Optical filter, lighting device and screen with optical filter

Abstract

A first embodiment of the invention relates to an optical filter (5) comprising a first optical element (1) and a second optical element (2). Wherein each of the first optical element (1) and the second optical element (2) comprises a plurality of light absorption transition dipole moments. The majority of the transition dipole moments are arranged at least in the first state in such a way that the moments fluctuate parallel to or around a first preferred direction selectable for the first optical element (1) and a second preferred direction selectable for the second optical element (2) with a tolerance of at most 20 degrees, such that the light incident on the first optical element (1) or the second optical element (2) is incident on the first optical element (1) or the second optical element (2). The optical elements (1, 2) are transmitted or at least partially absorbed depending on the direction of incidence and the polarization state thereof with respect to the respective optical elements (1, 2). The retardation layer (7) is arranged in the form of a C-plate or A-plate between the first optical element (1) and the second optical element (2) in such a way that at least 85% of the linearly or elliptically polarized light incident into the filter (5) at an angle of at least 35 DEG to the first or second preferred direction is absorbed, the angle between the electric field of the linear polarized light or the semi-major axis of the elliptical polarized light and the incident direction is less than 20 degrees when the semi-major axis and the incident direction are projected to the surface of the light filter (5). The invention also includes a second embodiment with a biaxial filter (5a) and an arrangement for a lighting device and a screen using the filter (5, 5a) according to the invention.

Description

Technical Field

[0001] In recent years, great progress has been made in extending the viewing angle of LCDs. However, in some cases, the large viewing area of ​​a display can often be a disadvantage. More and more information (for example, banking data or other personal and sensitive data) is available on mobile devices such as laptops and tablets. Therefore, people need to control who can see this sensitive data. They can choose a wide viewing angle (for example, when viewing travel photos or advertisements) to share the information on the screen with others. On the other hand, they need a narrow viewing angle (in privacy mode) when they want to work privately with the information on the display.

[0002] A similar problem exists in vehicle architecture, where the driver cannot be distracted by image content such as digital entertainment programs when the engine is on, while passengers want to watch such content while driving. Therefore, a display screen that can switch between corresponding display modes is required.

[0003] Additional films based on microlouvers have been used in mobile display devices to achieve visual data protection. However, the additional film is not controllable or switchable and can only be applied and removed by hand. When not needed, the additional film must be removed separately for display. In addition, another major disadvantage of using such louver films is that there will be a certain amount of light loss. Background Art

[0004] US6765550B2 describes such a privacy barrier based on micro shutters. The biggest disadvantage is that the filter needs to be mechanically removed or installed, and there is light loss in the privacy protection mode.

[0005] US5993940A describes a method for using a film having small strip-shaped prisms evenly arranged on the surface of the film to achieve a privacy mode, but the development and production costs are quite high.

[0006] In WO 2012 / 033583 A1, switching between the free mode and the confined mode is achieved by controlling the liquid crystal between the so-called "color" layers, which results in light loss and is also quite technically difficult.

[0007] US2012 / 0235891A1 describes a rather complex screen backlight. According to Figures 1 and 15 therein, not only multiple light guides are used, but also other complex optical elements, such as microlens elements 40 and prism structures 50, which can convert the light of the rear lighting into the light of the front lighting. This is both expensive and complex, and will also cause light loss. According to the variation described in Figure 17 of US2012 / 0235891A1, both light sources 4R and 18 generate light with a narrow luminous angle, and the light of the rear light source 18 must first be complexly converted into light with a large luminous angle. As mentioned above, this complex conversion will greatly reduce the brightness.

[0008] JP2007-155783A utilizes a special optical surface 19 that is complex to calculate and manufacture, so that light is deflected to different narrow or wide areas according to the incident angle of the light. The structure is similar to a Fresnel lens. In addition, there are some interfering edges that deflect the light in undesirable directions. Therefore, it is uncertain whether it can achieve meaningful light distribution.

[0009] US2013 / 0308185A1 describes a special stepped light guide that emits light from different directions onto a large surface, depending on which narrow side of the light guide is illuminated. This, combined with a transmissive image display device (e.g., a liquid crystal display), can produce a screen that can be switched between a free viewing mode and a restricted viewing mode. Disadvantages include: the restricted viewing effect can only be used for the left / right side or the top / bottom side, but not for the left / right / top / bottom side at the same time, which is necessary in some payment processes. In addition, even in the restricted viewing mode, residual light can still be seen from the obstructed viewing angle.

[0010] WO2015 / 121398A1 describes a screen with two operating modes, in which scattering particles present in the volume of the corresponding light guide plate are essentially used to achieve the switching of the operating modes. However, scattering particles made of polymers generally have the disadvantage of emitting light from two large surfaces, so that about half of the useful light will be emitted in the wrong direction, that is, towards the backlight, and this light cannot be recycled to a sufficient extent due to structural reasons. In addition, in some cases, especially at high concentrations, scattering particles made of polymers in the volume of the light guide plate will cause a scattering effect, thereby reducing the anti-peeping effect in the protected mode.

[0011] The core idea of ​​electrical birefringence technology is to use the switchable liquid crystal in the additional liquid crystal panel to "filter" the light emitted from the imaging layer at a specific angle. However, this technology has some disadvantages, including high additional energy consumption and cost, and difficult to adjust the best viewing angle (i.e., the best viewing position), which is usually fixed at around ±40°. In addition, the light absorption capacity of the liquid crystal structure is also insufficient, because for viewing angles greater than the optimal viewing angle, the light intensity is insufficiently attenuated, which results in the light intensity still reaching 3% of the maximum light intensity at viewing angles greater than ±40°.

[0012] The above methods and devices generally have the following disadvantages: they significantly reduce the brightness of the base screen, and / or require complex and expensive optical components for mode switching, and / or reduce the resolution in free-viewing and open modes, and / or cause visual artifacts on very high-resolution displays. Summary of the invention

[0013] The object of the present invention is to describe a filter with an optical element, wherein the light incident into the optical element is transmitted, partially absorbed or completely absorbed depending on its direction of incidence and polarization properties (but not its position). By using the filter with this optical element, the transmission of light should be able to be switched according to the angle (optionally, vertical relative to a sitting or standing observer), and optionally, it can be switched between at least two operating states. In particular, it should be possible to switch the transmission behavior for a specific direction. In the prior art, such optical elements are known to achieve directional selectivity through polarization properties, and when the absorption of the electric field is perpendicular to the film surface, its transmission diagram (for example, represented in pseudo-color in a polar coordinate system) will show an undesirable hourglass shape. Generally, the absorption of such optical elements does not significantly improve the privacy protection effect. Therefore, without further improvement, the privacy protection of this principle is limited. Due to the hourglass shape, the transmittance is used as a measure of privacy protection. For light linearly polarized in the horizontal direction, the transmittance at a vertical viewing angle greater than 5° is always higher than the transmittance at a vertical viewing angle less than 5°, which generally applies to the entire or at least most of the horizontal angle range. Therefore, the object of the present invention is in particular to reduce the transmittance when the vertical viewing angle is greater than 5° (i.e. to improve the anti-peeping ability), while maintaining the transmittance when the vertical viewing angle is less than 5°, and this effect can also be achieved at different horizontal angles. The concept of "vertical" here is used as an exemplary reference and refers to the direction when the observer (whether sitting or standing) looks at the filter. As a reference, the line connecting the observer's eyes defines the horizontal direction, and the direction perpendicular to this direction is defined as the vertical direction. Here, "vertical" is used as an exemplary reference for an observer, who is standing or sitting facing the optical filter, and the line connecting his eyes defines the horizontal direction as a reference, and the vertical direction as a reference is perpendicular to it.

[0014] This problem is solved by an optical filter according to a first embodiment of the present invention, which includes a first optical element and a second optical element, each of which contains a plurality of light-absorbing transition dipole moments. The two optical elements are arranged in a stacked manner relative to the incident direction of the light, i.e., the light passes through them in sequence. For a plate-shaped optical element having two large faces surrounded and connected by narrow edges, the light passes through the two large faces in the stacked optical elements in sequence. The stacked arrangement should not be understood as two optical elements being directly adjacent to each other - in some embodiments of the present invention, there is at least one other element between the optical elements, as described below.

[0015] The majority of transition dipole moments are in at least one first state parallel to a first preferred direction of the first optical element and to a second preferred direction of the second optical element with a tolerance of at most 20°, preferably at most 10°, or fluctuate or vary around these directions. In the context of the present invention, the characteristic "at least one first state" encompasses a number of possibilities: on the one hand, there may be only one state, which serves as a permanent configuration. On the other hand, there may also be two or more states. In this case, the direction of the transition dipole moment is variable, for example by means of a so-called host-guest liquid crystal cell. In this way, light incident on the first optical element or the second optical element is transmitted or at least partially absorbed, depending on its direction of incidence relative to the respective optical element and its polarization state.

[0016] A retardation layer is arranged between the first optical element and the second optical element, which retardation layer is in the form of a C-plate or A-plate for generating a phase retardation greater than λ / 4 for a given wavelength λ, for example a λ / 2 retardation layer. In any case, the phase retardation must be greater than λ / 4. The given wavelength can be a so-called "design wavelength", and the filter is optimized by means of corresponding design programs (for example LCDMaster, Techwiz, and / or using algorithms known in the prior art, such as A. Lien "Extended Jones matrix representation for the twisted nematic liquid-crystal display at obliqueincidence", Appl. Phys. Lett. 1990 Dec 24; 57(26): 2767-2769. https: / / doi.org / 10.1063 / 1.103781). Typically a "green" wavelength is chosen, such as λ=550 nm, but in principle any wavelength in the visible range can be used.

[0017] Therefore, when the angle between the electric field of the linearly polarized light or the major semi-axis of the elliptically polarized light and the incident direction (both of which are projected onto the surface of the filter) is less than 20°, linearly or elliptically polarized light (especially strongly elliptically polarized light) incident on the filter at an angle of at least 35° to the first preferred direction or the second preferred direction will be absorbed by at least 85%.

[0018] In this case, strongly elliptically polarized light means that the ratio of the semi-axis lengths is at least 1:4, but more preferably at least 1:10 or more. For example, it is conceivable that only linearly polarized light or strongly elliptically polarized light is incident on the filter.

[0019] The mechanism of action for improving privacy protection based on the combination of the above invention - the first optical element, the retardation layer (A plate or C plate) and the second optical element - can be conceptually explained in the following way: For ease of explanation, it is assumed that the absorption transition dipole moment is arranged perpendicular to the surface of the optical element, that is, the first preferred direction and the second preferred direction are both perpendicular to the corresponding optical element, so they are the same. The first optical element mainly absorbs the p-polarized electric field component. When linearly polarized light is incident on the first optical element, if the light is polarized only in the incident plane, the light is absorbed to the maximum extent. The larger the angle between the incident plane and the polarization direction (that is, the larger the corresponding vertical viewing angle), the higher the transmittance. Therefore, the angle-dependent two-dimensional transmission diagram of the first optical element presents an hourglass shape. Next, a retardation layer (A plate or C plate) made of a uniaxial birefringent material is inserted after the first optical element, and its extraordinary axis is arranged parallel or perpendicular to the surface. The light transmitted through the retardation layer is no longer polarized only along one of the two main axes of the retardation plate, so the s-polarized light entering through the retardation layer is at least partially converted into p-polarized light, which can be absorbed by the subsequent second optical element. The original hourglass shape is thus transformed into a rectangular shape that better limits the field of view within the privacy zone without reducing brightness.

[0020] The respective layer thicknesses of the material having a light-absorbing transition dipole moment in the first optical element and the second optical element may be different from each other. Alternatively, they may also be identical or nearly identical, of course, within the scope of technical feasibility. Typically, each optical element also has a transparent substrate (e.g., glass or polymer) on which the light-absorbing transition dipole moment material is located.

[0021] A transition dipole moment (also called a transition matrix element) is a quantum mechanical vector and a specific transition, i.e. a transition of a system from an initial state (usually the ground state) to a final state (usually an excited state), such as in atoms, molecules or solids. It corresponds to the electric dipole moment associated with that transition. The direction of the vector defines the polarization of the transition (or here synonymously the polarization direction), which in turn determines how the system interacts with electromagnetic waves with a given polarization and absorbs light of the corresponding polarization direction when transitioning from the ground state to an excited state. The magnitude of the vector corresponds to the strength of the interaction or the transition probability. Excited states relax by non-radiative processes.

[0022] The first preferred direction or the second preferred direction corresponds to an alignment direction of the transition dipole moment of the first optical element or the second optical element in a given light propagation direction, and the absorption is the same for any polarization direction of the light.

[0023] Preferably, the first preferred direction and the second preferred direction can also be identical, or their arrangement directions differ only by a few degrees (maximum 10°), and both can be particularly perpendicular to the respective optical element. Depending on the application, the first preferred direction and the second preferred direction can also differ by more than 10°.

[0024] The filter may also include a polarization filter, which is located in front of or behind the first optical element or the second optical element in the incident direction. When the linear polarization filter is located in front of the incident direction of the first optical element or the second optical element, the polarization filter ensures that the light incident on the corresponding optical element has linear polarization. On the contrary, when the linear polarization filter is located behind the incident direction of the first optical element or the second optical element, the polarization filter is used to eliminate unwanted polarization components in the light emitted from the first optical element or the second optical element. In addition, the use of a λ / 4 retardation layer can also be considered, for example, when the incident light is circularly polarized light, the layer converts the circularly polarized light into substantially linearly polarized light. As the corresponding design wavelength λ, for example, 550nm or 580nm can be selected. Other wavelength values ​​are also clearly feasible.

[0025] The optical filter may further include a device for selectively generating a first electric field EF1 or a second electric field EF2, so as to be able to basically realize two different operation modes. A liquid crystal layer is arranged in front of or behind the first optical element and / or the second optical element, and the first electric field EF1 or the second electric field EF2 acts on the liquid crystal layer and affects the polarization state of light passing through the liquid crystal layer according to the difference in the electric field, so that the transmission characteristics of the optical filter are different between the first operation mode B1 (applying the first electric field EF1) and the second operation mode B2 (applying the second electric field EF2). If more electric fields can be generated, other operation modes can also be realized.

[0026] In a preferred embodiment, when the second electric field EF2 is applied, the light transmitted by the liquid crystal layer remains substantially unchanged; and when the first electric field EF1 is applied, the incident light is circularly polarized or elliptically polarized, or the polarization direction of the light is rotated by 90°.

[0027] The second electric field EF2 may be, for example, 0 V / μm, whereas the first electric field EF1 may vary, for example, from 0.1 V / μm to 10 V / μm in a 1 kHz rectangular wave. Other embodiments are possible.

[0028] The object of the present invention is also solved by a second embodiment of the optical filter according to the present invention. The filter comprises a third optical element instead of the first optical element and the second optical element in the aforementioned embodiments. The third optical element also comprises a plurality of light-absorbing transition dipole moments. Likewise, most of the transition dipole moments are arranged parallel to the third preferred direction of the third optical element with a tolerance of a maximum of 20° (preferably a maximum of 10°) in at least the first state, or fluctuate or vary around the third preferred direction. In the present invention, the characteristic of "at least the first state" includes a variety of possibilities: on the one hand, there may be only one state, and it is a permanent configuration; on the other hand, the expression also explicitly includes the possibility of two or more states. In this case, the transition dipole moment is variable, for example, by a host-guest liquid crystal unit, so that the light incident into the third optical element is transmitted or at least partially absorbed depending on its incident direction relative to the third optical element and its polarization state.

[0029] The third optical element is made of a biaxial birefringent material, so that according to the present invention, the complex refractive indices of the three principal axes x, y, z in the third optical element are different from each other. Preferably, the three basic, linearly independent and mutually different principal axes x, y, z are located in a Cartesian coordinate system. Therefore, when the angle between the major semi-axis of the electric field of the linearly polarized light or the elliptically polarized light and the incident direction (both are projected onto the surface of the filter) is less than 20°, the linearly polarized light or elliptically polarized light (especially strongly elliptically polarized light) incident on the filter at an angle of at least 35° to the third preferred direction will be absorbed by at least 85%.

[0030] Likewise, strongly elliptically polarized light here also means that the ratio of the semi-axis lengths is at least 1:4, but more preferably at least 1:10 or greater.

[0031] In the second embodiment of the present invention, the filter may also include a linear polarization filter, which is located in front of or behind the third optical element in the incident direction. When the linear polarization filter is located in front of the incident direction of the third optical element, it ensures that the light incident on the third optical element has linear polarization. On the contrary, when the linear polarization filter is located behind the incident direction of the third optical element, it is used to eliminate the unwanted polarization component in the light emitted from the third optical element. In addition, it is also possible to consider the use of a λ / 4 retardation layer, for example, in the case where the incident light is circularly polarized light with a wavelength of λ, the layer converts the circularly polarized light into substantially linearly polarized light.

[0032] As mentioned above, the three principal axes x, y, z are different from each other, in that they are linearly independent. The third optical element has a layer thickness d - more precisely, this is the thickness of the layer with the light-absorbing transition dipole moment. In a first preferred variant of the second embodiment, the biaxial birefringent material of the third optical element has complex refractive indices n along the principal axes, respectively. x =n2–i*k2,n y =n1–i*k2 and n z =n1–i*k1, and these complex refractive indices satisfy the condition |n1–n2|*d≥λ / 4, where k1 / k2>10. “i” represents the imaginary part, n1, n2, n3 are the real refractive indices, and k1, k2, k3 are the corresponding absorption coefficients.

[0033] Refractive index n x 、n y 、n z The coordinate system orientation is only related to the arrangement direction of the main axes, and may be tilted and / or rotated relative to the coordinate system of the screen.

[0034] In a second preferred variation of the second embodiment, the biaxial birefringent material of the third optical element has complex refractive indices n along the principal axes, respectively. x =n2–i*k2, n y =-i*k2 and n z =n3-i*k1, which satisfies the condition |n1-n2|*d≥λ / 4, where k1 / k2>10 and |n2-n3|≤|n1-n2| / 2.

[0035] The working mechanism of the second embodiment of the present invention is described below:

[0036] The third optical element made of a biaxial birefringent material or a third optical element containing such a material in the second embodiment can also be conceptually regarded as a multilayer system consisting of alternating "transition dipole moment" layers and "A-type delay plate" layers. The layer with the transition dipole moment absorbs p-polarized light. Preferably, if the third preferred direction is perpendicular to the large surface of the optical element, the best privacy protection, i.e. the best angle-related horizontal transmission limit, can be achieved at a horizontal viewing angle with a vertical viewing angle component of 0°. However, if the vertical viewing angle deviates from 0°, the privacy protection effect at the same horizontal viewing angle will be reduced. This is related to the undesirable hourglass-shaped transmission diagram in the prior art mentioned in the purpose description. As mentioned above, since the layer with the transition dipole moment conceptually absorbs p-polarized light, the transmitted light is s-polarized. Conceptually, the A-type delay plate converts linearly polarized light into elliptically polarized light, which in turn can be (at least partially) absorbed by the next (conceptually) transition dipole moment layer, thereby improving the angle-related transmission limit. Conceptually, there is an alternating structure of multiple "transition dipole moment" layers and "A-type retardation plate" layers, so according to embodiments of the present invention, the angle-dependent transmission limitation is significantly improved relative to the prior art.

[0037] Alternatively, the biaxial third optical element can be manufactured similarly to the known biaxial film manufacturing method, for example, by polymerizing liquid crystal mesogens and then doping with dichroic dye molecules or a mixture thereof.

[0038] An exemplary method for manufacturing a first optical element, a second optical element, or a third optical element based on a host-guest effect, the method being based on a mixture of dichroic dyes or a combination of a dichroic dye mixture and a liquid crystal mixture or compound, and comprising the following manufacturing steps (e.g., as described below in paragraph 37 of US9481658B2 or WO2021 / 177308A1): using a low-birefringence or non-birefringence substrate and coating a thin film on its surface, which determines the direction of the molecules' arrangement relative to the surface, usually parallel or perpendicular to the surface. To this end, a polymer is usually used, preferably polyvinyl alcohol (PVAL / PVOH) or polyimide (PI). Optionally, the surface can be optically or mechanically treated to improve the quality of subsequent molecular arrangement. Subsequently, a mixture of a dichroic dye and a liquid crystal compound or polymer is coated on the substrate. Finally, the side chains are cross-linked by local illumination, so that the side chains are arranged along the surface to produce a birefringence effect.

[0039] Alternatively, the second manufacturing method variant uses thermotropic liquid crystal dichroic dyes, for example, as described in JP2011-237513A, and includes the steps of preparing the corresponding dyes and adding polar groups, then coating the dye mixture, and finally photo-aligning and curing the dye mixture by polarized light.

[0040] The following materials can be used in different manufacturing methods, for example (this list is not exhaustive):

[0041] A polymer substrate with low or no birefringence, preferably triacetate cellulose (TAC);

[0042] A dichroic substance or mixture, including but not limited to a dichroic dye (preferably, an azo dye) or dichroic metal nanoparticles (preferably, gold, silver, copper and aluminum), typically a single dye or a mixture of up to three different dyes to achieve absorption of a major part of the emission spectrum;

[0043] A polymer for surface treatment by alignment of a dye or liquid crystal substance, preferably polyvinyl alcohol or polyimide;

[0044] As a thermotropic liquid crystal compound or polymer, for example, refer to JP2011-237513A;

[0045] As a chemical crosslinking group (i.e., a crosslinking group) combined with a thermotropic liquid crystal compound or polymer, it includes but is not limited to a methacryloyl group, an epoxy group, an oxetane group, and a styryl group (preferably, a methacryloyl group), or it may be a polymerizable liquid crystal compound, for example, as described in JP6268730B2;

[0046] Polymerizable liquid crystal dichroic dyes include, but are not limited to, azo dyes.

[0047] At least one dye consists of dye molecules, wherein preferably each dye molecule is associated with a transition dipole or a transition dipole moment, i.e. each dye molecule corresponds to a transition dipole or a transition dipole moment. Typically, the mass fraction of the dye in each layer of the material of the corresponding optical element is 0.01% to 10%, preferably 0.1% to 5%. In special cases, the concentration of the liquid crystal dichroic dye can even reach 95%. Intermediate values ​​are also possible. Preferably, the thickness of each layer is in the range of 0.2 μm to 50 μm, more preferably in the range of 0.5 μm to 20 μm, all boundary values ​​being included. The dyes or dye mixtures of different layers in the optical element can be different, but do not have to be different.

[0048] According to two embodiments of the optical filter according to the invention, preferably, the first preferred direction, the second preferred direction or the third preferred direction respectively forms an angle of 0° to 45° with the surface normal of the optical element, all limit values ​​being included.

[0049] Furthermore, in embodiments of the present invention, the first preferred direction (and if present, the second preferred direction or the third preferred direction) may vary over the face of the respective optical element. According to the present invention, the weighted average preferred direction of all values ​​is regarded as the respective preferred direction. Preferably, the respective preferred directions of the plurality of transition dipole moments may be selected according to their position in the respective optical element.

[0050] In another preferred embodiment of the first optical element, the second optical element or the third optical element, the optical element is divided into different zones along a selectable reference line, wherein each zone has its own zone preferred direction which is selectable, which direction applies to all transition dipole moments in the zone, and all zone preferred directions are different from each other and have a maximum tolerance of ±10° in the observer direction. Thus, in each zone applicable thereto, all transition dipole moments are aligned parallel to the preferred direction of the zone with a maximum tolerance of ±10°. This arrangement has the advantage that an observer in restricted vision mode perceives a uniform illumination of the screen with the filter.

[0051] In a second embodiment, the filter may further include:

[0052] means for selectively generating a first electric field EF1 or a second electric field EF2;

[0053] The first electric field EF1 or the second electric field EF2 acts on the liquid crystal layer disposed in front of or behind the third optical element, and affects the polarization state of the light passing through it according to the difference in the electric field, so that the transmission characteristics of the filter are different between the first operation mode B1 (applying the first electric field EF1) and the second operation mode B2 (applying the second electric field EF2). It is also applicable that, although more than two different electric fields (corresponding to more possible operation modes) can be generated, only one of the electric fields can be applied at the same time.

[0054] For the two embodiments of the optical filter of the present invention, the switchable liquid crystal layer, in the first switching state, i.e. when the first electric field EF1 is applied, transmits the incident light basically unchanged; in the second switching state, i.e. when the second electric field EF2 is applied, the light is circularly polarized or elliptically polarized, or the polarization direction of the light is rotated 90°.

[0055] "Substantially", as used herein, means that at the interface, the orientation of the liquid crystal molecules is determined by the electric field and the surface induced forces. Therefore, an undesirable alignment of the liquid crystal molecules may result in (unwanted) changes in the polarization state.

[0056] The application of the optical filter of the present invention in a lighting device for a screen or on (or in) the screen is of particular importance. Therefore, the present invention also includes a lighting device for a screen, which can be operated in at least two operating modes (free viewing mode B1 and restricted viewing mode B2), and relative to the free viewing mode, light is emitted into a restricted angle range in the restricted viewing mode B2. The lighting device comprises:

[0057] A planar extended backlight, comprising the filter according to the first embodiment or the second embodiment, and the backlight is used to emit light;

[0058] A light guide plate located in front of the backlight in the viewing direction, the light guide plate having a light exit element on at least one large surface thereof and / or within its volume;

[0059] A light source disposed laterally on at least one narrow side of the light guide plate;

[0060] A linear polarization filter, which is located in front of the backlight or the light guide plate in the viewing direction of the observer, so that the light emitted from the backlight and passing through the filter and the linear polarization filter is limited in its propagation direction;

[0061] In the restricted viewing mode B2, the backlight is turned on and the light source is turned off; in the free viewing mode B1, at least the light source is turned on, that is, only the light source is turned on, or the light source and the backlight are turned on at the same time.

[0062] In the case where a liquid crystal layer is included in the filter of the above-mentioned lighting device, the operation modes B1 and B2 are respectively related to the state of the liquid crystal layer caused by the first electric field EF1 and the second electric field EF2. However, since the switching is only achieved by switching between the backlight and the light source, the above-mentioned lighting device can be switched between the above-mentioned operation modes even if there is no liquid crystal layer in the filter.

[0063] The present invention also includes a screen that can operate in at least two operating modes (free viewing mode B1 and restricted viewing mode B2). In the restricted viewing mode B2, light is emitted into a viewing angle range that is restricted for an observer relative to the free viewing mode B1. The screen includes:

[0064] A planar extended backlight, comprising the filter according to the first embodiment or the second embodiment (both comprising the above-mentioned liquid crystal layer), and the backlight is used to emit light, wherein, optionally, the backlight is configured to emit light directly;

[0065] A linear polarization filter is arranged in front of the backlight along the viewing direction of the screen observer, so that the light emitted from the backlight and passing through the filter and the linear polarization filter is limited in its propagation direction;

[0066] A transmissive display device, which is arranged in front of the filter in the viewing direction, in particular in front of the backlight and / or the polarization filter;

[0067] In the restricted viewing mode B2, the second electric field (EF2) is applied, and in the free viewing mode B1, the first electric field (EF1) is applied.

[0068] Preferably, the linear polarization filter is arranged in or on the transmissive display device, or is part of the transmissive display device.In addition, the filter can be integrated into the transmissive display device, in particular, it can share at least one common substrate with the transmissive display device.

[0069] Finally, the present invention also includes another screen that operates in at least two operating modes (free viewing mode B1 and restricted viewing mode B2), in which the light is emitted into a viewing angle range that is restricted for the observer relative to the free viewing mode B1. The screen includes:

[0070] Display devices, such as OLED panels, microLED panels, LCD panels, or any other type of screen;

[0071] The optical filter according to the first embodiment or the second embodiment of the present invention (both comprising the above-mentioned liquid crystal layer) is arranged on the front side of the display device along the viewing direction of the observer;

[0072] In the restricted viewing mode B2, the second electric field (EF2) is applied, and in the free viewing mode B1, the first electric field (EF1) is applied.

[0073] For the above-mentioned screen with a transmissive display device (especially a screen with an LCD panel), since it also has a backlight, a special embodiment can be adopted: the backlight is designed to have a substantially non-symmetrical (e.g., symmetrical around a vertical midline under the viewing angle of an observer) brightness distribution, but rather an asymmetric (e.g., in the horizontal direction) brightness distribution. That is, the backlight has an asymmetric brightness distribution, wherein preferably, its asymmetry exists relative to the horizontal direction of the observer.

[0074] This is particularly advantageous for application scenarios in vehicles, because through the design of the backlight, the light emitted toward the passenger window can be significantly reduced from a horizontal angle of approximately 25° or more relative to the mid-perpendicular, for example, to less than 10% of the peak brightness (preferably less than 2.5%), while the light toward the driver maintains a higher brightness. In this way, interfering reflections on the passenger window or the passenger side rearview mirror can be reduced or even avoided. Nevertheless, due to the installation of a filter in front of the display device, the screen can select an operating mode so that only the passenger can view the image content (restricted viewing mode B2), for example for playing a video, or both the driver and the passenger can view the image content (free viewing mode B1), for example for displaying a navigation map.

[0075] In addition, for such a screen, additional optical elements (e.g., a first optical element, a second optical element, or a third optical element) may be arranged in front of the transmissive display device along the viewing direction. If the transmissive display device is an LCD panel (e.g., emitting vertical linear polarized light suitable for polarized sunglasses), the upward and downward transmission will be limited from the observer's perspective, thereby minimizing reflections on the windshield.

[0076] For the aforementioned screen with a transmissive display device (especially an LCD panel), the filter according to the first embodiment or the second embodiment of the present invention (both of which have the above-mentioned liquid crystal layer) can be arranged not only in front of the display device along the observation direction, but also behind it.

[0077] In a specific embodiment, the optical filter of the present invention can be installed in front of the display device at a later time. Based on this, the user's existing display device can be retrofitted to have at least two operating modes: free viewing mode B1 and restricted viewing mode B2.

[0078] Such screens can be advantageously applied to mobile devices, land / air / water transport vehicles, payment terminals or access control systems. By switching between different operating modes, sensitive data can be protected (i.e., only the visual content is displayed to a single observer), or multiple observers can view the image content simultaneously.

[0079] Basically, when the above parameters change within a specific range, the performance of the present invention can still be maintained. It is understood that the features mentioned above and to be explained below are not only used in the indicated corresponding combinations, but can also be used in other combinations or alone without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] The present invention will be described in detail below using embodiments in conjunction with the accompanying drawings, which also disclose the essential technical features of the present invention. It should be particularly noted that these embodiments are only for illustrative purposes, and their contents should not be interpreted as limiting the scope of protection. For example, the description of an embodiment comprising multiple elements or components should not be interpreted as all of these elements or components being necessary for implementation. On the contrary, other embodiments may be implemented by using alternative elements or components, reducing the number of elements, or adding additional elements. Unless otherwise specified, elements or components in different embodiments may be combined with each other. Modifications and variations described in one embodiment may also be applicable to other embodiments. To avoid repetition, the same or corresponding elements in different drawings are represented by the same figure marks and will not be described one by one. Among them:

[0081] Figure 1A is a schematic diagram of an exemplary two-dimensional angle-dependent diagram of transmittance of an optical filter in the prior art;

[0082] Figure 1B is a schematic diagram of an exemplary two-dimensional angle-dependent representation of transmittance of an optical filter combined with a biaxial retardation plate in the prior art;

[0083] Figure 2A is a schematic diagram of the filter structure in the first embodiment;

[0084] Figure 2B It is a schematic diagram used to illustrate the projection of the major semi-axis of elliptical light and its incident direction on a plane;

[0085] Figure 3A It is a schematic diagram of an extended structure in a filter for selectively changing the transmission characteristics of the filter;

[0086] Figure 3B is a schematic diagram of an exemplary two-dimensional angle-dependent diagram of the transmittance of the optical filter in the first embodiment;

[0087] Figure 4 is a schematic diagram for explaining the principle of the complex refractive index of the biaxial third optical element;

[0088] Figure 5 is a schematic diagram of an exemplary two-dimensional angle correlation diagram of the transmittance of the optical filter under the first condition in the second embodiment;

[0089] Figure 6 is a schematic diagram of an exemplary two-dimensional angle correlation diagram of the transmittance of the optical filter under the second condition in the second embodiment;

[0090] Figure 7 is a schematic diagram of exemplary angle-dependent diagrams of transmittance in the horizontal direction of different filters when the vertical angle is 0°;

[0091] Figure 8 is a schematic diagram of an exemplary angle-dependent diagram of the transmittance in the horizontal direction of different filters when the vertical angle is 45°;

[0092] Fig. 9 is a schematic structural diagram of a lighting device for a screen using the filter in the first embodiment or the second embodiment;

[0093] Fig.10 is a schematic diagram of a first structure of a screen using the filter in the first embodiment or the second embodiment, the screen being operable in at least two operation modes (a free viewing mode B1 and a restricted viewing mode B2);

[0094] Fig.11 is a schematic diagram of a second structure of a screen using the filter in the first embodiment or the second embodiment, the screen being operable in at least two operation modes (a free viewing mode B1 and a restricted viewing mode B2); and

[0095] Fig.12 is a schematic diagram of a third structure of a screen using the optical filter in the first embodiment or the second embodiment, the screen being operable in at least two operation modes (free viewing mode B1 and restricted viewing mode B2). DETAILED DESCRIPTION

[0096] The drawings are merely illustrative and are not drawn to scale.

[0097] for Figure 1A , 1B , 3A, 5 and 6 are two-dimensional angular correlation diagrams of the transmittance of the filters shown in FIG. 3A, 3A, 3B and 3C, wherein the values ​​marked on each curve represent the normalized transmittance of the corresponding angle pair in the polar coordinate system (with "1" as the reference). For example, the value "0,12" in FIG. 3 indicates that the transmittance in the corresponding direction is 12%.

[0098] The optical filters known in the prior art achieve directional selectivity through polarization characteristics. When the electric field absorption direction is perpendicular to the film surface, the two-dimensional angle-dependent diagram of the transmittance in the polar coordinate system presents an undesirable hourglass shape. Figure 1A This is shown in a schematic diagram. Therefore, without further improvement, the privacy protection effect of this principle is limited. Due to this hourglass shape, the transmittance can be used as a measure of privacy protection. For light linearly polarized in the horizontal direction, the transmittance when the vertical viewing angle is greater than 5° is always higher than the transmittance when the vertical viewing angle is less than ±5°. Figure 1A This can be clearly seen in Figure 1. The term "vertical" is here an exemplary reference to an observer observing the filter, who can be standing or sitting. For reference, the line connecting the observer's eyes is defined as the horizontal direction, and correspondingly, the direction perpendicular to this direction is defined as the vertical direction.

[0099] Research in the prior art therefore combines such filters with a retardation layer of the B type ("B-plate"). Figure 1B is a schematic diagram of an exemplary two-dimensional angle-dependent diagram of the transmittance of a combination of an optical filter and a biaxial delay plate in the prior art. Although this solution achieves a certain degree of optimization, its effect still cannot meet the needs of all application scenarios.

[0100] Compared with the existing technology, Figure 2A is a schematic diagram of an exemplary structure of the optical filter 5 in the first embodiment.

[0101] In a first embodiment, an exemplary filter 5 comprises a first optical element 1 and a second optical element 2, wherein each of the two optical elements 1, 2 comprises a plurality of light-absorbing transition dipole moments, and the two optical elements 1, 2 are arranged in a stacked form (i.e., as elements of a stacked structure) relative to the incident direction 4 of the light. Among them, most of the transition dipole moments are parallel to a first preferred direction of the first optical element with a tolerance of a maximum of 20° (preferably a maximum of 10°) and parallel to a second preferred direction of the second optical element in at least a first state, or fluctuate or vary around these directions. In the present invention, the characteristic of "at least a first state" includes a variety of possibilities: on the one hand, there may be only one state, which is a permanent configuration. On the other hand, there may also be two or more states. In this case, the direction of the transition dipole moment is variable. Therefore, light incident into the first optical element 1 or the second optical element 2 is transmitted or at least partially absorbed depending on its incident direction relative to the corresponding optical element 1, 2 and its polarization state. A retardation layer 7 in the form of a C-plate or an A-plate, for example a λ / 2 retardation layer, is arranged between the first optical element 1 and the second optical element 2; in any case, the phase retardation must be greater than λ / 4, so that linear or elliptically polarized light (especially strongly elliptically polarized light) incident on the filter 5 at an angle of at least 35° relative to the first preferred direction or the second preferred direction is absorbed by at least 85%, wherein the angle between the electric field of the linearly polarized light or the major semi-axis 6 of the elliptically polarized light and the incident direction 4 (both are projected onto the surface of the filter 5) is less than 20°.

[0102] For ease of understanding, Figure 2B The schematic diagram shows the projection principle of the major semi-axis 6 of the exemplary elliptical light and its incident direction 4 on the surface plane of the filter 5. The major semi-axis projection 6a and the incident direction projection 4a form an angle α. The angle α needs to be less than 20° to realize the mechanism of the present invention.

[0103] The mechanism of action for improving privacy protection based on the combination of the above invention - the first optical element 1, the retardation layer 7 (A plate or C plate) and the second optical element 2 - can be conceptually explained in the following way: For ease of explanation, it is assumed that the absorption transition dipole moment is arranged substantially perpendicular to the surface of the optical element, that is, the respective preferred directions are perpendicular to its surface (this is a preferred case). The first optical element 1 mainly absorbs the p-polarized electric field component. When linearly polarized light is incident on the first optical element 1, if the light is polarized only in the incident plane, the light is absorbed to the maximum extent. The larger the angle between the incident plane and the polarization direction (that is, the larger the corresponding vertical viewing angle), the higher the transmittance. Therefore, the two-dimensional angle-dependent transmission diagram of the first optical element 1 presents an hourglass shape. Next, a retardation layer 7 (A plate or C plate) made of a uniaxial birefringent material is inserted after the first optical element 1, and its extraordinary axis is arranged parallel or perpendicular to the surface. The light transmitted through the retardation layer 7 is no longer polarized only along one of the two main axes of the retardation plate, so the s-polarized light entering through the retardation layer 7 is at least partially converted into p-polarized light, which can be absorbed by the subsequent second optical element 2. The original hourglass shape was thus transformed into a rectangular form that better limits the field of view within the protected area without reducing brightness.

[0104] Figure 3B FIG. 1 is a schematic diagram of an exemplary two-dimensional angle dependence diagram of the transmittance of the optical filter in the first embodiment. Figure 1A and Figure 1B The hourglass shape shown allows identification of the narrower waist and reduced transmission values.

[0105] The first preferred direction and the second preferred direction for the first optical element and the second optical element can be identical or differ only by a small angle (maximum 10°) in their arrangement directions. This preferably applies to all the examples described in the figures.

[0106] Figure 3A The schematic diagram of an extended structure of the filter 5 for selectively changing the transmission characteristics of the filter is further shown. The filter 5 of the extended structure additionally includes: (not shown in the figure) a device for selectively generating at least the first electric field EF1 or the second electric field EF2, such as a double ITO layer with a driving circuit; a liquid crystal layer 3 arranged at the front / rear end of the first optical element 1 and / or the second optical element 2. The first electric field EF1 or the second electric field EF2 is applied to the liquid crystal layer 3, thereby affecting the polarization state of the transmitted light thereof, so that the filter 5 exhibits different transmission characteristics between the first operating mode B1 in which the first electric field EF1 is applied and the second operating mode B2 in which the second electric field EF2 is applied.

[0107] Here, too, there is a retardation layer 7 (A plate or C plate). Figure 3AAs shown, the filter 5 may further include a polarization filter P, and the filter 5 is located in front of or behind the first optical element 1 or the second optical element 2 in the incident direction.

[0108] In a preferred embodiment, when the second electric field EF2 is applied, the light transmitted by the liquid crystal layer 3 remains substantially unchanged; and when the first electric field EF1 is applied, the incident light is circularly polarized or elliptically polarized, or the polarization direction of the light is rotated by 90°.

[0109] also, Figure 4 A schematic diagram is provided for illustrating the complex refractive index of a third optical element made of a biaxial birefringent material, in particular for the filter 5a in the second embodiment. Exemplarily, the filter 5a comprises a third optical element comprising a plurality of light-absorbing transition dipole moments, wherein most of the transition dipole moments are parallel to a third preferred direction of the third optical element or fluctuate around the direction in at least a first state with a tolerance of a maximum of 20° (preferably a maximum of 10°). In the present invention, the characteristic of "at least a first state" includes a variety of possibilities: on the one hand, there may be only one state, which is a permanent configuration. On the other hand, there may also be two or more states. In this case, the direction of the transition dipole moment is variable. Therefore, the light incident into the third optical element is transmitted or at least partially absorbed depending on its incident direction relative to the third optical element and its polarization state. As previously described, the third optical element is made of a biaxial birefringent material, so the complex refractive indexes of the three linearly independent principal axes inside it are different from each other. Preferably, the three basic principal axes x, y, z are located in a Cartesian coordinate system. Therefore, linearly or elliptically polarized light (especially strongly elliptically polarized light) incident on the filter 5a in the second embodiment at an angle of at least 35° relative to the third preferred direction is absorbed by at least 85%, wherein the angle between the electric field of the linearly polarized light or the major semi-axis of the elliptically polarized light and the incident direction (both are projected onto the surface of the filter 5a) is less than 20°.

[0110] For example, Figure 4 , the wavelength delay of light with wavelength λ satisfies λ=|(n1-n2)|*d. For example, λ=550nm or 580nm, or it can be all wavelengths in the visible wavelength range.

[0111] For example, Figure 5 , which is a schematic diagram of an exemplary two-dimensional angular correlation diagram of the transmittance of the filter 5a in the second embodiment when the wavelength retardation of light with a wavelength of λ satisfies the first condition λ / 2=|(n1-n2)|*d.

[0112] In a first preferred variant of the second embodiment, the third optical element has a complex refractive index n in three different principal axes x, y, z respectively. x=n2–i*k2, n y =-i*k2 and n z =-i*k1, these complex refractive indices satisfy the condition |n1-n2|*d>λ / 4, where k1 / k2>10. Figure 5 In the case of the results shown, the retardation was set by way of example to 5 / 3*λ. Figure 5 Marked as d.

[0113] The coordinate direction of the refractive index is related only to the arrangement direction of the main axis, and is tilted and / or rotated as necessary with respect to the coordinate system of the screen having the filter 5a in the second embodiment.

[0114] In a second preferred variant of the second embodiment, the third optical element has a complex refractive index n in three different principal axes x, y, z respectively. x =n2–i*k2, n y =-i*k2 and n z =n3–i*k1, these complex refractive indices satisfy the condition |n1-n2|*d>λ / 4, where k1 / k2>10 and |n2-n3|<|n1-n2| / 2. Figure 6 A schematic diagram of an exemplary two-dimensional angular correlation diagram of the transmittance of the filter under the second condition in the second embodiment is shown, wherein the wavelength delay of light with a wavelength of λ satisfies the condition λ / 4=|n1-n2|*d, and in this case, the delay is exemplarily set to 5 / 6λ.

[0115] In its second embodiment, the filter 5a may further include: a device for selectively generating at least a first electric field EF1 or a second electric field EF2; a liquid crystal layer 3 arranged in front of or behind the third optical element, the first electric field EF1 or the second electric field EF2 acting on the liquid crystal layer and affecting the polarization state of the transmitted light thereof according to the different electric fields, so that the filter 5 has different transmission characteristics between the first operating mode B1 in which the first electric field EF1 is applied and the second operating mode B2 in which the second electric field EF2 is applied.

[0116] The beneficial effects of the solution of the present invention are achieved by Figure 7 and Figure 8 exhibit. Figure 7 An exemplary angle-dependent principle diagram of horizontal transmittance of different filters when the vertical angle is 0° (normalized to a maximum of 10°=1) is shown. Figure 8 The transmittance at a vertical angle of 45° is shown.

[0117] Figure 7 and Figure 8 Meet the following requirements:

[0118]

[0119]

[0120] like Figure 7 As shown, at a vertical angle of 0°, the effect of the present invention on the transmittance is negligible, and this is true over the entire horizontal angle range. All transmittance values ​​below 10-2 are generally suitable for practical applications, so the differences between different filters at angles greater than about 60° are negligible.

[0121] In contrast, Figure 8 As shown, at a vertical angle of 45°, the difference in transmittance over the entire horizontal angle range is very significant. Compared to prior art filters, all exemplary filters described herein reduce transmittance significantly more strongly in the particularly important (horizontal) angle ranges of -50° to -25° and 25° to 50°.

[0122] The application of the present invention to a lighting device for a screen or a screen is of particular importance. Fig. 9 A schematic diagram showing the structural principle of a screen lighting device using the optical filter 5 of the first embodiment or the optical filter 5a of the second embodiment is shown.

[0123] A lighting device for a screen can be operated in at least two operation modes (a free viewing mode B1 and a restricted viewing mode B2), wherein light is emitted into a restricted angle range in the restricted viewing mode B2 relative to the free viewing mode. The lighting device comprises:

[0124] a planar extended backlight 8, which includes the optical filter 5 according to the first embodiment or the optical filter 5a according to the second embodiment, and the backlight 8 is used to emit light;

[0125] A light guide plate 9 located in front of the backlight 8 in the viewing direction, having light emitting elements on at least one large surface thereof and / or within its volume;

[0126] A light source 10 disposed laterally on at least one narrow side of the light guide plate 9;

[0127] A linear polarization filter P located in front of the backlight 8 or the light guide plate 9 in the viewing direction of the observer, so that the light emitted from the backlight 8 and passing through the filter 5 or 5a and the linear polarization filter P is limited in its propagation direction;

[0128] In the restricted viewing mode B2 , the backlight 8 is turned on and the light source 10 is turned off; in the free viewing mode B1 , at least the light source 10 is turned on.

[0129] In the case where the liquid crystal layer 3 is included in the filter 5 or 5a of the above-mentioned lighting device, respectively, the operation modes B1 and B2 are related to the state of the liquid crystal layer 3 caused by the first electric field EF1 and the second electric field EF2, respectively. In this case, the light output efficiency in the free viewing mode B1 is improved because in the free viewing mode B1, when the light source 10 and the backlight 8 are turned on at the same time, not only the light transmitted from the backlight 8 passes through the filter 5 or 5a, but also the light guide plate 9 emits light from the side. However, even if the liquid crystal layer 3 is not present in the filter 5 or 5a, the lighting device can be switched between the above-mentioned operation modes because the switching is achieved only by switching between the backlight 8 and the light source 10.

[0130] The invention also includes a screen that can be operated in at least two operating modes, a free viewing mode B1 and a restricted viewing mode B2, in which the light is emitted into a viewing angle range that is restricted for an observer relative to the free viewing mode B1. Fig.10 A schematic diagram of a first structure of such a screen is shown. The screen comprises:

[0131] a planar extended backlight 8 comprising the filter 5 according to the first embodiment or the filter 5a according to the second embodiment (in this case both comprising the above-mentioned liquid crystal layer 3), and for emitting light, wherein the backlight 8 is optionally arranged to emit light directly (e.g. a so-called matrix backlight);

[0132] A linear polarization filter P, which is arranged in front of the backlight 8 along the viewing direction of the screen observer, so that the light emitted from the backlight 8 and passing through the filter 5 or 5a and the linear polarization filter P is limited in its propagation direction;

[0133] a transmissive display device 11 (preferably an LCD panel) which is located in front of the backlight 8 and / or the polarization filter P in the viewing direction;

[0134] In the restricted viewing mode B2, the second electric field (EF2) is applied, and in the free viewing mode B1, the first electric field (EF1) is applied.

[0135] Preferably, the linear polarization filter P is disposed in or on the transmissive display device 11, or is a part thereof.

[0136] In addition, the present invention also includes another screen with a second structure, which operates in at least two operation modes (free viewing mode B1 and restricted viewing mode B2). In restricted viewing mode B2, light is emitted into a viewing angle range restricted for an observer relative to free viewing mode B1. Fig.11 A schematic diagram of such a screen is shown, which includes:

[0137] A display device 12, such as an OLED panel, a microLED panel, an LCD panel or any other type of screen; an optical filter 5 or 5a according to the first embodiment or the second embodiment of the present invention (in this case, both include the liquid crystal layer 3), which is arranged in front of the display device 12 along the viewing direction of the observer;

[0138] wherein, in the restricted viewing mode B2, the second electric field (EF2) is applied, and in the free viewing mode B1, the first electric field (EF1) is applied; and

[0139] Optionally, a polarization filter P and / or a retardation layer (not shown in the figure), preferably a λ / 4 type retardation layer (which converts circularly polarized light into substantially linear or strongly elliptically polarized light), is arranged between the display device 12 and the filter 5 or 5a of the present invention.

[0140] at last, Fig.12 A schematic diagram of a third structure of a screen using the filter 5 of the first embodiment or the filter 5a of the second embodiment is shown, and the screen can operate in at least two operating modes (free viewing mode B1 and restricted viewing mode B2). For the aforementioned screen with a transmissive display device 11 (especially an LCD panel), since it also has a backlight 8a, an embodiment of this third structure can be adopted: the backlight 8a is designed so that it basically does not have a symmetrical brightness distribution around the vertical center line of the observer's viewing angle, but rather embodies an asymmetric brightness distribution in the horizontal direction. This can be achieved by using a turning film, an asymmetric diffuser and / or an asymmetric louver film. In other words, the above-mentioned backlight 8a has an asymmetric brightness distribution, wherein, preferably, the asymmetry exists relative to the horizontal direction of the observer.

[0141] This third structure of the screen is particularly suitable for application scenarios in vehicles, because through the design of the backlight 8a, the light emitted toward the co-pilot window can be significantly reduced from a horizontal angle of 25° or more (relative to the vertical center line), for example, reduced to less than 10% of the peak brightness (preferably less than 2.5%), while the light toward the driver maintains a higher brightness. In this way, interfering reflections on the co-pilot window or the co-pilot side rearview mirror can be reduced or even avoided. Nevertheless, since a filter 5 or 5a is installed in front of (or behind) the image display device 11, the screen can be selectively operated so that only the co-pilot can view the image content (restricted viewing mode B2, such as for playing videos), or both the driver and the co-pilot can view the image content (free viewing mode B1, such as for displaying navigation maps).

[0142] The invention solves the problem posed: a filter with an optical element is described, wherein light incident on the optical element is transmitted, partially absorbed or completely absorbed depending on its incident direction and polarization properties. By using the filter with this optical element, the transmittance of the light can be adjusted according to the angle (optionally vertical relative to a sitting or standing observer) and can be selectively switched between at least two operating states. At the same time, a reduction in the transmittance at a vertical viewing angle greater than 5° (i.e., an increase in privacy protection) is achieved, while maintaining the transmittance at a vertical viewing angle less than 5°.

[0143] The present invention described above can be used in combination with a display device and is suitable for use in scenarios where private data needs to be displayed and / or entered. For example, entering a PIN code or display data on an ATM or payment terminal, entering a password, or reading an email on a mobile device. As described above, the present invention can also be used in passenger cars to selectively prevent image content from interfering with the driver or co-driver.

[0144] Reference numerals list

[0145] 1. First optical element

[0146] 2 Second optical element

[0147] 3 LCD

[0148] 4 Direction of incident light

[0149] 4a Projection of incident direction 4

[0150] 5 Optical Filter of First Embodiment

[0151] 5a Optical filter of the second embodiment

[0152] 6 Optical Filter of Second Embodiment

[0153] 6a Projection of the major semi-axis 6

[0154] 7 Delay Layer

[0155] 8 Backlight

[0156] 8a Backlight with asymmetrical light density distribution

[0157] 9 Light guide plate

[0158] 10 Light Source

[0159] 11 Transmissive display device

[0160] 12 Display device

[0161] P Polarization filter (linear)

Claims

1. A filter (5), characterized in that: include: A first optical element (1) and a second optical element (2), each of the first optical element (1) and the second optical element (2) comprising a plurality of light-absorbing transition dipole moments, and the first optical element (1) and the second optical element (2) are stacked along the incident direction (4) of light, wherein most of the transition dipole moments in the plurality of transition dipole moments are arranged, at least in a first state, to be parallel to or fluctuate around a first preferred direction selectable for the first optical element (1) and a second preferred direction selectable for the second optical element (2) with a tolerance of up to 20°, so that light incident on the first optical element (1) or the second optical element (2) is transmitted or at least partially absorbed according to its incident direction (4) and its polarization state relative to the corresponding optical element (1, 2); A retardation layer (7) is arranged between the first optical element (1) and the second optical element (2) in the form of a C plate or an A plate. For a predetermined wavelength λ, a phase difference greater than λ / 4 is generated by the retardation layer (7), so that at least 85% of the linearly polarized light or elliptically polarized light incident on the filter (5) at an angle of at least 35° to the first preferred direction or the second preferred direction is absorbed, wherein the angle between the electric field of the linearly polarized light or the major semi-axis (6) of the elliptically polarized light and the incident direction (4) when projected onto the surface of the filter (5) is less than 20°.

2. The optical filter (5) according to claim 1, characterized in that Also includes: means for selectively generating a first electric field (EF1) or a second electric field (EF2); A liquid crystal layer (3) is arranged on the upper side or the lower side of the first optical element (1) and / or the second optical element (2), wherein the first electric field (EF1) or the second electric field (EF2) acts on the liquid crystal layer, and the liquid crystal layer affects the polarization state of light passing through the liquid crystal layer according to the first electric field (EF1) or the second electric field (EF2), so that the filter (5) has different transmission characteristics in a first operating mode B1 with the first electric field (EF1) applied and a second operating mode B2 with the second electric field (EF2) applied.

3. An optical filter (5a), comprising: A third optical element, comprising a plurality of light-absorbing transition dipole moments, wherein most of the transition dipole moments in the plurality of transition dipole moments are arranged, at least in a first state, to be parallel to or fluctuate around a third preferred direction selectable for the third optical element with a tolerance of up to 20°, so that light incident on the third optical element is transmitted or at least partially absorbed according to its incident direction relative to the third optical element and its polarization state, characterized in that The third optical element is made of a biaxial birefringent material so that the three complex refractive indices of the three principal axes (x, y, z) in the third optical element are different from each other, thereby at least 85% of the linearly polarized light or elliptically polarized light incident on the filter (5a) at an angle of at least 35° to the third preferred direction is absorbed, wherein the angle between the electric field of the linearly polarized light or the major semi-axis (6) of the elliptically polarized light and the incident direction (4) when projected onto the surface of the filter (5a) is less than 20°.

4. The optical filter (5a) according to claim 3, characterized in that The three principal axes (x, y, z) of the third optical element have complex refractive indices n respectively. x =n2–i*k2,n y =n1–i*k2 and n z =n1–i*k1, which satisfies the condition |n1–n2|*d≥λ / 4, where k1 / k2>10.

5. The optical filter (5a) according to claim 3, characterized in that The three principal axes (x, y, z) of the third optical element have complex refractive indices n respectively. x =n2–i*k2,n y =n1–i*k2 and n z =n3–i*k1, which satisfies the condition |n1–n2|*d≥λ / 4, where k1 / k2>10 and |n2–n3|≤|n1–n2| / 2.

6. The optical filter (5a) according to any one of claims 3 to 5, further comprising: means for selectively generating a first electric field (EF1) or a second electric field (EF2); A liquid crystal layer (3) is arranged on the upper side or the lower side of the third optical element, wherein the first electric field (EF1) or the second electric field (EF2) acts on the liquid crystal layer, and the liquid crystal layer affects the polarization state of light passing through the liquid crystal layer according to the first electric field (EF1) or the second electric field (EF2), so that the filter (5a) has different transmission characteristics in 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.

7. An illumination device for a screen, capable of operating in at least one operating mode B1 corresponding to a free viewing mode and one operating mode B2 corresponding to a restricted viewing mode, wherein: Compared with the free viewing mode, the viewing angle range of light emitted in the restricted viewing mode is limited, and the lighting device includes: A planar extended backlight (8) comprising a filter (5, 5a) according to any one of claims 1 to 6 and for emitting light; A light guide plate (9) which is arranged on the front side of the backlight (8) along the viewing direction of the observer, and the light guide plate (9) has a coupling-out element on at least one large surface and / or within its volume; a light source (10) arranged laterally on at least one narrow surface of the light guide plate (9); and a linear polarization filter (P) arranged in front of the backlight (8) or in front of the light guide plate (9) along the viewing direction so that the light emitted by the backlight (8) and passing through the filter (5, 5a) and the linear polarization filter (P) is limited in its propagation direction, Wherein, in the operation mode B2, the backlight (8) is turned on and the light source (10) is turned off, and in the operation mode B1, at least the light source (10) is turned on.

8. A screen capable of operating in at least one operating mode B1 corresponding to a free viewing mode and one operating mode B2 corresponding to a restricted viewing mode, wherein: Compared with the free viewing mode, the viewing angle range of light emitted in the restricted viewing mode is limited, and the screen includes: A planar extended backlight (8) comprising the filter (5) according to claim 2 or the filter (5a) according to claim 6 and for emitting light; a linear polarization filter (P) disposed on the front side of the backlight (8) in the viewing direction of the observer so that the light emitted by the backlight and passing through the filter (5, 5a) and the linear polarization filter (P) is limited in its propagation direction; and a transmissive display device (11) arranged in front of the filter (5, 5a) along the viewing direction, Wherein, in the operation mode B2, the second electric field (EF2) is applied, and in the operation mode B1, the first electric field (EF1) is applied.

9. The screen according to claim 8, characterized in that The linear polarization filter (P) is arranged in the transmissive display device (11) or is a part of the transmissive display device (11).

10. A screen capable of operating in at least one operating mode B1 corresponding to a free viewing mode and one operating mode B2 corresponding to a restricted viewing mode, wherein: Compared with the free viewing mode, the viewing angle range of light emitted in the restricted viewing mode is limited, and the screen includes: Display device (12); The optical filter (5) according to claim 2 or the optical filter (5a) according to claim 6, which is arranged on the front side of the display device (12) in the viewing direction of the observer, Wherein, in the operation mode B2, the second electric field (EF2) is applied, and in the operation mode B1, the first electric field (EF1) is applied.

11. A screen capable of operating in at least one operating mode B1 corresponding to a free viewing mode and one operating mode B2 corresponding to a restricted viewing mode, wherein: Compared with the free viewing mode, the viewing angle range of light emitted in the restricted viewing mode is limited, and the screen includes: A transmissive display device (11); a backlight (8a) disposed on the rear side of the transmissive display device (11) along a viewing direction of an observer, the backlight (8a) having an asymmetric light density distribution; and The optical filter (5) according to claim 2 or the optical filter (5a) according to claim 6, Wherein, in the operation mode B2, the second electric field (EF2) is applied, and in the operation mode B1, the first electric field (EF1) is applied.

12. The screen according to claim 11, characterized in that The asymmetric light density distribution exists in the horizontal direction from the viewing angle of the observer.

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