, so that the transmission properties of the switchable light filter (5) differ between a first operating mode B1 and a second operating mode B2, wherein light which, from the perspective of an observer, is incident on the liquid crystal layer (3) from behind,has an angular spectrum restricted perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer (3) and the first optical element (1), is at least partially fanned out parallel to the second plane due to the anamorphic diffuser (D), whereby insufficient absorption effects of the optical element (1) for light from certain directions are at least partially compensated for in one of the operating modes B1 or B2. The invention further discloses screens that can be operated in at least two operating modes B1 or B2.

Description

Technical field of the invention

[0001] In recent years, great strides have been made in widening the viewing angle of LCDs. However, there are often situations in which the very large viewing area of ​​a screen can be a disadvantage. Information, such as banking details and other personal and sensitive data, is also becoming increasingly available on mobile devices such as notebooks and tablet PCs. Accordingly, people need control over who can see this sensitive data. They must be able to choose between a wide viewing angle – a public mode – in order to share information on their display with others, e.g., when viewing holiday photos or for advertising purposes. On the other hand, they need a narrow viewing angle – in a private mode – if they want to keep the image information confidential.

[0002] A similar problem arises in vehicle construction: The driver must not be distracted by image content, such as digital entertainment programs, when the engine is running, while the passenger also wants to consume these while driving. Therefore, a screen that can switch between the corresponding display modes is required.

[0003] Additional films based on micro-louvres have already been used for mobile displays to achieve visual privacy. However, these films were not switchable or reversible; they always had to be applied and removed manually. They also had to be transported separately from the display when not in use. A major disadvantage of using such louvre films is the associated light loss.

[0004] US Pat. No. 6,765,550 B2 describes such a privacy screen using micro-louvres. The biggest disadvantage is the mechanical removal or installation of the filter, as well as the loss of light in protected mode.

[0005] US 2012 / 0235891 A1 describes a very complex backlight in a display. Fig. 1 and Fig. 15 not only uses multiple light guides, but also other complex optical elements such as microlens elements 40 and prism structures 50, which transform the light from the rear illumination on its way to the front illumination. This is expensive and technically complex to implement and also involves 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, with the light from the rear light source 18 first undergoing a complex conversion into light with a wide illumination angle. This complex conversion, as already noted above, significantly reduces brightness.

[0006] US 2013 / 0308185 A1 describes a special light guide with steps that emits light over a large area in different directions depending on the direction from which it is illuminated from a narrow side. In conjunction with a transmissive image display device, e.g. an LC display, a screen that can be switched between free and restricted view modes can be created. A disadvantage here is, among other things, that the restricted view effect can only be created for left / right or for top / bottom, but not for left / right / top / bottom simultaneously, as is necessary for certain payment transactions. In addition, even in restricted view mode, residual light is still visible from blocked viewing angles.

[0007] WO 2015 / 121398 A1 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 polymer scattering particles selected there generally have the disadvantage that light is coupled out of both large surfaces, causing approximately half of the useful light to be emitted in the wrong direction, namely toward the backlight, where it cannot be adequately recycled due to the design. Furthermore, the polymer scattering particles distributed in the volume of the light guide can, under certain circumstances, particularly at higher concentrations, lead to scattering effects that reduce the privacy effect in the protected operating mode.

[0008] The applicant's DE 10 2019 009 071 A1 describes a lighting device with at least two operating modes. This lighting device does not allow switching self-illuminating screen types such as OLED panels between a free and a restricted viewing mode.

[0009] DE 11 2015 000 351 T5 discloses a display device for holographic reconstruction. Even with this display device, which includes an illumination device, it is not possible to switch self-illuminating screen types such as OLED panels between a free and a restricted viewing mode.

[0010] Finally, US 2008 / 0278663 A1 describes an anamorphic lens array used for anamorphic light distribution of the light emitted by a screen. This arrangement does not allow screens to be switched between a free and a restricted view 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 requirements, and the difficulty of changing the ±40° sweet spot, i.e., the optimal viewing position. The absorption coefficient of the LC structures is also insufficient, as the attenuation of the light intensity increases again for viewing angles greater than the sweet spot, so that the light intensity for viewing angles greater than ±40° is up to 3% of the maximum light intensity.

[0012] The aforementioned methods and arrangements generally have the disadvantage that they significantly reduce the brightness of the basic 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 have visual artifacts on very high-resolution displays. Description of the invention

[0013] The object of the invention is therefore to describe 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 depending on its position. Switchable light filters using the optical element are intended to influence the transmission of light in an angle-dependent manner, allowing switching between at least two operating states. In particular, insufficient absorption effects in certain directions are to be at least partially compensated for. Corresponding screens are also to be disclosed.

[0014] This object is achieved according to the invention by a switchable light filter comprising - a first optical element which transmits or at least partially absorbs light depending on its polarization state and / or its direction of incidence, wherein a first preferred direction can be selected for the first optical element, which is arranged at an angle α to the perpendicular bisector of the first optical element, and wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, - means arranged behind the first optical element from the perspective of an observer for limiting the angular spectrum of light incident on the switchable light filter, wherein this limitation of the angular spectrum is effective at least perpendicular to the first plane (preferably, this first plane contains approximately or exactly the horizontal direction when looking at the switchable light filter, at least from the perspective of a seated or standing observer; however, the first plane may also be different therefrom), - means for selectively generating a first electric field EF1 or a second electric field EF2 (e.g. two ITO layers with electronic control), - an anamorphic diffuser D, which, from the perspective of an observer, is arranged in front of the optical element and which develops its maximum scattering effect along a second plane which is at an angle of at least + / -45° (preferably at least + / -75°, but particularly preferably more than + / -87°, i.e. (quasi) perpendicular) to the first plane, - a linear polarization filter P, which has its transmission maximum for light which is linearly polarized parallel to a first direction, and which is optionally arranged between the anamorphic diffuser D and said means for limiting the angular spectrum of light (said first direction is preferably approximately horizontal from the perspective of a seated or standing observer), - a liquid crystal layer arranged between the linear polarization filter P and 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, - optionally a retardation layer R (e.g. a biaxially birefringent layer) located between the linear polarizer P and the first optical element, - so that the transmission properties of the switchable light filter 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, - and such that light which, from the perspective of an observer, is incident on the liquid crystal layer from behind has a restricted angular spectrum perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer and the first optical element, is at least partially spread out parallel to the second plane due to the anamorphic diffuser D, whereby in one of the two operating modes B1 or B2 (preferably B2), insufficient absorption effects of the optical element (further information on the so-called "hourglass" behavior of the first optical element will be provided below) for light from directions which deviate from the first preferred direction by at least 20° are at least partially compensated, and whereby in one of the two operating modes B1 or B2 (preferably B2), a luminance profile of the light penetrating the switchable light filter is present for an observer,which is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane (which corresponds to a so-called “4-way privacy effect” because visibility is restricted from below, above, left and right).

[0015] From the viewer's perspective, the linear polarization filter P can, in principle, be located behind all other components or directly behind the diffuser D. The general sequence of optical element, liquid crystal cell, and linear polarizer P can also be reversed from the viewer's perspective with respect to the optical element and linear polarizer P. This does not change the optical effect.

[0016] A key element in the general functioning of the invention, which also applies to the screens described below, is that insufficient absorption effects of the optical element for light from directions that deviate from the first preferred direction by at least 20°—in particular for light from planes of incidence that are rotated by more than 10° or more than 20° relative to the selectable first plane—are at least partially compensated for. Typically, an optical element for linearly polarized light (assumption to simplify the explanation of the effect: horizontally linearly polarized light from the viewer's perspective) has an "hourglass shape" of the transmission values ​​in polar coordinates. This means that the absorption becomes weaker when a vertical angular component is added from a horizontal lateral view.Thanks to the invention, this "hourglass shape" is now truncated in the vertical direction, corresponding to the effect of said means for limiting the angular spectrum of light, whereby this limitation of the angular spectrum takes place at least perpendicular to the first plane. This means that the light penetrating the optical element does not have to be absorbed (at least not strongly) at the critical angles (obliquely from the horizontal and vertical directions), because there is little or no light present there due to said limitation of the angular spectrum.On the other hand, the anamorphic diffuser D, which is arranged in front of the optical element from the viewpoint of an observer and develops its maximum scattering effect along a second plane which is at an angle of at least + / -45° (preferably at least + / -75°, but particularly preferably at least + / -87°, i.e. (quasi) perpendicular) to the first plane, subsequently ensures that the light is again fanned out along the second plane, which is particularly preferably approximately perpendicular to the first plane.

[0017] By means of the inventive approach, the so-called public-privacy contrast (ie, the ratio of the respective transmission values ​​between the two operating modes B1 and B2) increases significantly, particularly in relevant horizontal angle ranges, e.g. -35° to -60° and / or +35° to +60°, especially when vertical angular components of the light incidence direction are present from the viewpoint of an observer.

[0018] In the context of the invention, "limitation of the angular spectrum" or "limited angular range" means that the corresponding luminance or light transmission is concentrated to at least 80% or 90% in a defined angular range, while outside the defined limited angular range, residual light or residual transmission may still exist, which is usually due to technical reasons. Ideally, such residual light or residual transmission is minimal and decreases at larger angles.

[0019] In a first embodiment of the switchable light filter, the first optical element comprises a linear polarization filter. This transmits light polarized parallel to the first direction with maximum transmission. In conjunction with the liquid crystal layer, this corresponds to a "dual-cell" approach known in the prior art. In one state, the liquid crystal layer ensures angle-dependent polarization of the light passing through it, such that the linear polarization filter, as an optical element, absorbs light at angles with increasing distance from the perpendicular bisector. In another state, the liquid crystal layer changes the light's polarization properties only comparatively slightly, or ideally not at all, so that light can also pass through the linear polarization filter, as an optical element, at larger angles.The first preferred direction is chosen here, for example, parallel to the perpendicular bisector of the optical element. "Dual-cell" arrangements exhibit absorption weaknesses, particularly at larger angles (e.g., several percent transmission at approximately 55° horizontally), which are counteracted by the mechanisms according to the invention.

[0020] In contrast, in a second embodiment of the switchable light filter, the optical element comprises a plurality of light-absorbing transition dipole moments, wherein the majority of these transition dipole moments are permanently or at least in a first state aligned parallel to the selectable first preferred direction with a tolerance of at most + / -20° or vary around it, so that light incident on the first optical element is transmitted or at least partially absorbed depending on its direction of incidence relative to the optical element and its polarization state. Generally, s-polarized light is transmitted by such an optical element except for losses in almost all directions, while p-polarized light is absorbed at angles of incidence greater than approximately 20° from the first preferred direction, with the absorption increasing with the magnitude of the deviation.

[0021] The multitude of light-absorbing transition dipole moments are typically arranged in a layer at least 0.2 micrometers thick. Multiple layers of transition dipole moments may also be present, separated, for example, by OCA ("Optically Clear Adhesive") and / or substrates. The layer thicknesses of the transition dipole moments can range, for example, from 0.2 µm to 50 µm, preferably 0.2 µm to 20 µm, particularly preferably 1 µm to 10 µm. In a preferred embodiment, two layers of transition dipole moments are present in the first optical element, each of which is between 1 µm and 10 µm thick, wherein both each have a substrate (e.g. consisting of TAC or another non- or slightly birefringent material with a thickness preferably less than 100 µm), wherein furthermore the two substrates with the respective layers of transition dipole moments are glued together by means of an OCA (Optically Clear Adhesive).

[0022] Furthermore, the anamorphic diffuser D can exhibit a polarization dependence, meaning that light incident on it is scattered by the diffuser D in a first polarization state (the luminance curve is broadened in the angular range) and transmitted in a second polarization state (virtually scatter-free). This applies, for example, to grating structures that are smaller than one wavelength. Advantageously, the first polarization state, which causes scattering by the anamorphic diffuser, corresponds to the one exhibited by the light in operating mode B1 after passing through the liquid crystal layer 3.

[0023] Furthermore, there may be at least one birefringent layer between the optical element and the linear polarizer P.

[0024] In general, for the switchable light filter, the potentials between the electrodes for generating the first electric field EF1 and the second electric field EF2 differ by at least 2 volts.

[0025] In addition, the first optical element and / or the liquid crystal layer and / or the means for selectively generating a first electric field EF1 or a second electric field (EF2) can be divided into several separately switchable segments, so that local switchability between the respectively possible operating modes is enabled.

[0026] The invention also includes a first screen comprising an image display device and a switchable light filter as described above. The linear polarization filter P can be arranged in the image display device or be part of it.

[0027] Furthermore, the invention includes a second screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular spectrum which is restricted compared to the free view mode, comprising - a backlight extending over a large area and emitting light with a limited angular spectrum (whereby the backlight may optionally be designed to be directly luminous), whereby this limitation of the angular spectrum is effective at least perpendicular to the first plane (preferably, this first plane corresponds approximately or exactly to the horizontal direction when looking at the backlight, at least from the perspective of a seated or standing observer, but the first plane may also be arranged differently), which includes the perpendicular to a selectable point (preferably the center of the area; in the case of a curved backlight, this point may also be located elsewhere) on the backlight, - a transmissive image display device comprising, on the viewer side, a linear polarization filter P having its transmission maximum for light that is linearly polarized parallel to a first direction, wherein the first direction is preferably horizontal from the perspective of a seated or standing viewer, and wherein the linear polarization filter P can also be located behind all other components, - means for selectively generating a first electric field EF1 or a second electric field EF2, and a liquid crystal layer on which the first electric field EF1 or the second electric field EF2 acts and which, depending thereon, influences the polarisation state of light passing through it, - optionally a retardation layer R (e.g. a biaxially birefringent layer), - a first optical element which transmits or at least partially absorbs light depending on its polarization state and / or its direction of incidence, wherein a first preferred direction can be selected for the first optical element, which is arranged at an angle α to a perpendicular to the first optical element, and wherein the angle α is measured in the selectable first plane, - and an anamorphic diffuser D which develops its maximum scattering effect along a second plane which is at an angle of at least + / -45° (preferably at least + / -75°, but particularly preferably more than + / -87°, i.e. (quasi) perpendicular) to the first plane, - so that the luminance profiles of the light emitted by the second screen 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,- and such that light which, from the perspective of a viewer, is incident on the liquid crystal layer from behind has a restricted angular spectrum perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer and the first optical element, is at least partially fanned out parallel to the second plane due to the anamorphic diffuser D, whereby in one of the operating modes B1 or B2 (preferably B2), insufficient absorption effects (see also the description of the "hourglass" above) of the optical element for light from directions which deviate from the first preferred direction by at least 20° are at least partially compensated, and whereby in one of the operating modes B1 or B2 (preferably B2), a luminance profile of the light emitted by the second screen is present for a viewer,which is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane (which corresponds to a so-called “4-way privacy effect” because visibility is restricted from below, above, left and right).

[0028] In a first embodiment of the second screen, the first optical element comprises a linear polarization filter. This transmits light polarized parallel to the first direction, which, in conjunction with the liquid crystal layer, corresponds to a "dual-cell" approach known in the prior art. Here, the liquid crystal layer, in one state, ensures angle-dependent polarization of the light passing through it, which the linear polarization filter, as an optical element, absorbs light at angles with increasing distance from the perpendicular bisector. In another state, the liquid crystal layer changes the light's polarization properties only comparatively slightly, or ideally not at all, so that light can also pass through the linear polarization filter, as an optical element, at larger angles.

[0029] In contrast, in a second embodiment of the second screen, the optical element comprises a plurality of light-absorbing transition dipole moments, wherein the majority of these transition dipole moments are permanently or at least in a first state aligned parallel to the selectable first preferred direction with a tolerance of at most + / -20° or vary around it, so that light incident on the first optical element is transmitted or at least partially absorbed depending on its direction of incidence relative to the optical element and its polarization state. In general, s-polarized light is transmitted by such an optical element except for losses in almost all directions, while p-polarized light is absorbed at angles of incidence greater than approximately 20° from the first preferred direction, with the absorption increasing with the magnitude of the deviation.

[0030] The second screen described above may also include an anamorphic diffuser D that exhibits polarization dependence. Details of this have been described above.

[0031] Furthermore, the invention also includes a third screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular spectrum which is restricted compared to the free view mode, comprising - an image display device that emits light in a restricted angular spectrum (e.g. an OLED, microLED or LCD panel with optical means for a corresponding restriction of the angular spectrum), wherein this restriction of the angular spectrum is effective at least perpendicular to a selectable first plane (preferably, this first plane corresponds approximately or exactly to the horizontal direction when looking at the third screen, at least from the perspective of a seated or standing observer, but the first plane can also be located differently), which includes the perpendicular to a selectable point (preferably the center of the surface of the third screen; in the case of a curved image display device, this point can also be located elsewhere) on the image display device, - in the viewing direction of a viewer in front of the image display device, a linear polarization filter P which has its transmission maximum for light which is linearly polarized parallel to a first direction (the first direction is preferably horizontal from the perspective of a seated or standing viewer; the linear polarization filter P can also be located behind all other components), - means for selectively generating a first electric field EF1 or a second electric field EF2, and a liquid crystal layer on which the first electric field EF1 or the second electric field EF2 acts and which, depending thereon, influences the polarisation state of light passing through it, - optionally a retardation layer R (e.g. a biaxially birefringent layer), - a first optical element which transmits or at least partially absorbs light depending on its polarization state and / or its direction of incidence, wherein a first preferred direction can be selected for the first optical element, which is arranged at an angle α to a perpendicular of the first optical element, and wherein the angle α is measured in the selectable first plane, - and an anamorphic diffuser D which develops its maximum scattering effect along a second plane which is at an angle of at least + / -45° (preferably at least + / -75°, but particularly preferably at least + / -87°, i.e. at least approximately perpendicular) to the first plane, - so that the luminance profiles of the light emitted by the third screen 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, - and such that light which, from the perspective of a viewer, is incident on the liquid crystal layer from behind has a restricted angular spectrum perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer and the first optical element, is at least partially fanned out parallel to the second plane due to the anamorphic diffuser D, whereby in one of the operating modes B1 or B2 (preferably B2), insufficient absorption effects (see also the "hourglass" effects described above) of the optical element for light from directions which deviate from the first preferred direction by at least 20° are at least partially compensated, and whereby in one of the operating modes B1 or B2 (preferably B2), a luminance profile of the light emitted by the third screen is present for a viewer,which is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane (which corresponds to a so-called “4-way privacy effect” because visibility is restricted from below, above, left and right).

[0032] The following additional design features may optionally apply to the switchable light filter described above as well as to the first, second or third screen:

[0033] The liquid crystal layer can have a scattering effect in at least one operating mode, preferably in operating mode B1.

[0034] Advantageously, the transition dipole moments of a first optical element are formed as at least one, at least two, at least three, or more dichroic dyes mixed with the liquid crystals in a guest-host arrangement. For permanent transition dipole moments, the liquid crystals can preferably be fixed via a curing process.

[0035] In contrast, the transition dipole moments can also be embedded in a liquid crystal layer in a non-fixed manner as a guest-host arrangement, so that the transition dipole moments can be varied in their orientation and / or their magnitude between the first and at least one second state depending on the influence of this liquid crystal layer.

[0036] In the case that the transition dipole moments are variable, e.g. via so-called guest-host liquid crystal cells, such guest-host liquid crystal cells can correspond directly to the aforementioned liquid crystal layer (the liquid crystal layer and the first optical element would then together be one component), but do not have to.

[0037] The dichroic dye molecules generally align parallel to the liquid crystal molecules.

[0038] Alternatively, the first optical element may be formed as a laminate of layers of polymer film polarizers.

[0039] The aforementioned configurations also ensure that the first optical element is non-periodic in its structure. This is very advantageous because, in interaction with the pixel structures of screens, there is no risk of artifacts such as moiré effects.

[0040] In a preferred embodiment, light penetrating the liquid crystal layer is transmitted essentially unchanged when the second electric field EF2 is applied, while when the first electric field EF1 is applied, the incident light is circularly or elliptically polarized, or the polarization of the light is rotated by 90°. Essentially, this 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, leading to an undesirable, slight change in polarization.

[0041] For liquid crystal layer designs as TN liquid crystal cells, the following applies: The alignment of the liquid crystal molecules typically differs by 90° at the large surfaces that define the liquid crystal layer. Such alignment is supported by PMI or PVA and additional mechanical or optical surface treatment. For TN liquid crystal layers, it is also generally true that when switching between the electric fields EF1 and EF2, the majority of the liquid crystals in the liquid crystal layer are rotated out of 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 25° to 40° in plane.

[0042] The first electric field EF1 can, for example, have a field strength other than 0 V / µm, approximately on the order of 1 V / µm, e.g., as a square wave at 1 kHz, while the second electric field EF2, for example, has a field strength of 0 V / µm. However, the configuration can also be exactly the opposite, or such that both fields EF1 and EF2 are not field-free.

[0043] If the liquid crystal layer is arranged behind the first optical element in the viewing direction, the liquid crystal layer is preferably illuminated by linearly polarized light or elliptically polarized light, with a ratio of the 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 polarization filters in the light path, or by λ / 4 layers when using circularly polarized light.

[0044] In a further embodiment, at least two first optical elements are present, with a birefringent layer optionally being arranged between the at least two such first optical elements. Furthermore, the at least two first optical elements could, if appropriate, have different thicknesses of the layers containing the plurality of light-absorbing transition dipole moments, but this is not required.

[0045] The first preferred direction can, for example, enclose an angle between 0° and 45° with a surface normal of the first optical element. It is also possible for the first preferred direction to vary across the surface of the first optical element.

[0046] It can also be advantageous if the first optical element is divided into different areas (A1, A2, ...) along a selectable reference line. A separate preferred area direction can be selected for each area (A1, A2, ...), which applies to all transition dipole moments within an area (A1, A2, ...). All preferred area directions are pairwise different and point in the direction of the observer, up to a maximum tolerance of ±10 degrees. This arrangement has the advantage that the observer perceives a corresponding screen, or a screen with a switchable light filter, as homogeneously illuminated in restricted viewing mode.

[0047] 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.

[0048] The first preferred direction corresponds to the orientation of the transition dipole moments of the first optical element for a given propagation direction of light, in which the absorption is the same for any polarization of the light.

[0049] A first exemplary manufacturing variant for a first optical element with dipole moments 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 manufacturing steps (with reference to US 9,481,658 B2 or WO2021 / 177308A1 paragraph 37ff) - A low- or non-birefringent substrate 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 molecular alignment. - Application of the mixture of dichroic dye and thermotropic liquid crystalline compounds or polymers. - By irradiating light, the side chains are locally condensed, causing birefringence along the surface.

[0050] An alternative, second manufacturing variant uses thermotropic, liquid crystalline dichroic dyes (reference to JP2011-237513A), and includes the following steps: - Preparation 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.

[0051] The following materials, for example, are suitable for various manufacturing variants, although this list is not exhaustive: - As polymer substrate with low or no birefringence: preferably 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 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 2011-237513A as an example. - As chemical groups for crosslinking, which are bound to the thermotropic liquid crystalline compounds or polymers (crosslinking): metaacrylic groups, epoxy groups, oxetanyl groups, and styrene groups, preferably methacrylic groups. Alternatively, these can be polymerizable liquid crystal compounds, such as those described in JP 6268730 B2. - Polymerizable liquid crystalline dichroic dyes, for example azo dyes

[0052] The at least one dye consists of dye molecules, wherein a transition dipole or transition dipole moment is advantageously associated with each dye molecule, i.e. each dye molecule corresponds to a transition dipole or transition dipole moment. Typically, a dye has a mass fraction of at least 0.01%, preferably from 1% to 15%, of the material of the respective layers of the optical element in question. In special cases, in the case of liquid-crystalline dichroic dyes, the concentration can even reach 95%. The thickness of the layers is preferably in the range from 0.2 µm to 50 µm, more preferably in the range from 0.5 µm to 20 µm, all boundary values ​​included. The dyes or dye mixtures for different layers within an optical element can be different, but do not have to be.

[0053] For certain applications in which viewers wearing (usually vertically) linearly polarized sunglasses look at a previously described screen or a screen with a first switchable light filter, it can be advantageous to arrange a λ / 4 layer in the light path in the viewing direction in front of the screen to convert linearly polarized light into circularly polarized light. Depending on the condition of the first liquid crystal layer, left- or right-rotating, circularly polarized light would then be generated, which - with reduced brightness - is visible through the said sunglasses. In the case of crossed linear polarizations of sunglasses and the light emanating from the screen, if no such λ / 4 layer is present, it could otherwise happen, depending on the condition, that a viewer wearing such sunglasses essentially sees no image.

[0054] A switchable light filter as described above or a first, second, or third screen as described above is advantageously used in a mobile device, a motor vehicle, aircraft, or watercraft, in a payment terminal, or in an access system. Switching between the aforementioned operating modes is possible to protect sensitive data, i.e., to display it in a way that is perceptible to only one viewer, or alternatively, to display image content simultaneously for 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 indicated, but also in other combinations or on their own, without departing from the scope of the present invention. Short description of the drawings

[0057] The invention is explained in more detail below using exemplary embodiments with reference to the attached drawings, which also disclose features essential to the invention. These exemplary embodiments are merely illustrative and are not to be interpreted as restrictive. For example, a description of an embodiment with a large number of elements or components should not be interpreted to mean that all of these elements or components are necessary for implementation. Rather, other embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different exemplary embodiments may be combined with one another unless otherwise stated. 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 by the same reference numerals and are not explained more than once. They show: Fig. 1 a schematic diagram of the construction of a switchable light filter in a first embodiment, Fig. 2 a schematic diagram of the construction of a switchable light filter in a second embodiment, Fig. 3 a schematic diagram of a first screen with a switchable light filter in a first embodiment, Fig. 4 a schematic diagram of a first screen with a switchable light filter in a second embodiment, Fig. 5 a schematic diagram of a second screen, Fig. 6a a schematic diagram of a third screen in a first embodiment, Fig. 6b a schematic diagram of a third screen in a second embodiment, Fig. 7 a schematic diagram to explain the position of exemplary levels, Fig. 8 a schematic diagram of a first optical element, Fig. 9a normalized transmission values ​​in polar coordinates of an exemplary prior art light filter in a public viewing mode, Fig. 9b normalized transmission values ​​in polar coordinates of an exemplary light filter in the prior art in a view-restricted viewing mode, Fig. 10a normalized transmission values ​​in polar coordinates of an exemplary light filter in a public viewing mode, Fig. 10b normalized transmission values ​​in polar coordinates of an exemplary light filter in a view-restricted viewing mode, as well as Fig. 11 exemplary normalized luminance distributions in comparison of the invention to the prior art. Detailed description of the drawings

[0058] The drawings are not to scale and only represent basic principles. Fig. 8 only selected dipole moments are shown, while in reality a multitude of dipole moments exist.

[0059] It shows Fig. 1 a schematic diagram of the construction of a switchable light filter in a first embodiment, comprising - a first optical element 1, which transmits or at least partially absorbs light depending on its polarization state and / or its direction of incidence, wherein a first preferred direction can be selected for the first optical element 1, which is arranged at an angle α to the perpendicular bisector of the first optical element 1, and wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, - means 7 arranged behind the first optical element 1 from the perspective of an observer for limiting the angular spectrum of light incident on the switchable light filter 5, this limitation of the angular spectrum being effective at least perpendicular to the first plane (in this example, this first plane, from the perspective of a seated or standing observer, contains the horizontal direction when looking at the switchable light filter 5), - means (not shown in the drawing) for selectively generating a first electric field EF1 or a second electric field EF2 (e.g. two ITO layers with electronic control), - an anamorphic diffuser D, which is arranged in front of the optical element 1 from the viewpoint of an observer and which develops its maximum scattering effect along a second plane which is at an angle of 90° (i.e. perpendicular) to the first plane, in this example, - a linear polarization filter P, which has its transmission maximum for light that is linearly polarized parallel to a first direction (here, from the viewpoint of an observer, the first direction is horizontal), - a liquid crystal layer 3 arranged between the linear polarization filter P and the first optical element 1, 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, - a retardation layer R (e.g. a biaxially birefringent layer) which is located between the linear polarizer P and the first optical element 1, here between the linear polarizer P and the liquid crystal layer 3, - 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, - and such that light which, from the perspective of an observer, is incident on the liquid crystal layer 3 from behind has a restricted angular spectrum perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer 3 and the first optical element 1, is at least partially spread out parallel to the second plane due to the anamorphic diffuser D, whereby in one of the two operating modes B1 or B2 (preferably B2), insufficient absorption effects of the optical element 1 (further information on the so-called "hourglass" behavior of the first optical element will be provided below) for light from directions which deviate from the first preferred direction by at least 20° are at least partially compensated, and whereby in one of the two operating modes B1 or B2 (preferably B2), a luminance profile of the light penetrating the switchable light filter 5 is present for an observer,which is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane (which corresponds to a so-called “4-way privacy effect” because visibility is restricted from below, above, left and right).

[0060] To explain an example position of the first and second selectable level, the Fig. 7 shows a schematic diagram. The first selectable plane is perpendicular to the first optical element 1, since it contains its bisector. In this example, the second selectable plane is perpendicular to the first selectable plane and perpendicular to the first optical element 1. The diagram also shows how the angle α is measured within the first selectable plane.

[0061] An essential element for the general functioning of the invention, which is also applied to the screens below, is that insufficient absorption effects of the optical element 1 for light from directions that deviate from the first preferred direction by at least 20° - in particular for light from planes of incidence that are rotated by more than 10° or more than 20° with respect to the selectable first plane - are at least partially compensated. Typically, an optical element 1 in the prior art has an "hourglass shape" of the transmission values ​​in polar coordinates for linearly polarized light (assumption for the simpler explanation of the effect: (from the perspective of an observer) horizontally linearly polarized light is incident upon it). This is in Fig. 9b. The hourglass shape is also roughly maintained when differently polarized light is incident on an optical element 1 in the prior art, see Fig. 9a. With regard to Fig. 9b, the said hourglass shape undesirably causes the absorption to become weaker when a vertical angular component is added from a horizontal lateral view. Due to the present invention, this "hourglass shape" is now, so to speak, cut off in the vertical direction, corresponding to the effect of the said means 7 for limiting the angular spectrum of light, wherein this limitation of the angular spectrum takes place at least perpendicular to the first plane (i.e., in this example, vertically from the perspective of an observer). This means that the light penetrating the optical element 1 does not have to be absorbed (at least not strongly) at the critical angles (obliquely from the horizontal and vertical directions) because, due to the said limitation of the angular spectrum, no or hardly any light is present there.On the other hand, the anamorphic diffuser D, which is arranged in front of the optical element 1 from the point of view of an observer and develops its maximum scattering effect along a second plane which is at an angle of at least + / -45°, but preferably as in . Fig. 7, is perpendicular to the first selectable plane, then again for a fanning out of the light along the second plane.

[0062] To better understand the results achieved, Fig. 10a a normalized transmission value in polar coordinates of an exemplary light filter in a public viewing mode (here corresponding to operating mode B1) and Fig. 10b in a view-restricted viewing mode (here corresponding to operating mode B2). While compared to the state of the art in operating mode B1 (cf. Fig. 9a) by the present invention (see example in Fig. 10a) essentially only the vertical transmission components have been reduced, in operating mode B2 for a restricted view mode a significantly better visual protection behavior is available. While in the state of the art (see example according to Fig. 9b) the above-described hourglass shape is present, in which, for example, the standardized transmission for the exemplary angle H-40°V-30° is still above 0.05, in the invention in the example according to Fig. 10b for all vertical angle components added to the horizontal angle H-40°, the normalized transmission is clearly below 0.05. The comparison of the drawings Fig. 9b and Fig. Figure 10b also shows in general how, especially for light from planes of incidence that are rotated by more than 10° or more than 20° relative to the first plane, the absorption is increased, thus the transmission is reduced.

[0063] The example according to Fig. 10b also verifies the desired result of the inventive measures, whereby in one of the two operating modes B1 or B2 (here: B2), insufficient absorption effects of the optical element 1 for light from directions that deviate from the first preferred direction by at least 20° are at least partially compensated, and whereby in one of the two operating modes B1 or B2 (here: B2), a viewer sees a luminance profile of the light penetrating the switchable light filter 5 that is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane (which corresponds to a so-called "4-way privacy effect" because visibility is restricted from below, above, left and right).

[0064] Furthermore, Fig. 11 exemplary standardized luminance distributions comparing the invention with the prior art, simulated for horizontal viewing directions with a viewing angle perpendicular to the vertical (V0°). Even for this horizontal angle spectrum, which is combined exclusively with vertical-perpendicular angle components, the privacy effect is increased, i.e., the standardized transmission is reduced, especially for horizontal viewing angles with absolute values ​​greater than 30°.

[0065] By means of the inventive approach, the so-called public-privacy contrast (ie, the ratio of the respective transmission values ​​between the two operating modes B1 and B2) increases significantly, particularly in relevant horizontal angular ranges, e.g. -35° to -60° and +35° to +60°, especially when vertical angular components of the viewing direction are present from the viewpoint of an observer.

[0066] From the observer's perspective, the linear polarization filter P can in principle also be located behind all other components or directly behind the diffuser D. The general sequence of the optical element 1, liquid crystal cell 3, optional retardation layer R, linear polarizer P can also be reversed. However, the optical effect remains unchanged. Such an arrangement is shown in Fig. Figure 2 as a schematic diagram of the structure of a switchable light filter 5 in a second embodiment. Accordingly, the explanations given for Fig. Figure 1 apply here.

[0067] In the context of the invention, "limitation of the angular spectrum" or "limited angular range" means that the corresponding luminance or light transmission is concentrated to at least 80% or 90% in a defined angular range, while outside the defined limited angular range, residual light or residual transmission may still exist, which is usually due to technical reasons. Ideally, such residual light or residual transmission is minimal and decreases at larger angles.

[0068] In a first embodiment of the switchable light filter 5, the first optical element 1 comprises a linear polarization filter. This transmits light polarized parallel to the first direction with maximum transmission, which, in conjunction with the liquid crystal layer 3, corresponds to a "dual-cell" approach known in the prior art. Here, the liquid crystal layer 3 ensures an angle-dependent polarization of the light penetrating it in one state, such that the linear polarization filter as the optical element 1 absorbs light at angles with increasing distance from the perpendicular bisector, while in another state the liquid crystal layer 3 changes the light's polarization properties only comparatively little, or ideally not at all, so that light can penetrate the linear polarization filter as the optical element 1 even at larger angles.

[0069] In contrast, in a second embodiment of the switchable light filter 5, the optical element 1 comprises a plurality of light-absorbing transition dipole moments, wherein the majority of these transition dipole moments are permanently or at least in a first state aligned parallel to the selectable first preferred direction with a tolerance of at most + / -20° or vary around it, so 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 optical element 1 and its polarization state. In general, s-polarized light is transmitted by such an optical element 1 except for losses in almost all directions, while p-polarized light with angles of incidence greater than approximately 20° from the first preferred direction is absorbed, with the absorption increasing with the magnitude of the deviation. Fig. Figure 8 shows a schematic diagram of a first optical element 1, where the dot-dash line indicates the perpendicular bisector of the first optical element 1, and the dashed line indicates the first preferred direction, between which angle α is spanned. The solid arrows indicate aligned transition dipole moments. In the actual implementation, there are considerably more than just the few transition dipole moments shown here.

[0070] The multitude of light-absorbing transition dipole moments are typically arranged in a layer at least 0.2 micrometers thick. Multiple layers of transition dipole moments may also be present, separated, for example, by OCA ("Optically Clear Adhesive") and / or substrates. The layer thicknesses of the transition dipole moments can range, for example, from 0.2 µm to 50 µm, preferably 0.2 µm to 20 µm, particularly preferably 1 µm to 10 µm. In a preferred embodiment, two layers of transition dipole moments are present in the first optical element 1, each of which is between 1 µm and 10 µm thick, each of which has a substrate (e.g. consisting of TAC or another non-birefringent or slightly birefringent material with a thickness preferably less than 100 µm), wherein furthermore the two substrates with the respective layers of transition dipole moments are glued together by means of an OCA (Optically Clear Adhesive).

[0071] In general, for the switchable light filter 5, the potentials between the electrodes for generating the first electric field EF1 and the second electric field EF2 differ by at least 2 volts.

[0072] In addition, the first optical element 1 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) can be divided into several separately switchable segments, so that local switchability between the respectively possible operating modes is enabled.

[0073] The invention also includes a first screen comprising an image display device 9 and a switchable light filter 5 as described above. The linear polarization filter P can be arranged in the image display device 9 or be a part of it. Fig. 3 a schematic diagram of a first screen with a switchable light filter in a first embodiment and Fig. 4 a schematic diagram of a first screen with a switchable light filter in a second embodiment.

[0074] Furthermore, Fig. 5 shows a schematic diagram of a second screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular spectrum which is restricted compared to the free view mode, comprising - a surface-like extended backlight 8b which emits light with a limited angular spectrum (wherein the backlight 8b can optionally be designed to be directly luminous), whereby this limitation of the angular spectrum is effective at least perpendicular to the first plane (in this example, this first plane corresponds at least from the perspective of a seated or standing observer approximately or exactly to the horizontal direction when looking at the backlight 8b), which includes the perpendicular to a selectable point (preferably the surface center; in the case of a curved backlight, this point can also be located elsewhere) on the backlight 8b, - a transmissive image display device 9b, which comprises a linear polarization filter P on the viewer side, which has its transmission maximum for light that is linearly polarized parallel to a first direction, wherein the first direction is preferably horizontal from the perspective of a seated or standing viewer, and wherein the linear polarization filter P can also be located behind all other components, - means (not shown in the drawing) for selectively generating a first electric field EF1 or a second electric field EF2, and 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, - retardation layer R (e.g. a biaxially birefringent layer), - a first optical element 1, which transmits light or at least partially absorbs it depending on its polarization state and / or its direction of incidence, wherein a first preferred direction can be selected for the first optical element 1, which is arranged at an angle α to a perpendicular to the first optical element 1, and wherein the angle α is measured in the selectable first plane, - and an anamorphic diffuser D, which develops its maximum scattering effect along a second plane which is at an angle of at least + / -45°, in this example + / -90°, i.e. perpendicular to the first plane, - so that the luminance profiles of the light emitted by the second screen 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, - and such that light which, from the perspective of a viewer, is incident on the liquid crystal layer 3 from behind has a restricted angular spectrum perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer 3 and the first optical element 1, is at least partially spread out parallel to the second plane due to the anamorphic diffuser D, whereby in one of the operating modes B1 or B2 (preferably B2), insufficient absorption effects (see also the description of the "hourglass" above) of the optical element 1 for light from directions which deviate from the first preferred direction by at least 20° are at least partially compensated, and whereby in one of the operating modes B1 or B2 (preferably B2), a luminance profile of the light emitted by the second screen is present for a viewer,which is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane (which corresponds to a so-called “4-way privacy effect” because visibility is restricted from below, above, left and right).

[0075] The operation of this second screen, as well as the third one described below, corresponds to the operation of the switchable light filter 5 explained in more detail above. Instead of using means 7 (as in the switchable light filter 5) to limit the angular spectrum, in the second screen the angular spectrum is already limited in the background lighting 8b. In the third screen (see Fig. 6a and Fig. 6b) the angular spectrum is already suitably limited by the image display device 9c.

[0076] In a first embodiment of the second screen, the first optical element 1 comprises a linear polarization filter. This transmits light that is polarized parallel to the first direction, which, in conjunction with the liquid crystal layer 3, corresponds to a "dual-cell" approach known in the prior art. Here, the liquid crystal layer 3 ensures angle-dependent polarization of the light penetrating it in one state, whereby the linear polarization filter as the optical element 1 absorbs light at angles with increasing distance from the perpendicular bisector, while in another state the liquid crystal layer 3 changes the light's polarization properties only comparatively slightly or ideally not at all, so that light can penetrate the linear polarization filter as the optical element 1 even at larger angles.

[0077] In contrast, in a second embodiment of the second screen, an optical element 1 comprises a plurality of light-absorbing transition dipole moments, as already explained above.

[0078] The second screen described above may also include an anamorphic diffuser D that exhibits polarization dependence. Details of this have been described above.

[0079] Furthermore, the invention also includes a third, as a schematic diagram in Fig. 6a shown screen in a first embodiment variant, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted in an angular spectrum that is restricted compared to the free view mode, comprising - an image display device 9c that emits light in a restricted angular spectrum (e.g., an OLED, microLED, or LCD panel with optical means, such as a lenticular array or a privacy filter, for a corresponding restriction of the angular spectrum; however, the restriction of the angular spectrum can also be intrinsically present in the structure or stack of an OLED / microLED or other panel), wherein this restriction of the angular spectrum is effective at least perpendicular to a selectable first plane (in this example, this first plane corresponds to the horizontal direction when looking at the third screen from the perspective of a seated or standing observer), which includes the perpendicular to a selectable point (preferably the center of the surface of the third screen; in the case of a curved image display device, this point can also be located elsewhere) on the image display device 9c, - in the viewing direction of a viewer in front of the image display device 9c, a linear polarization filter P which has its transmission maximum for light which is linearly polarized parallel to a first direction (the first direction is preferably horizontal from the perspective of a seated or standing viewer), - means (not shown in the drawing) for selectively generating a first electric field EF1 or a second electric field EF2, and 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, - a retardation layer R (e.g. a biaxially birefringent layer), - a first optical element 1, which transmits light or at least partially absorbs it depending on its polarization state and / or its direction of incidence, wherein a first preferred direction can be selected for the first optical element 1, which is arranged at an angle α to a perpendicular of the first optical element 1, and wherein the angle α is measured in the selectable first plane, - and an anamorphic diffuser D, which develops its maximum scattering effect along a second plane which is at an angle of at least + / -45°, in this example at an angle of 90°, i.e. perpendicular to the first plane, - so that the luminance profiles of the light emitted by the third screen 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, - and such that light which, from the perspective of a viewer, is incident on the liquid crystal layer 3 from behind has a restricted angular spectrum perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer 3 and the first optical element 1, is at least partially spread out parallel to the second plane due to the anamorphic diffuser D, whereby in one of the operating modes B1 or B2 (preferably B2), insufficient absorption effects (see also the "hourglass" effects described above) of the optical element 1 for light from directions which deviate from the first preferred direction by at least 20° are at least partially compensated, and whereby in one of the operating modes B1 or B2 (preferably B2), a luminance profile of the light emitted by the third screen is present for a viewer,which is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane (which corresponds to a so-called “4-way privacy effect” because visibility is restricted from below, above, left and right).

[0080] Based on the structure according to Fig. 6a, the general sequence of optical element 1, optional retardation layer R, liquid crystal cell 3, and linear polarizer P can also be reversed. The optical effect does not change. Such a setup is shown in Fig. 6b shows a schematic diagram of a second design variant of the second screen.

[0081] The following additional design features may optionally apply to the switchable light filter 5 described above as well as to the first, second or third screen:

[0082] The liquid crystal layer 3 can have a scattering effect in at least one operating mode, preferably in operating mode B1.

[0083] Advantageously, the transition dipole moments of a first optical element 1 are formed as at least one, at least two, at least three, or more dichroic dyes mixed with the liquid crystals in a guest-host arrangement. For permanent transition dipole moments, the liquid crystals can preferably be fixed via a curing process.

[0084] The aforementioned configurations also ensure that the first optical element 1 has a non-periodic structure. This is very advantageous because, in interaction with pixel structures of image display devices, there is no risk of artifacts such as moiré effects.

[0085] In a preferred embodiment, light penetrating the liquid crystal layer 3 is transmitted essentially unchanged when the second electric field EF2 is applied, while when the first electric field EF1 is applied, the incident light is circularly or elliptically polarized, or the polarization of the light is rotated by 90°. Essentially, this 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, leading to an undesirable, slight change in polarization.

[0086] The first electric field EF1 can, for example, have a field strength other than 0 V / µm, approximately on the order of 1 V / µm, e.g., as a square wave at 1 kHz, while the second electric field EF2, for example, has a field strength of 0 V / µm. However, the configuration can also be exactly the opposite, or such that both fields EF1 and EF2 are not field-free.

[0087] The first preferred direction can, for example, enclose an angle between 0° and 45° to a surface normal of the first optical element.

[0088] The first preferred direction corresponds to the orientation of the transition dipole moments of a first optical element 1 for a given propagation direction of light, in which the absorption is the same for any polarization of the light.

[0089] The at least one dye consists of dye molecules, wherein a transition dipole or transition dipole moment is advantageously associated with each dye molecule, i.e. each dye molecule corresponds to a transition dipole or transition dipole moment. Typically, a dye has a mass fraction of at least 0.01%, preferably from 1% to 15%, of the material of the respective layers of the optical element in question. In special cases, in the case of liquid-crystalline dichroic dyes, the concentration can even reach 95%. The thickness of the layers is preferably in the range from 0.2 µm to 50 µm, more preferably in the range from 0.5 µm to 20 µm, all boundary values ​​included. The dyes or dye mixtures for different layers within an optical element can be different, but do not have to be.

[0090] For certain applications in which viewers wearing (usually vertically) linearly polarized sunglasses look at the aforementioned first, second, or third screens, it may be advantageous to further arrange a λ / 4 layer in the light path in the viewing direction in front of the screen to convert linearly polarized light into circularly polarized light. Depending on the state of the first liquid crystal layer 3, left- or right-rotating, circularly polarized light would then be generated, which - with reduced brightness - is visible through said sunglasses. In the case of crossed linear polarizations of sunglasses and the light emanating from the screen, if no such λ / 4 layer is present, it could otherwise happen, depending on the state, that a viewer wearing such sunglasses essentially sees no image.

[0091] The invention solves the stated problem: An optical element has been 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 depending on its position, but rather on its position. Switchable light filters that utilize the optical element influence the transmission of light in an angle-dependent manner, allowing switching between at least two operating states. Furthermore, in particular, insufficient absorption effects in certain directions are at least partially compensated. Corresponding screens have been disclosed.

[0092] The invention described above, in conjunction with an image display device, can be advantageously used wherever confidential data is displayed and / or entered, such as when entering a PIN or displaying data at ATMs or payment terminals, entering a password, or reading emails on mobile devices. As described above, the invention can also be used in cars to selectively withhold distracting image content from the driver or passenger. List of reference symbols 1 first optical element 3 Liquid crystal layer 5 switchable light filters 7 Means of limiting the angular spectrum 8b Backlight 9 Image display device 9b Image display device 9c Image display device D anamorphic diffuser P Polarizing filter R delay layer

Claims

[1] Switchable light filter (5) comprising - a first optical element (1) which transmits or at least partially absorbs light depending on its polarization state and / or its direction of incidence, wherein a first preferred direction can be selected for the first optical element (1), which is arranged at an angle α to the perpendicular bisector of the first optical element (1), and wherein the angle α is measured in a selectable first plane containing said perpendicular bisector, - means (7) arranged behind the first optical element (1) from the perspective of an observer for limiting the angular spectrum of light incident on the switchable light filter (5), this limitation of the angular spectrum being effective at least perpendicular to the first plane, - means for selectively generating a first electric field EF1 or a second electric field EF2, - an anamorphic diffuser (D) which, from the perspective of an observer, is arranged in front of the first optical element (1) and which develops its maximum scattering effect along a second plane which is at an angle of at least + / -45° to the first plane, - a linear polarization filter (P) having its transmission maximum for light linearly polarized parallel to a first direction, and optionally arranged between the anamorphic diffuser (D) and said means (7) for limiting the angular spectrum of light, - optionally a retardation layer (R) located between the linear polarization filter (P) and the first optical element 1, - a liquid crystal layer (3) arranged between the linear polarization filter (P) and the first optical element (1), 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, - and such that light which, from the perspective of an observer, is incident on the liquid crystal layer (3) from behind, has an angular spectrum which is restricted perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer (3) and the first optical element (1), is at least partially fanned out parallel to the second plane due to the anamorphic diffuser (D), as a result of which, in one of the operating modes B1 or B2, insufficient absorption effects of the first optical element (1) for light from directions which deviate from the first preferred direction by at least 20° are at least partially compensated, and as a result of which, in one of the operating modes B1 or B2, a luminance profile of the light penetrating the switchable light filter (5) is present for an observer which is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane. [2] Switchable light filter (5) according to claim 1, characterized bythat the first optical element (1) is a linear polarization filter. [3] Switchable light filter (5) according to claim 1, characterized by in that the first optical element (1) comprises a plurality of light-absorbing transition dipole moments, the majority of these transition dipole moments being permanently or at least in a first state with a tolerance of at most + / -20° parallel to the selectable first preferred direction or varying around it, so that light which is 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. [4] Switchable light filter (5) according to one of the preceding claims, characterized by that the anamorphic diffuser (D) has a polarization dependence. [5] Switchable light filter (5) according to one of the preceding claims, characterized by that there is at least one birefringent layer between the first optical element (1) and the linear polarization filter (P). [6] First screen comprising an image display device (9) and a switchable light filter (5) according to one of the preceding claims. [7] First screen according to claim 6, characterized by that the linear polarization filter (P) of the switchable light filter (5) is arranged in the image display device (9) or is a part of it. [8] Second screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode in which light is emitted in an angular spectrum which is restricted compared to the free view mode, - a backlight (8b) extending over a large area and emitting light with a restricted angular spectrum, said restriction of the angular spectrum being effective at least perpendicular to a selectable first plane including a perpendicular to a selectable point on the backlight (8b), - a transmissive image display device (9b) which comprises, on the viewer side, a linear polarization filter (P) which has its transmission maximum for light which is linearly polarized parallel to a first direction, - means for selectively generating a first electric field EF1 or a second electric field EF2 and 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, - optionally a delay layer (R), - a first optical element (1) which transmits or at least partially absorbs light depending on its polarization state and / or its direction of incidence, wherein a first preferred direction can be selected for the first optical element (1), which is arranged at an angle α to a perpendicular to the first optical element (1), and wherein the angle α is measured in the selectable first plane, - and an anamorphic diffuser (D) which develops its maximum scattering effect along a second plane which is at an angle of at least + / -45° to the first plane, - so that the luminance profiles of the light emitted by the second screen 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, - and such that light which, from the perspective of a viewer, is incident on the liquid crystal layer (3) from behind, has an angular spectrum which is restricted perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer (3) and the first optical element (1), is at least partially fanned out parallel to the second plane due to the anamorphic diffuser (D), as a result of which, in one of the operating modes B1 or B2, insufficient absorption effects of the first optical element (1) for light from directions which deviate from the first preferred direction by at least 20° are at least partially compensated, and as a result of which, in one of the operating modes B1 or B2, a viewer sees a luminance profile of the light emitted by the second screen which is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane. [9] Second screen according to claim 8, characterized bythat the first optical element (1) is a linear polarization filter. [10] Second screen according to claim 8, characterized by in that the first optical element (1) comprises a plurality of light-absorbing transition dipole moments, the majority of these transition dipole moments being permanently or at least in a first state with a tolerance of at most + / -20° parallel to the selectable first preferred direction or varying around it, so that light which is 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. [11] Second screen according to claim 9 or 10, characterized by that the anamorphic diffuser (D) has a polarization dependence. [12] Third screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode in which light is emitted in an angular spectrum which is restricted compared to the free view mode, comprising - an image display device (9c) which emits light with a restricted angular spectrum, this restriction of the angular spectrum being effective at least perpendicular to a selectable first plane which includes a perpendicular to a selectable point on the image display device (9c), - in the viewing direction of a viewer in front of or in the image display device (9c), a linear polarization filter (P) which has its transmission maximum for light which is linearly polarized parallel to a first direction, - means for selectively generating a first electric field EF1 or a second electric field EF2 and 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, - optionally a delay layer (R), - a first optical element (1) which transmits or at least partially absorbs light depending on its polarization state and / or its direction of incidence, wherein a first preferred direction can be selected for the first optical element (1), which is arranged at an angle α to a perpendicular of the first optical element (1), and wherein the angle α is measured in the selectable first plane, - and an anamorphic diffuser (D) which develops its maximum scattering effect along a second plane which is at an angle of at least + / -45° to the first plane, - so that the luminance profiles of the light emitted by the third screen 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, - and such that light which, from the perspective of a viewer, is incident on the liquid crystal layer (3) from behind, has an angular spectrum which is restricted perpendicular to the first plane, wherein the same light, after penetrating the liquid crystal layer (3) and the first optical element (1), is at least partially fanned out parallel to the second plane due to the anamorphic diffuser (D), as a result of which, in one of the operating modes B1 or B2, insufficient absorption effects of the first optical element (1) for light from directions which deviate from the first preferred direction by at least 20° are at least partially compensated, and as a result of which, in one of the operating modes B1 or B2, a viewer sees a luminance profile of the light emitted by the third screen which is simultaneously restricted in the angular spectrum parallel and perpendicular to the first plane. [13] Switchable light filter (5) or first, second or third screen according to one of the preceding claims, characterized by that the liquid crystal layer (3) has a scattering effect in at least one operating mode B1 or B2.

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