Switchable light filter, illumination device, and screen

Abstract

The invention relates to a switchable light filter (5) comprising a first optical element (1) which in turn comprises a plurality of light-absorbing transition dipole moments such that light which is incident into the first optical element (1) is transmitted depending on its direction of incidence with respect to the first optical element (1) and its polarisation state or is at least partially absorbed, means for generating at least two different electric fields, a liquid crystal layer (3) on which the electric field in question acts such that the transmission properties of the switchable light filter (5) can be modified by changing the applied electric field between a first operating mode B1 and a second operating mode B2, wherein the relative transmission in each case in the two operating modes B1 and B2 at at least one point on the optical element (1) is described by a transmission TB1(β) for the operating mode B1, and TB2(β) for the operating mode B2, these each being normalised such that TB1(α)=1 and TB2(α)=1 for the values of the transmission. Depending on the operating mode, the light filter (5) influences how light is transmitted depending on angle of incidence and polarisation state. The invention furthermore discloses illumination devices and screens which use an above-described 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. State of the art

[0005] US Patent 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 conjunction with a transmissive image display device, such as an LCD, this allows for the creation of a screen that can be switched between a free and restricted viewing mode. 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 certain other technologies. 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° angle. 0 Sweet spot, i.e., the best possible 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, so that the light intensity is insufficient for viewing angles greater than + / -40°. 0 up to 3% of the maximum light intensity.

[0012] The aforementioned methods and arrangements generally share the disadvantage that they significantly reduce the brightness of the main 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.

[0013] The object of the invention is therefore to describe a light filter with an optical element in which light incident on the optical element is filtered depending on its direction of incidence and its polarization properties - not primarily However, depending on its position, light is transmitted or partially or completely absorbed. The light filters used in the optical element 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 states. In particular, the transmission behavior should be switchable for specific directions.

[0014] In a first embodiment according to the invention, this problem is solved by a 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 0.2 micrometers thick, wherein the absorbing transition dipole moments are preferably formed by dichroic dyes where the dye mass density is typically more than 1% or more than 10%,• wherein the majority of the transition dipole moments are aligned parallel to or fluctuate around a first preferred direction selectable for the first optical element, at least in a first state (in this context, the property "at least in a first state" encompasses several possibilities: firstly, that there can be exactly one state; this is a permanent configuration. However, the formulation explicitly also includes the possibility of two or more states. In this case, the transition dipole moments are variable, e.g., via so-called guest-host liquid crystal cells) with a tolerance of a maximum of 20° (other tolerance values, such as 5° or 10°, are also possible), wherein the first preferred direction is arranged at a predetermined angle α to the perpendicular bisector of the first optical element, e.g.,with a=0°, a=+ / -2° or magnitude(a)>2°, wherein the angle a is measured in a selectable first plane which contains the said perpendicular bisector, and wherein the angle a is preferably measured parallel to an edge of the first optical element, such as the lower edge. • so that light which enters the first optical element with an incidence direction and a polarization state is transmitted or at least partially absorbed depending on its incidence direction relative to the first optical element and its polarization state, Means for selectively generating a first electric field EF1 or a second electric field EF2, a liquid crystal layer arranged 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, 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, wherein the preferred polarization transmission direction of the first polarization filter is preferably aligned parallel to an edge of the first optical element, preferably the lower edge, so that the transmission properties of the switchable light filter can be adjusted between a first operating mode B1, in which the first electric field EF1 (e.g.a distinction is made between a first operating mode B1 and a second operating mode B2, in which the second electric field EF2 (e.g. with a field strength other than 0 V / pm, approximately on the order of 1 V / pm, e.g. as a square wave with 10 kHz) is applied, wherein the respective relative transmission in the two operating modes B1 and B2 at at least one point on the first optical element (preferably at several points, particularly advantageously on at least half the area of ​​the optical element) is adjusted to a selectable tolerance of e.g. 3% or 5% by a first transmission Tßi(ß) for the first operating mode B1 and a second transmission T, respectively. B 2(ß) is described for the second operating mode B2, which depends on an angle ß and is normalized in such a way that the transmission values ​​for the given angle a T B i(a)=1 and T B2 (a)=1 holds true if light • at angles β with a-60° < β < a-40° or a+40° < β < a+60° into the first optical element 1, in the first operating mode B1, in which the first electric field EF1 is applied, an s-polarized component of said light to at least a normalized first transmittance value T B i(ß) ^0.25 (preferred T B i(ß) ^0,3) is transmitted, as well as • at angles β with a-60° < β < a-40° or a+40° < β < a+60° into the first optical element, in the second operating mode B2, in to which the second electric field EF2 is applied, the p-polarized component of said light to at most a normalized second transmission value T B2 (β) < 0.2 (preferably T B2 (β) < 0, 1 , especially preferred T B 2(ß) 0.05, and advantageously also for all angles ß < a-40° or a+40° < ß) is transmitted.

[0015] S-polarized light refers to light whose electric field is oriented perpendicular to the plane of incidence on a medium, and p-polarized light refers to light oriented parallel to the plane of incidence. The plane of incidence is defined by the normal to the interface of the medium and the direction of incidence. An important mean-effect relationship exists as follows: By switching between the first operating mode B1, in which the first electric field EF1 (e.g., with a field strength of 0 V / pm) is present, and the second operating mode B2, in which the second electric field EF2 (e.g., with a field strength other than 0 V / pm, approximately on the order of 1 V / pm, e.g., as a square wave with 10 kHz) is present, only in operating mode B2 is s-polarized light incident on the liquid crystal layer essentially converted into p-polarized light, which then incidents on the first optical element, and vice versa.In combination with the layer of the first optical element, which is at least 0.2 micrometers thick and has absorbing transition dipole moments, the transmission variations mentioned for T result. B i(ß) or T B 2(ß) the aforementioned operating modes. Furthermore, if the absorbing transition dipole moments are formed by dichroic dyes, where the dye mass density is generally more than 1%, this provides additional support.

[0016] Further operating modes B3, B4, etc., with electric fields EF3, EF4, etc., differing from those of EF1 and EF2, may be explicitly provided. Furthermore, the operating modes B1, B2, etc., may also differ locally on the switchable light filter.

[0017] To standardize the transmission T Bi (a)=1 and T B2(a)=1, the following should be noted: The angles β and a are, of course, measured in the same previously described plane. It is also fundamentally possible that T B i(a)>1 and / or T B2 (a)>1 applies for angle β^a. In many cases, T applies. B i(a)<1 and / or T B2 (a)<1 applies for angle ß^a.

[0018] 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 directly correspond to the aforementioned liquid crystal layer, but do not have to.

[0019] In a preferred embodiment, light penetrating the liquid crystal layer is transmitted essentially unchanged when the first electric field EF1 is applied, while when the second electric field EF2 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, leading to an undesirable, slight change in polarization.

[0020] 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, it is generally the case 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 around 20° to 30°.

[0021] If the liquid crystal layer is positioned behind the first optical element in the viewing direction, linearly polarized or elliptically polarized light with a major-to-minor axis ratio of at least 4:1 (preferably at least 5:1 or more) is preferentially incident. This can be achieved, for example, by linear polarizing filters in the light path, or by X / 4 layers in the case of circularly polarized light.

[0022] It is advantageous that the first optical element (and any further such optical element, if present) and / or the liquid crystal layer is divided into several separately switchable segments, thus enabling local switching between the respective possible operating states.

[0023] In a further embodiment, the switchable light filter comprises at least two first optical elements, with an optional retarder between at least two such first optical elements are arranged. Furthermore, the at least two first optical elements could, but do not necessarily have, different thicknesses of the layers containing the multitude of light-absorbing transition dipole moments.

[0024] In certain embodiments of the invention, there may be at least one first specific angle βi for which the transmission of the p-polarized component of said light is not equal to the transmission of the s-polarized component of said light. Preferably, this condition applies to an entire angular range of angles β and particularly preferably even to all angles α β.

[0025] In other embodiments of the invention, it can be the case that in both operating modes B1 and B2 there is at least one second specific angle β2 for which the transmission of the s-polarized component of said light is greater than that of the p-polarized component of said light. Preferably, this condition applies to an entire range of angles β2, and particularly preferably even to all angles.

[0026] The invention also comprises a lighting device in a first embodiment 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 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.

[0027] In addition, the invention also comprises a screen in a first embodiment 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 restricted for a viewer compared to the free viewing mode, comprising - a lighting device as described above, which in particular includes a switchable light filter with the structural features and properties 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 display device, which is arranged in front of the switchable light filter in the viewing direction, - 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.

[0028] Preferably, the first or second linear polarization filter P is arranged in or is part of the transmissive image display device.

[0029] Furthermore, the invention comprises a screen in a second embodiment, which can be operated in at least a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode, in which light is emitted 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 with the structural features and properties as described above, - 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.

[0030] 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".

[0031] Furthermore, the invention comprises a screen in a third and a fourth embodiment, which can be operated in at least a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode, in which light is emitted into a viewing angle area that is more restricted for a viewer compared to the free viewing mode, comprising - a transmissive image display device, preferably an LC panel, - a backlight downstream of the transmissive image display device in the viewing direction, wherein said backlight has a luminance distribution whose peak brightness is emitted in a direction that forms an angle of at least 3° with the first preferred direction, advantageously even 5° or 8°, (the backlight can have a permanent or variable luminance curve), - in the direction of viewing, in front of (third embodiment) or behind (fourth embodiment) the image reproduction device, a switchable light filter according to the invention with the structural features and properties as described above, - 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.

[0032] A backlight with such properties can be created, for example, if it also contains a turning film and / or a partially mirrored or asymmetric prism grid above a light guide with diffuser, BEF or DBEF.

[0033] For such a screen of the third or fourth configuration, it can further advantageously apply – for at least one partial area – that in the second operating mode B2, for an angular range of at least a - 4° < β < a + 4° (preferably even for a - 6° < β < a + 6° or a - 8° < β < a + 8°, up to a maximum of a - 20° < β < a + 20°), for all angles β contained in said angular range, the product of the second transmission T – which, like the first transmission TBI (β), is angle-dependent – ​​holds true. B 2(β), an angle-dependent luminance Lv(β) of the backlight at angle β, and an angle-dependent transmission T Bw(ß) of the image display device deviates by at most + / -10% from the value for said product for the angle a = ß at the angle ß.

[0034] Due to the compliance of this product - within the stated tolerance - the perceived homogeneity is increased for a viewer, since, for example, in the case of a strong drop in transmission of the first optical element or the image reproduction device when viewing at an angle, such as to side areas of the screen, when the viewer looks directly at the screen from the center, this is compensated for by the corrective luminance distribution of the backlight.

[0035] Alternatively, for such a screen of the third or fourth configuration - for at least one partial area - it can optionally apply that in the second operating mode B2 for an angular range of at least a - 4° < ß < a + 4° (preferably even for a - 6° < ß < a + 6° or a - 8° < ß < a + 8°, up to a maximum of a - 20° < ß < a + 20°) for all angles ß contained in said angular range, the product of the second transmission T B2 (ß) and the luminance L v (ß) the backlight at angle ß deviates by at most + / -10% from the value for said product for angle a = ß.

[0036] Here too, the compensating effect described above applies due to the disruptive luminance distribution of the backlight, but neglecting the transmission behavior of the image reproduction device.

[0037] Finally, the invention also includes a screen in a fifth embodiment, wherein the screen can be operated in at least a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode, in which light is emitted into a viewing angle area that is more restricted for a viewer compared to the free viewing mode. - a transmissive image display device, preferably an LC panel, - a backlight downstream of the transmissive image display device in the viewing direction, wherein said backlight has an asymmetric luminance distribution, wherein said asymmetry preferably has respect to the horizontal direction from the viewer's point of view, - a switchable light filter according to the invention, with the structural features and properties as described above, positioned in front of or behind the image display device in the direction of viewing, - 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.

[0038] The backlight is designed such that it does not exhibit a symmetrical luminance distribution (e.g., around the vertical center line perpendicular to the imaginary line connecting the eyes from the viewer's perspective), but rather an asymmetrical luminance distribution (e.g., horizontally). In other words, the aforementioned backlight 8a has an asymmetrical luminance distribution, preferably with respect to the horizontal direction from the viewer's perspective. Such a design is possible, for example, using deterministically coupled light guides and / or turning films, which shift the peak brightness.

[0039] This variant is advantageous for use in vehicles because the backlight design significantly reduces light that would otherwise be emitted towards the passenger window, particularly at horizontal angles of 25 degrees or more (from the vertical), for example, to less than 10% – preferably less than 2.5% – of the peak brightness, while maintaining a deliberately high luminance towards the driver. This reduces or even eliminates distracting reflections in the passenger window or, if applicable, on the exterior mirror closest to the passenger. Nevertheless, thanks to the light filter positioned in front of the display unit, the screen can be operated in two ways: either so that only the passenger can see the screen content (operating mode B2), for example, for moving images, or so that both the driver and passenger can see the screen content (operating mode B1), for example, for navigation maps.

[0040] For some of the aforementioned screens of the first to fourth configurations, it may be advantageous when used in passenger 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 fluctuate 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, oci = 0°, oci = +1-2° or magnitude(oci)>2° can apply), where 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.

[0041] 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.

[0042] Furthermore, the invention comprises a lighting device for a screen in a second embodiment, which can be operated in at least a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode, in which light is emitted in an angular range that is restricted compared to the free viewing mode, comprising - an extensive, area-like backlight that emits light into a limited angular range and is optionally designed to emit light directly, as well as - a plate-shaped light guide located in front of the backlight in the direction of viewing, which has coupling elements on at least one of the large surfaces and / or within its volume, - light sources arranged laterally on at least one narrow side of the light guide, and - optionally a linear polarizing filter, - a switchable light filter according to the invention arranged in the viewing direction in front of the backlight (this also includes a position in front of a screen with which the lighting device is used), having the structural features and properties as described above, - wherein in the second operating mode B2 the backlight is switched on and the lamps are switched off, and wherein in the first operating mode B1 at least the lamps are switched on, and 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.

[0043] 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 reasons. Ideally, such 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 emitted within a restricted angular range. This also applies to the variant described below with a light guide that emits or couples light predominantly within a restricted angular range. In contrast to this embodiment of the invention, luminance curves of backlightings are defined, particularly over horizontal (possibly...(also vertical) angular ranges, typically bell-shaped, although there does not necessarily have to be a real concentration of luminance around a smaller angular range.

[0044] Finally, the invention comprises a lighting device for a screen in a third embodiment, which can be operated in at least a first operating mode B1 for a free viewing mode and a second operating mode B2 for a restricted viewing mode, in which light is emitted in an angular range that is restricted compared to the free viewing mode. - an extensive, area-like backlight that emits light into an unrestricted angular range and is optionally designed to emit light directly (e.g., using a locally dimmable LED matrix lighting unit), as well as - a plate-shaped light guide located in front of the backlight in the direction of viewing, which has coupling elements on at least one of its large surfaces and / or within its volume, wherein said coupling elements couple light coupled laterally into at least one narrow side of the light guide predominantly (i.e., more than half, preferably more than 80% or 90%) into a restricted angular range, - light sources arranged laterally on at least one narrow side of the light guide, and - optionally a linear polarizing filter, - a switchable light filter according to the invention, arranged in the viewing direction in front of the backlight, preferably in front of the light guide, with the structural features and properties as described above, - wherein in the second operating mode B2 the backlight is switched off and the lamps are switched on, and wherein in the first operating mode B1 at least the backlight is switched on (in addition, the lamps may optionally also be switched on), and - 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.

[0045] The lighting devices of the first, second and third embodiments are advantageously combined with a transmissive screen, such as an LC panel, to create a screen that 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 an angular range that is more limited than in the free viewing mode.

[0046] Advantageously, the transition dipole moments of a first optical element (or, if present, also of a second or further optical element) are formed as one or more dichroic dye(s) 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.

[0047] In contrast, the transition dipole moments can also be 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.

[0048] The dichroic dye molecules generally align themselves parallel to the liquid crystal molecules.

[0049] Alternatively, the first optical element can be designed as a laminate of layers of polymer film polarizers.

[0050] 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.

[0051] 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.

[0052] Furthermore, it is possible that at least two such preferred directions differ by more than 10° in a selectable plane and / or that the respective preferred direction of a transition dipole moment can be selected depending on its position in the first optical element.

[0053] Furthermore, it can be advantageous if the first optical element is divided into different areas (A1, A2, ...) along a selectable reference line, with a separate preferred direction selectable for each area (A1, A2, ...), which applies to all transition dipole moments lying within that area (A1, A2, ...). All preferred directions are pairwise distinct and point towards a viewer 3 with a maximum tolerance of + / -10 degrees. This arrangement has the advantage that the viewer perceives a screen with a light filter as homogeneously illuminated in restricted viewing mode.

[0054] 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.

[0055] 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.

[0056] The first and second preferred directions can also be identical or differ in their orientation by only a few degrees (maximum 10°), and both can be perpendicular to the optical element in question. This is a preferred case. However, depending on the application, it is also possible for the first and second preferred directions to differ from each other by more than 10°.

[0057] The 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.

[0058] A first exemplary fabrication 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 fabrication (with reference to US 9,481,658 B2 or WO2021 / 177308A1 paragraph 37ff) 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 light is irradiated, the side chains condense locally, causing birefringence along the surface.

[0059] An alternative, second manufacturing variant uses thermotropic, liquid-crystalline dichroic dyes (reference to JP201 1-237513A), 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.

[0060] 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 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 -237513A as an example. - Chemical groups for crosslinking, bonded to thermotropic liquid crystalline compounds or polymers: metaacryloylic groups, epoxy groups, oxetanyl groups, and styrene groups, preferably methacryloylic groups. Alternatively, polymerizable liquid crystalline compounds, such as those described in JP 6268730B2, can be used. Polymerizable liquid crystalline dichroic dyes, for example, azo dyes, are also suitable.

[0061] The at least one dye consists of dye molecules, wherein advantageously a transition dipole or transition dipole moment is associated with each dye molecule, i.e., each dye molecule corresponds to a transition dipole or transition dipole moment. Typically, a dye has a mass fraction of at least 0.01%, preferably 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 thickness of the layers is preferably in the range of 0.2 pm to 50 pm, more preferably in the range of 0.5 pm to 20 pm, including all boundary values. The dyes or dye mixtures for different layers within an optical element can be different, but they don't have to be.

[0062] 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.

[0063] In principle, the performance of the invention is maintained if the parameters described above are varied within certain limits.

[0064] 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

[0065] 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 a shows the principle diagram of the operation of an exemplary switchable light filter in operating modes B1 and B2, Fig. 1b shows exemplary normalized transmission graphs for an optical element in combination with a linear polarization filter, left without and right with a delay plate (B-plate). Fig. 2 shows the schematic diagram of an exemplary construction of a switchable light filter in a first embodiment, Fig. 3 shows an exemplary normalized transmission graph (a schematic diagram based on a measurement) over the horizontal angular range for a switchable light filter in the first operating mode B1 , Fig. 4 shows an exemplary normalized transmission graph (a schematic diagram based on a measurement) over the horizontal angular range for a switchable light filter in the second operating mode B2, Fig. 5 shows the schematic diagram of an exemplary construction of a switchable light filter in a second embodiment, Fig. 6 shows the schematic diagram of an exemplary setup of a lighting device in a first embodiment, with a switchable light filter, Fig. 7 shows the schematic diagram of an exemplary design of a screen in a first embodiment, which can be operated in at least two operating modes B1 and B2, with a switchable light filter, Fig. 8 shows the schematic diagram of an exemplary design of a screen in a second embodiment, which can be operated in at least two operating modes B1 and B2, with a switchable light filter, Fig. 9a shows the schematic diagram of an exemplary construction of a screen in a third embodiment, which can be operated in at least two operating modes B1 and B2, with a switchable light filter, Fig. 9b shows the schematic diagram of an exemplary design of a screen in a fourth embodiment, which can be operated in at least two operating modes B1 and B2, with a switchable light filter, Fig. 9c shows an exemplary normalized luminance distribution for a backlight, such as can be used in a screen of the third or fourth design, Figs. 10a to 10d show exemplary luminance and transmission graphs relating in particular, but not exclusively, to a screen of the third or fourth design. Fig. 11 shows the schematic diagram of an exemplary design of a screen in a fifth embodiment, which can be operated in at least two operating modes B1 and B2, with a switchable light filter, Fig. 12 shows the schematic diagram of an exemplary construction of a lighting device in a second embodiment, with a switchable light filter, as well as Fig. 13 shows the schematic diagram of an exemplary construction of a lighting device in a third embodiment, with a switchable light filter. Detailed description of the drawings

[0066] The drawings are not to scale and merely represent schematic diagrams. For exemplary one- or two-dimensional angle-dependent representations of transmission, e.g., of light filters, as shown in Figures 1b, 3, 4, 9c, and 10b to 10d, the numbers assigned to the lines represent the transmission normalized to "1" for the corresponding pair of angles in polar coordinates. For example, in Figure 1b, the number "0.90" corresponds to a transmission of 90% for the corresponding direction.

[0067] Figure 1a shows the schematic diagram of the operation of an exemplary switchable light filter 5 in a first operating mode B1 and a second operating mode B2. The exemplary light filter 5 comprises a first optical element 1, which in turn comprises • a plurality of light-absorbing transition dipole moments arranged in a layer at least 0.2 micrometers thick, wherein the absorbing transition dipole moments are preferably formed by dichroic dyes where the dye mass density is more than 1% or more than 10%, respectively, • wherein the majority of the transition dipole moments are aligned parallel to a first preferred direction selectable for the first optical element 1 with a tolerance of at least 20° (in connection with the embodiment examples which are described below with reference to the drawings, the property “at least in a first state” is intended to include exactly one possibility, namely that there is exactly one state; that is, it is a permanent embodiment) with a tolerance of at most 20° (other tolerance values ​​are also possible, e.g. 5° or 10°) or around this fluctuates around, wherein the first preferred direction is arranged at a predetermined angle a to the perpendicular bisector of the first optical element 1, e.g. with oc=0°, a=+ / -2° or magnitude(a)>2°, wherein the angle a is measured in a selectable first plane which contains the said perpendicular bisector, and wherein the angle a is preferably measured parallel to an edge of the first optical element 1, such as the upper edge, • so that light which enters the first optical element 1 with an incidence direction and a polarization state is transmitted or at least partially absorbed depending on its incidence direction relative to the first optical element 1 and its polarization state, (Not shown in the drawing) means for selectively generating a first electric field EF1 or a second electric field EF2, a liquid crystal layer 3 arranged in the direction of view behind the first optical element 1 in this example, 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, so that the transmission properties of the switchable light filter 5 change between a first operating mode B1, in which the first electric field EF1 (e.g. with a field strength of 0 V / pm) is present, and a second operating mode B2, in which the second electric field EF2 (e.g. with a field strength other than 0 V / pm, approximately on the order of 1 V / pm, e.g.as a rectangular wave with 10 kHz), distinguishing the respective relative transmission in the two operating modes B1 and B2 at at least one point on the optical element 1 (preferably at several points, particularly advantageous on at least half the area of ​​the optical element) to a selectable tolerance of e.g. 3% or 5% by a first transmission Tßi(ß) for the first operating mode B1 or second transmission T. B 2(ß) is described for the second operating mode B2, which depends on an angle ß and is normalized in such a way that the transmission values ​​for the given angle a T B i(a)=1 and T B2 (a)=1 holds true if light • at angles β with a-60° < β < a-40° or a+40° < β < a+60° into the optical element 1, in the first operating mode B1, in which the first electric field EF1 is applied, an s-polarized component of said light is formed into at least one first normalized Transmission value T B i(ß) ^0.25 (preferred T B i(ß) >0,3) is transmitted, as well as • at angles β with a-60° < β < a-40° or a+40° < β < a+60° into the optical element 1, in the second operating mode B2, in which the second electric field EF2 is applied, a p-polarized component of said light at most a normalized second transmission value T B2 (β) < 0.2 (preferably T B2 (β) < 0, 1 , especially preferred T B 2(ß) 0.05, and advantageously also for all angles ß < a-40° or a+40° < ß) is transmitted.

[0068] To normalize the transmission T BI (a)=1 and T B2 (a)=1, the following should be noted: The angles β and a are, of course, measured in the same previously described plane. It is also fundamentally possible that T B i(a)>1 and / or T B2 (a)>1 applies for angle β^a. In many cases, T applies. B i(a)<1 and / or T B2(a)<1 applies for angle ß^a.

[0069] An important means-and-effect relationship exists in the following fact: By switching between the first operating mode B1, in which the first electric field EF1 (e.g., with a field strength of 0 V / pm) is applied, and the second operating mode B2, in which the second electric field EF2 (e.g., with a field strength other than 0 V / pm, approximately on the order of 1 V / pm, e.g., as a square wave with 10 kHz) is applied, only in operating mode B2 is s-polarized light incident on the liquid crystal layer 3 essentially converted into p-polarized light, which then incidents on the first optical element 1. This is shown in Fig. 1a: On the left is operating mode B1, where the s-polarized component of the light incident on the liquid crystal layer 3 remains s-polarized after passing through it. In contrast, on the right side of the Fig.Figure 1a shows how the s-polarized component of the light becomes p-polarized after passing through the liquid crystal layer. In combination with the layer of the first optical element, which is at least 0.2 micrometers thick and has absorbing transition dipole moments, the transmission variations mentioned for T result. B i(ß) or T B2 (ß) the aforementioned operating modes. This is indicated by the arrows in Fig. 1a: On the left, i.e., in operating mode B1, the s-polarized light passes through the first optical element 1 even at oblique angles, while on the right, i.e., in operating mode B2, the p-polarized component is transmitted very little laterally, and is thus stylized by the arrow only in a vertical direction.

[0070] In a preferred embodiment, light passing through the liquid crystal layer 3 is transmitted essentially unchanged when the first electric field EF1 is applied, while when the second electric field EF2 is applied, the incident light is circularly or elliptically polarized, or its polarization is rotated by 90°. Figure 1 shows, as described above, operating mode B1 on the left. In this mode, the first electric field E1 is applied, and the light incident from below, indicated by "sss", is essentially unaltered in its polarization by the liquid crystal layer 3. Accordingly, the s-polarized light can pass through the optical element 1 with the transition dipole moments, and indeed (at least) in all directions indicated here in the plane of the paper. While there are losses in transmission, the condition described above for the first transmission value T is met. Bi(ß) is satisfied for the corresponding angles ß with a-60°< ß < a-40° or a+40° < ß < a+60°.

[0071] For clarity, the angle 'a' is shown with a value greater than 0° in Fig. 1. However, for all subsequent considerations, 'a' will generally be considered 0°. Values ​​of 'a' of 70° allow for tilting the preferred transmission direction of the switchable light filter 5.

[0072] Furthermore, Fig. 1 on the right shows operating mode B2. In this mode, the second electric field E2 is applied, and the light incident from below, indicated by "sss", is altered in its polarization by the liquid crystal layer 3, essentially being converted into p-polarized light. Consequently, the resulting p-polarized light can only penetrate the optical element 1 with its transition dipole moments to a limited extent. Due to the orientation of the transition dipole moments, the transmission is limited; in particular, the condition described above for the transmission value Tβ2(β) is met for the corresponding angles β with a-60° < β < a-40° or a+40° < β < a+60°. Preferably, this condition is met for all angles β < a-40° or a+40° < β.

[0073] To further understand the mode of operation of the invention, Fig. 1b shows exemplary normalized transmission graphs for an optical element 1 in combination with a linear polarization filter, on the left without and on the right with a retarder plate (B-plate). It is evident that the use of a retarder plate (e.g., B-plate as calculated here) is particularly advantageous for vertical angles with a An absolute amount of approximately 15° or more ensures reduced transmission and is therefore advantageous.

[0074] The diagrams are drawn in a polar coordinate system for the hemisphere into which light is emitted. The straight lines correspond to the horizontal and vertical directions, respectively. Transmission is restricted, particularly in the horizontal direction. For the circles shown, the polar angle from the inside (i.e., at the intersection of the lines) to the outside is 25°, 45°, and 90°.

[0075] Advantageously, the transition dipole moments are formed as one or more dichroic dye molecules mixed with the liquid crystals in a guest-host arrangement. For permanent transition dipole moments, the liquid crystals can be fixed via a curing process. The dichroic dye molecules generally align parallel to the liquid crystal molecules.

[0076] The aforementioned configurations also ensure that the first optical element 1 has a non-periodic structure. This is highly advantageous because it eliminates the risk of artifacts such as moiré effects when used in conjunction with pixel structures of screens.

[0077] 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.

[0078] Furthermore, Fig. 2 shows a schematic diagram of an exemplary setup of a switchable light filter 5 in a first embodiment. The liquid crystal layer 3 is arranged in the viewing direction (from above) in front of the first optical element 1. This first embodiment also includes a first linear polarization filter (X) located in the viewing direction in front of the liquid crystal layer 3, the preferred polarization transmission direction of which is preferably aligned parallel to an edge of the optical element 1, preferably the lower edge.

[0079] Fig. 3 shows an exemplary normalized transmission graph (as a schematic diagram based on a measurement) over the horizontal angular range for a switchable light filter 5 in the first operating mode B1, in which the first electric field EF1 The diagram shows the following: Since oc=0° was assumed, the following applies as desired: when light enters the optical element 1 (and, in principle, the switchable light filter 5) at angles β with a-60° < β < a-40° or a+40° < β < a+60°, the s-polarized component of said light has a transmission value T of at least one normalized value. B i(ß) >0.25 is transmitted.

[0080] In contrast, Fig. 4 shows an exemplary normalized transmission graph (as a schematic diagram based on a measurement) over the horizontal angular range for a switchable light filter 5 in the second operating mode B2, in which the second electric field EF2 is applied. Here it is evident that when light at angles β with a-60° < β < a-40° or a+40° < β < a+60° enters the optical element 1 (or the switchable filter 5), in a second operating mode B2, in which the second electric field EF2 is applied, the p-polarized component of said light (the p-polarization here arises in particular from the influence of the liquid layer 3) is reduced to a maximum normalized transmittance value T B2 (ß) 0.2 (in this case even T B2 (ß) s 0,15) is transmitted.

[0081] In certain embodiments of the invention, there may be at least one angle βi for which the transmission of the p-polarized component of said light is not equal to the transmission of the s-polarized component of said light. Preferably, this condition applies to an entire angular range of angles β (e.g., from -60° < β < -40° and / or +40° < β < +60°), and particularly preferably even to all angles.

[0082] In other embodiments of the invention, it can be the case that in both operating modes B1 and B2 there is at least one angle β2 for which the transmission of the s-polarized component of said light is greater than that of the p-polarized component of said light. Preferably, this condition applies to an entire angular range of angles β2 (e.g., from -60° < β < -40° and / or +40° < β < +60°), and particularly preferably even to all angles α β.

[0083] Figure 5 shows a schematic diagram of an exemplary setup of a switchable light filter in a second embodiment. Here, the switchable light filter 5 comprises at least two first optical elements 1, 1', wherein a retarder R is arranged between at least two such first optical elements 1, T. The use of a retarder R is purely optional, but is implemented here as an example. Furthermore, the first two optical elements 1, 1' could, but do not have to, have different thicknesses of the layers containing the multitude of light-absorbing transition dipole moments. If the retarder R is arranged between the first two optical elements 1, 1', it allows for improved transmission limitation, particularly in the second operating mode B2.

[0084] Figure 6 shows a schematic diagram of an exemplary setup of a lighting device in a first embodiment, with a switchable light filter 5. This 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 limited for a viewer compared to the free viewing mode, comprises - an area-wide backlight 8 that emits light and is optionally designed to be directly illuminated (e.g. with an LED matrix, for local dimming), as well as - a light filter 5 arranged in the direction of viewing in front of the backlight 8 as described above.

[0085] Thus, in a first embodiment, such a lighting device can also be operated in the two aforementioned operating modes B1 and B2 if the switchable light filter 5 used therein is used accordingly in the respective operating mode.

[0086] Furthermore, Fig. 7 shows the schematic diagram of an exemplary design of a screen in a first embodiment, which can be operated in at least two operating modes B1 and B2, with a switchable light filter 5. Such 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 limited for a viewer compared to the free viewing mode, comprises in a first embodiment - a lighting device as described above for Fig. 6, - and, if no first linear polarizing filter X is arranged in the switchable light filter 5 of the lighting device, one in the viewing direction in front the second linear polarizing filter P arranged in the background light 8, whereby light emanating from the background light and passing through the second linear polarizing filter P is restricted in its directions of propagation in conjunction with the optical element 1 of the switchable light filter 5, and - a transmissive image display device 1 1 , which is arranged in the viewing direction in front of the 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.

[0087] Preferably, the first or second linear polarization filter P, X is arranged in or is part of the transmissive image display device 11.

[0088] Furthermore, the invention comprises a screen in a second embodiment that 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 that is more limited for the viewer compared to the free viewing mode. Figure 8 shows a schematic diagram of an exemplary setup of such a screen in a second embodiment. This includes - an image display device 12, whereby basically any type of image display device is eligible, for example LC panel, OLED, microLED and others, - in the direction of view in front of the image display device 12 a light filter 5 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] Figures 9a and 9b show schematic diagrams of exemplary configurations of a screen in a third and a fourth embodiment, which can be operated in at least two modes B1 and B2, with a switchable light filter. Such a screen comprises - a transmissive image display device 11 , preferably an LC panel, - a backlight 8a downstream of the transmissive image display device 1 in the viewing direction, wherein said backlight 8a has a luminance distribution whose peak brightness is emitted in a direction which forms an angle of at least 3° with the first preferred direction, advantageously even 5° or 8°, (the backlight 8a can have a permanent or variable luminance curve), - in the direction of viewing in front of (see Fig. 9a, third embodiment) or behind (see Fig. 9b, fourth embodiment) the image display device 1 1 a light filter 5 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.

[0091] A backlight 8a with such properties can be created, for example, if it also contains a turning film and / or a partially mirrored or asymmetric prism grid above a light guide with diffuser, BEF or DBEF.

[0092] Fig. 9c shows an exemplary normalized luminance distribution for a backlight 8a, such as can be used in a screen of the third or fourth design: Here the peak brightness is shifted to the right by approximately 5° in the horizontal direction.

[0093] For such a screen of the third or fourth configuration, it can further advantageously apply – for at least one partial area – that in the second operating mode B2, for an angular range of at least a - 4° < ß < a + 4° (preferably even for a - 6° < ß < a + 6° or a - 8° < ß < a + 8°, up to a maximum of a - 20° < ß < a + 20°), for all angles ß contained in said angular range, the product of T B 2(ß), luminance Lv(ß) of the backlight 8a at the angle ß and transmission Tßw(ß) of the image reproduction device 1 1 at the angle ß deviates by at most + / -10% from the value for said product for the angle a = ß.

[0094] Due to the compliance of this product - within the stated tolerance - the perceived homogeneity is increased for a viewer, since in the case of a strong angle-dependent transmission drop of the optical element 1 or the image reproduction device 1 1 when viewing at an angle, e.g. on lateral areas. of the screen, when the viewer looks directly at the screen from the center, is compensated for by the corrective luminance distribution of the backlight 8a.

[0095] In this context, reference is made to Figures 10a to 10d, which show exemplary luminance and transmission graphs of individual components or groups of components with regard to, in particular, but not exclusively, a screen of the third or fourth embodiment. These show two different parameter combinations for the luminance of the backlight 8a, the transmission of the transmissive image display unit 11, and the transmission of the optical element 1. The condition that the product of T B 2(ß), Lv(ß) and T BW The condition that the value of β deviates by a maximum of 10% for angles β where a - 4° < β < a + 4° is satisfied for both sets of parameters. In the case represented by the solid lines, the luminance Lv(β) and transmittance T of the image display device are β. B w(ß) shows no angular dependence.

[0096] For the second case (dashed lines), the angular dependence of T is B 2(ß) more pronounced, so that the condition can only be met if the luminance Lv(ß) increases at least locally with the difference of the viewing angle ß from the angle a.

[0097] Alternatively, for such a screen of the third configuration - for at least one sub-area - it can optionally apply that in the second operating mode B2 for an angular range of at least a - 4° < ß < a + 4° (preferably even for a - 6° < ß < a + 6° or a - 8° < ß < a + 8°, up to a maximum of a - 20° < ß < a + 20°) for all angles ß contained in said angular range, the product of T B2 (ß) and luminance Lv(ß) of the backlight 8a at the angle ß deviates by at most + / -10% from the value for said product for the angle a = ß.

[0098] Here too, the compensating effect described above applies due to the disruptive luminance distribution of the backlight, but disregarding the transmission behavior T. BW (ß) of the image display device 1 1.

[0099] Finally, the invention also includes a screen in a fifth embodiment, wherein the screen has at least two operating modes: B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is directed into a Compared to the free viewing mode, the image is projected onto a screen with a limited viewing angle for the viewer. Figure 1 shows a schematic diagram of an exemplary setup of such a screen in its fifth embodiment, comprising... - a transmissive image display device 11 , preferably an LC panel, - a backlight 8x downstream of the transmissive image display device 1 1 in the viewing direction, wherein said backlight 8x has an asymmetric luminance distribution, wherein said asymmetry preferably has respect to the horizontal direction from the viewpoint of a viewer, - in the direction of view in front of or behind the image display device 11, a light filter 5 as described above, - and, if no first linear polarizing filter X is arranged in the switchable light filter 5, a linear polarizing filter P arranged in the viewing direction in front of the backlight 8x or in the image display unit 1 1 - as realized in the embodiment shown - , whereby light emanating from the backlight 8x and passing through the second linear polarizing filter P is restricted in its directions of propagation in conjunction with the optical element 1 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.

[0100] The backlight 8x is designed such that it does not exhibit a symmetrical luminance distribution (e.g., around the vertical center line from the viewer's perspective), but rather an asymmetrical luminance distribution (e.g., in the horizontal direction). In other words, the aforementioned backlight 8a has an asymmetrical luminance distribution, with the asymmetry preferably present in the horizontal direction from the viewer's perspective. Such a design is possible, for example, using deterministically coupled light guides and / or turning films, which shift the peak brightness.

[0101] This variant is advantageous for use in vehicles because, in particular, light that would be emitted towards the passenger window is prevented by the design of the backlighting from horizontal angles of approximately 25 degrees or The luminance (relative to the vertical center line) can be significantly reduced, for example to less than 20% – preferably less than 2.5% – of the peak brightness, while a deliberately high luminance is maintained in the direction of the driver. In this way, disturbing reflections in the passenger window or, if applicable, on the exterior mirror closest to the passenger are reduced or even avoided. Nevertheless, due to the switchable light filter 5 located in front of the display unit 11, the screen can be operated in such a way that either only the passenger can see the image content (operating mode B2), for example for moving images, or that both the driver and passenger can see the image content (operating mode B1), for example for navigation maps.

[0102] For some of the screens described above in the first to fifth configurations, it may be advantageous when used in passenger cars if an optional second optical element 2 is arranged in the viewing direction in front of the transmissive image display unit 1 1 (see also Fig. 11), which comprises: • 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 2 with a tolerance of at most 20° (alternatively 10°) or fluctuate around it, the second preferred direction being arranged at an angle oci to the perpendicular bisector of the second optical element 2 (where, for example, oci = 0°, oci = +1-2° or magnitude(oci) > 2°), wherein the angle oci is measured in a selectable second plane containing said perpendicular bisector; the second plane 2 is preferably perpendicular to the first plane of the first optical element 1. • so that light which enters the second optical element 2 is transmitted or at least partially absorbed depending on its direction of incidence relative to the second optical element 2 and its polarization state.

[0103] 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.

[0104] Furthermore, the invention comprises a lighting device for 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 within a narrower angular range compared to the free viewing mode. Figure 12 shows a schematic diagram of an exemplary setup of a lighting device in such a second embodiment. This includes - an area-like extended backlight 8b, which emits light into a restricted angular range and which is optionally designed to emit light directly, as well as - a plate-shaped light guide 9 located in the direction of viewing in front of the backlight 8b, which has coupling elements on at least one of the large surfaces and / or within its volume, - light sources 10 arranged laterally on at least one narrow side of the light guide 9, and - optionally a linear polarization filter P (not shown in the drawing), - a light filter 5 arranged in the direction of viewing in front of the backlight 8b (this also includes a position in front of a screen with which the lighting device is used), as described above, - wherein in operating mode B2 the backlight 8b is switched on and the light sources 10 are switched off, and wherein in operating mode B1 at least the light sources 10 are switched on, and - 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.

[0105] Furthermore, a transmissive image reproduction device 11, e.g. an LC panel, is also present here in the direction of viewing (i.e. from above in the plane of the sheet).

[0106] Within the context 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 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 added to the backlighting, which emits light within a limited angular range. This also applies to the variant described below with a light guide that emits or couples light predominantly into a limited angular range. In contrast to this embodiment of the invention, luminance curves of backlights are typically bell-shaped over angular ranges, particularly horizontal (and possibly also vertical) ranges, although there does not necessarily have to be a true concentration of luminance around a smaller angular range.

[0107] Finally, the invention comprises a lighting device for a screen in a third 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 within a narrower angular range compared to the free viewing mode. Figure 13 shows a schematic diagram of an exemplary setup of a lighting device in a third embodiment. This includes - an area-wide, extended backlight 8c that emits light in an unrestricted angular range and is optionally designed to emit light directly (e.g. by means of a locally dimmable LED matrix lighting unit), as well as - a plate-shaped light guide 9c located in the viewing direction in front of the backlight 8c, which has coupling elements on at least one of the large surfaces and / or within its volume, wherein said coupling elements couple light coupled laterally into at least one narrow side of the light guide 9c predominantly (i.e. more than half, preferably more than 80% or 90%) into a restricted angular range, - light sources 10 arranged laterally on at least one narrow side of the light guide 9c, and - optionally a linear polarization filter P (not shown in the drawing), - a light filter 5 arranged in the viewing direction in front of the backlight 8c, preferably in front of the light guide 9c, as described above, - wherein in operating mode B2 the backlight 8c is off and the light sources 10 are switched on, and wherein in operating mode B1 at least the backlight 8c is switched on (in addition, the light sources 10 may optionally also be switched on), and - 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.

[0108] The lighting devices of the first, second and third embodiments are advantageously combined with a transmissive image reproduction device 11 (see, for example, Fig. 12 and Fig. 13), such as an LC panel, to produce a screen that can be operated in at least two operating modes: B1 for a free viewing mode and B2 for a restricted viewing mode, in which light is emitted in an angular range that is more restricted than in the free viewing mode.

[0109] 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. The light filters, which utilize the optical element, 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 states. In particular, the transmission behavior for specific directions is switchable.

[0110] 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 for PIN entry or data display at ATMs or payment terminals, for password entry, or when reading emails on mobile devices. As described above, the invention can also be used in cars to selectively shield the driver or passenger from distracting visual content. List of reference signs 1. First optical element 2. Second optical element 3 Liquid crystal layer 5 switchable light filters 8 Backlight 8a Backlight 8b Backlight 8c Backlight 8x Backlight 9, 9c light guide 10 light source 11 Image display unit P, X polarization filter

Claims

Patent claims 1 . Switchable light filter (5), comprising a first optical element (1 ), in turn comprising • a multitude of light-absorbing transition dipole moments arranged in a layer at least 0.2 micrometers thick, • wherein the majority of the transition dipole moments are aligned, at least in a first state, with a tolerance of a maximum of 20° parallel to a first preferred direction that can be selected for the first optical element (1) or fluctuate around this, wherein the first preferred direction is arranged at a predetermined angle a to the perpendicular bisector of the first optical element (1), wherein the angle a is measured in a selectable first plane that contains said perpendicular bisector, • so that light which is incident on the first optical element (1) with a direction of incidence and a state of polarization is transmitted or at least partially absorbed depending on its direction of incidence relative to the first optical element (1) and its state of polarization, Means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged behind or in front of the first optical element (1) in the viewing direction, on which the first electric field (EF1) or the second electric field (EF2) acts and which, depending thereon, influences the polarization state of light passing through it, a first linear polarization filter (X) arranged in front of the liquid crystal layer (3) in the viewing direction, if the liquid crystal layer (3) is arranged in front of the first optical element (1) in the viewing direction, so that the transmission properties of the switchable light filter (5) differ between a first operating mode B1, in which the first electric field (EF1) is applied, and a second operating mode B2, in which the second electric field (EF2) is applied,wherein the respective relative transmission in the two operating modes B1 and B2 at at least one point on the first opti-, - 37 - see element (1 ) up to a selectable tolerance by a first transmission T B i(ß) for the first operating mode B1 or a second transmission T B 2(ß) for the second operating mode B2, which depend on an angle ß and are standardized so that for the values of the transmission for the given angle a Tßi(a)=1 and T B2 (a)=1 holds, in that when light • at angles (ß) with a-60°< ß < a-40° or a+40° < ß < a+60° into the first optical element (1 ), in the first operating mode B1 , in which the first electric field (EF1 ) is applied, an s-polarized portion of said light to at least one standardized first transmission value T B i(ß) >0.25 is transmitted, and • at angles (ß) with a-60°< ß < a-40° or a+40° < ß < a+60° into the first optical element (1 ), in the second operating mode B2, in which the second electric field (EF2) is applied, a p-polarized portion of said light to at most a standardized second transmission value T B2 (ß) < 0.2 is transmitted.

2. Switchable light filter (5) according to claim 1, characterized in that the first optical element (1) and / or the liquid crystal layer (3) is divided into a plurality of separately switchable segments, so that local switchability between the respectively possible operating states is enabled.

3. Switchable light filter (5) according to one of the preceding claims, characterized in that it comprises at least two first optical elements (1 , T), wherein optionally a retarder is arranged between at least two such first optical elements (1 , 1 ').

4. Switchable light filter (5) according to one of the preceding claims, characterized in that there is at least one first specific angle (ßi) for which the transmission of the p-polarized portion of said light is unequal to the transmission of the s-polarized portion of said light.

5. Switchable light filter (5) according to one of claims 1 or 2, characterized in that in both operating modes B1 and B2 it has at least one second - 38 - specific angle (ß2) for which the transmission of the s-polarized portion of said light is greater than that of the p-polarized portion of said light.

6. Lighting device for a screen which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - a surface-like backlight (8) which emits light and - a switchable light filter (5) according to one of claims 1 to 5, arranged in front of the background lighting (8) in the viewing direction.

7. A screen which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - a lighting device according to claim 6 with a switchable light filter (5) which comprises a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged behind or in front of the first optical element (1) in the viewing direction, and a first linear polarization filter (X) lying in front of the liquid crystal layer (3) in the viewing direction, if the liquid crystal layer (3) is arranged in front of the first optical element (1) in the viewing direction, - if no first linear polarization filter (X) is arranged in the switchable light filter (5) of the illumination device, a second linear polarization filter (P) arranged in front of the background illumination (8) in the viewing direction, whereby light emanating from the background illumination and penetrating the second linear polarization filter (P) is restricted in its propagation directions, and - a transmissive image display device (11) which is arranged in front of the switchable light filter (5) in the viewing direction, wherein in the second operating mode B2 the second electric field (EF2) is applied and in the first operating mode B1 the first electric field (EF1) is applied.

8. Screen according to claim 7, characterized in that, if present, the first linear polarization filter (X) and otherwise the second linear polarization filter (P) are arranged in the transmissive image display device (11) or are part of it.

9. A screen which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view 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 5, which comprises a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged behind or in front of the first optical element (1) in the viewing direction, and a first linear polarization filter (X) lying in front of the liquid crystal layer (3) in the viewing direction, if the liquid crystal layer (3) is arranged in front of the first optical element (1) in the viewing direction, - wherein in the second operating mode B2 the second electric field (EF2) is applied and in the first operating mode B1 the first electric field (EF1) is applied.

10. Screen according to claim 8, characterized in that the switchable light filter (5) can be subsequently attached by a user and / or reversibly to the image display device (12).

11. Screen which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - a transmissive image display device (11), - a backlight (8a) arranged downstream of the transmissive image display device (11) in the viewing direction, said backlight (8a) having a luminance distribution whose peak brightness is radiated in a direction which forms an angle of at least 3° with the first preferred direction, - in the viewing direction in front of or behind the image display device (11), a switchable light filter (5) according to one of claims 1 to 5, which comprises a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged behind or in front of the first optical element (1) in the viewing direction, and a first linear polarization filter (X) lying in front of the liquid crystal layer (3) in the viewing direction, if the liquid crystal layer (3) is arranged in front of the first optical element (1) in the viewing direction, wherein the transmission properties of the switchable light filter (5) differ between the first operating mode B1 and a second operating mode B2,wherein the respective relative transmission in the two operating modes B1 and B2 at at least one point on the first optical element (1 ) is determined up to a selectable tolerance by a first transmission Tßi(ß) for the first operating mode B1 or a second transmission T, B 2(ß) for the second operating mode B2, which depend on an angle ß and are each normalized so that for the values of the transmission for a given angle a Tßi(a)=1 and T B2 (a)=1 applies, - wherein in the second operating mode B2 the second electric field (EF2) is applied and in the first operating mode B1 the first electric field (EF1) is applied.

12. Screen according to claim 1 1 , characterized in that in the second operating mode B2 for an angular range of at least a - 4° < ß < a + 4° for all angles ß contained in said angular range, the product of the second transmission T B2(ß), an angle-dependent luminance L v (ß) of the background illumination (8a) at the angle ß and an angle-dependent transmission Tßw(ß) of the image display device (11) at the angle ß deviates by a maximum of + / -10% from the value for said product for the angle a = ß.

13. Screen according to claim 1 1 , characterized in that in the second operating mode B2 for an angular range of at least a - 4° < ß < a + 4° for all in The angle ß contained in said angular range is the product of the second transmission T B 2(ß) and the angle-dependent luminance L v (ß) of the backlight (8a) at the angle ß deviates by a maximum of + / -10% from the value for said product for the angle a = ß.

14. A screen which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - a transmissive image display device (11), preferably an LC panel, - a backlight (8x) arranged downstream of the transmissive image display device (11) in the viewing direction, said backlight (8x) having an asymmetric luminance distribution, said asymmetry preferably being present with respect to the horizontal direction from the viewpoint of a viewer, - in the viewing direction in front of or behind the image display device (11), a switchable light filter (5) according to one of claims 1 to 5, which comprises a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged in the viewing direction behind or in front of the first optical element (1), and a first linear polarization filter (X) lying 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), - wherein in the second operating mode B2 the second electric field (EF2) is applied and in the first operating mode B1 the first electric field (EF1) is applied.

15. Screen according to one of claims 7 to 14, characterized in that a second optical element (2) is arranged in front of the transmissive image display device (11) in the viewing direction, which second optical element comprises: • a multitude of light-absorbing transition dipole moments, • wherein the majority of the transition dipole moments are aligned at least in a first state with a tolerance of a maximum of 20° parallel to a second preferred direction selectable for the second optical element (2) or - 42 - fluctuates around it, wherein the second preferred direction is arranged at an angle oci to the perpendicular bisector of the second optical element (1 ), wherein the angle oci is measured in a selectable second plane containing said perpendicular bisector, • so that light which is incident on the second optical element (2) is transmitted or at least partially absorbed depending on its direction of incidence relative to the second optical element (2) and its polarization state.

16. Lighting device for a screen, which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, comprising - a surface-like extended backlight (8b) which radiates light in a limited angular range and which is optionally designed to be directly luminous, and - a plate-shaped light guide (9) located in front of the background lighting (8b) in the viewing direction, which has output coupling elements on at least one of the large surfaces and / or within its volume, - illuminating means (10) arranged laterally on at least one narrow side of the light guide (9), and - a switchable light filter (5) according to one of claims 1 to 5, arranged in front of the background lighting (8b) in the viewing direction, which comprises a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged behind or in front of the first optical element (1) in the viewing direction, and a first linear polarization filter (X) lying in front of the liquid crystal layer (3) in the viewing direction, if the liquid crystal layer (3) is arranged in front of the first optical element (1) in the viewing direction, - wherein in the second operating mode B2 the background lighting (8b) is switched on and the lighting means (10) are switched off, and wherein in the first operating mode B1 at least the lighting means (10) are switched on, and - 43 - wherein in the second operating mode B2 the second electric field (EF2) is applied and in the first operating mode B1 the first electric field (EF1) is applied.

17. Lighting device for a screen, which can be operated in at least a first operating mode B1 for a free view mode and a second operating mode B2 for a restricted view mode, in which light is emitted in an angular range that is restricted compared to the free view mode, comprising - a surface-like extended backlight (8c), which radiates light in an unrestricted angular range and which is optionally designed to be directly luminous, and - a plate-shaped light guide (9c) located in front of the background lighting (8c) in the viewing direction, which has output coupling elements on at least one of the large surfaces and / or within its volume, wherein said output coupling elements output light coupled laterally into at least one narrow side of the light guide (9c) predominantly into a limited angular range, - illuminating means (10) arranged laterally on at least one narrow side of the light guide (9c), and - a switchable light filter (5) according to one of claims 1 to 5, arranged in front of the background lighting (8c) in the viewing direction, which comprises a first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged behind or in front of the first optical element (1) in the viewing direction, and a first linear polarization filter (X) lying in front of the liquid crystal layer (3) in the viewing direction, if the liquid crystal layer (3) is arranged in front of the first optical element (1) in the viewing direction, - wherein in the second operating mode B2 the background lighting (8c) is switched off and the lighting means (10) are switched on, and wherein in the first operating mode B1 at least the background lighting (8c) is switched on, and - wherein in the second operating mode B2 the second electric field (EF2) is applied and in the first operating mode B1 the first electric field (EF1) is applied. - 44 -

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