SWITCHABLE LIGHT FILTER AND USE THEREOF

Abstract

An optical element which can be utilized in various versions and implementations of switchable light filters having, in addition, polarization filters and/or other means for varying the polarization characteristics of light, for example, liquid crystal layers. A combination of switchable light filter and imaging display device makes possible a private viewing effect that can be switched on and off. The optical element comprises a first layer or a first layer and a plurality of further layers, each layer including a material with a plurality of light-absorbing transition dipole moments. At least in a first state, each transition dipole moment is oriented, with a tolerance of 10° at the maximum, parallel to a selectable preferential direction or fluctuates around it so that light which is incident in the optical element is transmitted or at least partially absorbed depending on its incident direction.

Claims

1. A switchable light filter comprising: a first optical element which comprises a first layer or a first layer and a plurality of further layers wherein each layer comprises a material with a plurality of light-absorbing transition dipole moments, wherein, at least in a first state, each transition dipole moment is oriented, with a tolerance of 10° at a maximum, parallel to a first preferential direction selectable for the first optical element or fluctuates around this, so that light which is incident in the first optical element is transmitted or at least partially absorbed depending on its incident direction relative to the layers and depending on its polarization state, a polarization filter which is arranged upstream or downstream of the first optical element considered in incident direction, an electric field generator configured to selectively generate a first electric field or a second electric field a liquid crystal layer arranged between the first optical element and the polarization filter, the liquid crystal layer being acted upon by the first electric field or the second electric field and, depending thereon, influencing the polarization state of light passing through it so that: in a first operating mode which the first electric field is applied and with a first sub-operating mode and a second sub-operating at least 24% of unpolarized light which is incident in the switchable light filter parallel to the first preferential direction is transmitted and, at least 85% of unpolarized light which is incident in the switchable light filter at an angle of greater than 30° to the first preferential direction is absorbed, wherein the absorption takes place exclusively in a first direction in the first sub-operating mode and exclusively in a second direction perpendicular to the first direction in the second sub-operating mode, in a second operating mode in which the second electric field is applied and with the first sub-operating mode and the second sub-operating mode, at least 24% of unpolarized light which is incident in the switchable light filter parallel to the first preferential direction is transmitted and, at least 85% of unpolarized light which is incident in the switchable light filter at an angle of greater than 30° to the first preferential direction is absorbed, wherein the absorption takes place exclusively in the second direction in the first sub-operating mode and exclusively in the second direction perpendicular to the first direction in the second sub-operating mode, so that the directions of absorption differ by 90°, respectively, for each of the two sub-operating modes for the first operating mode and the second operating mode.

2. A switchable light filter comprising: a first optical element which comprises a first layer or a first layer and a plurality of further layers wherein each layer comprises a material with a plurality of light-absorbing transition dipole moments, wherein, at least in a first state, each transition dipole moment is oriented, with a tolerance of 10° at a maximum, parallel to a first preferential direction selectable for the first optical element or fluctuates around this, so that light which is incident in the first optical element is transmitted or at least partially absorbed depending on its incident direction relative to the layers and depending on its polarization state, wherein the transition dipole moments in each of the layers can be varied with respect to their orientation and/or their amount between the first state and at least a second state in order to alternatively put the respective layer in at least two different states, a polarization filter which is arranged upstream or downstream of the first optical element, an electric field generator configured to selectively generate a first electric field or a second electric field, wherein the first state is generated for the first optical elementby applying the first electric field and the second state is generated for the second optical element by applying the second electric field so that in a first operating mode in which the first electric field is applied and the transition dipole moments of the layers of the first optical element are oriented along the first preferential direction and which has a first sub-operating mode and a second sub-operating mode, at least 24% of unpolarized light which is incident in the switchable light filter parallel to the first preferential direction is transmitted and at least 85% of unpolarized light which is incident in the switchable light filter at an angle of greater than 30° to the first preferential direction is absorbed wherein the absorption takes place exclusively in a first direction in the first sub-operating mode and exclusively in a second direction perpendicular to the first direction in the second sub-operating mode, and in a second operating modein which the second electric field is applied and the transition dipole moments of the layers of the first optical element are oriented parallel to a surface of the polarization filter and perpendicular to a transmission direction of the polarization filter, at least 24% of unpolarized light which is incident in the switchable light filter at any angle to the first preferential direction is transmitted.

3. A switchable light filter comprising: a first optical element and a second optical element, wherein each of the two optical elements comprises: a first layer or a first layer and a plurality of further layers wherein each layer comprises a material with a plurality of light-absorbing transition dipole moments, wherein, at least in a first state, each transition dipole moment is oriented, with a tolerance of 10° at a maximum, parallel to a first preferential direction selectable for the first optical element and to a second preferential direction selectable for the second optical element or fluctuates around these, so that light which is incident in the first optical element or second optical element is transmitted or at least partially absorbed depending on its incident direction relative to the layers and depending on its polarization state, wherein the first preferential direction and the second preferential direction of the transition dipole moments differ from one another by less than 40°, a liquid crystal layerarranged between the first optical element and the second optical element, which liquid crystal layer influences the polarization state of light passing through it depending on a first electric field or second electric field acting on the liquid crystal layer, an electric field generator configured to selectively generate the first electric field or second electric field, electively, a polarization filter arranged above or below one of the two optical elements, or no polarization filter so that either, in a case where a polarization filter present, in a first operating mode in which the first electric field is applied and with a first sub-operating mode, at least 24% of unpolarized light which is incident in the switchable light filter parallel to the first preferential direction or second preferential direction is transmitted and, at least 85% of unpolarized light which is incident in the switchable light filter an angle of greater than 30° to the corresponding preferential direction is absorbed, wherein the absorption takes place exclusively in a first direction in the first sub-operating mode and exclusively in a second direction perpendicular to the first direction in the second sub-operating mode, wherein either the first direction or the second direction is perpendicular to a polarization direction of the polarization filter, or, in a case where there is no polarization filter, in a first operating mode in which the first electric field is applied, at least 24% of unpolarized light which is incident in the switchable light filter at any angle is transmitted, and regardless of whether or not there is a polarization filter, in a second operating modein which the second electric field is applied, at least 24% of unpolarized light which is incident in the switchable light filter parallel to the first preferential direction or to the second preferential direction is transmitted and at least 85% of unpolarized light which is incident in the switchable light filter at an angle of greater than 30° to the corresponding preferential direction is absorbed.

4. A switchable light filter comprising a first optical element and a second optical element, wherein each of the two optical elements comprises a first layer or a first layer and a plurality of further layers wherein each layer comprises a material with a plurality of light-absorbing transition dipole moments, wherein, at least in a first state, each transition dipole moment is oriented, with a tolerance of 10° at a maximum, parallel to a first preferential direction selectable for the first optical element and to a second preferential direction selectable for the second optical element or fluctuates around these, so that light which is incident in the first optical element or second optical element is transmitted or at least partially absorbed depending on its incident direction relative to the layers and depending on its polarization state, wherein the transition dipole moments in each of the layers may be varied with respect to their orientation and/or their amount between the first state and at least a second state in order to alternatively put the respective layer in at least two different states, an electric field generator configured to selectively generate a first electric field or a second electric field wherein for each of the two optical elements the first state is generated by applying the first electric field and the second state is generated by applying the second electric field an optically anisotropic layer for rotating a polarization direction of light passing through the optically anisotropic by 90° arranged between the two optical elements electively, a polarization filter arranged above or below one of the two optical elements, or no polarization filter so that: in a first operating mode in which the first electric field is applied, at least 24% of unpolarized light which is incident in the switchable light filter at any angle relative to the switchable light filter is transmitted, wherein in the first operating mode, the transition dipole moments of the two optical elements are aligned perpendicular to one another and, in a case where a polarization filteris present, polarization filter transition dipole elements of the polarization filter aligned parallel to the transition dipole moments of the switchable optical element located closest to the polarization filter, and in a second operating mode in which the second electric field is applied, at least 24% of unpolarized light which is incident in the switchable light guide parallel to the first preferential direction or second preferential direction is transmitted and, at least 85% of unpolarized light which is incident in the switchable light guide at an angle of greater than 30° to the corresponding preferential direction is absorbed, wherein in the second operating mode, the transition dipole moments of the polarization filter, if present, and the transition dipole moments of the switchable optical element located closest to the polarization filter are oriented perpendicular to one another, and the transition dipole moments of the two optical elements are oriented parallel to one another.

5. The switchable light filter according claim 1 wherein at least one polarization compensation layer is arranged upstream and/or downstream of the liquid crystal layer.

6. The switchable light filter according to claim 1 wherein the preferential directions form an angle of between 0° and 45° to a surface normal of the first layer.

7. The switchable light filter according to claim 1 wherein the switchable light filter is divided into a plurality of separately switchable segments so that localized switching between the respective possible operating states is made possible.

8. The switchable light filter according claim 1 wherein each of the layers of the first optical element and/or of the second optical element, if any, is constructed non-periodically.

9. A display screen comprising a switchable light filter according to claim 1 and an imaging display unit arranged downstream or upstream of the switchable light filter from a perspective of an observer.

10. An optical element comprising: a first layer, or a first layer and a plurality of further layers wherein each layer comprises a material with a plurality of light-absorbing transition dipole moments, wherein, at least in a first state, each transition dipole moment is oriented, with a tolerance of 10° at a maximum, parallel to a selectable preferential direction or fluctuates around this, so that light which is incident in the optical element is transmitted or at least partially absorbed depending on its incident direction relative to the layers.

11. The optical element according to claim 10, wherein the preferential directions form an angle between 0° and 45° to a surface normal of the first layer.

12. The optical element according to claim 10 wherein each of the layers is constructed aperiodically with respect to its structure.

13. The optical element according to claim 10, wherein every transition dipole moment is oriented in the respective preferential direction within a tolerance of 10° at the maximum around the latter.

14. The optical element according to claim 10 wherein at least two such preferential directions in a selectable plane differ by more than 10°.

15. The optical element according to claim 10 whereint the transition dipole moments in every layer can be varied with respect to their orientation and/or their amount between the first state and at least a second state so that the respective layer can be brought alternatively into at least two different states.

16. The optical element according to claim 15, wherein the optical element is divided into a plurality of segments separately switchable between the first state and at least the second state.

17. The optical element according to claim 10 whereint the respective preferential direction of a transition dipole moment is selectable depending on its position in the respective layer.

18. The optical element according to claim 10 wherein every layer is divided along a selectable reference line on the respective layer into different regions, wherein every region can have its own selected regional preferential direction which applies to all of the transition dipole moments of the corresponding layer located within a region and wherein all of the regional preferential directions differ pairwise and face in a direction of an observer to within a tolerance of ±10° at the maximum.

19. The optical element according to claim 10 wherein the material contains at least one dye, preferably a dichroic dye mixture.

20. The optical element according to claim 10 wehrein the material contains liquid crystals.

21. The optical element according to claim 10 wherein the optical element is formed as a laminate of layers of polymer film polarizers.

22. An illumination device for a display screen, which illumination device can be operated in at least two operating modes: a first operating mode for a free viewing mode and a second operating mode for a limited viewing mode, comprising: a two-dimensionally extensive backlight which contains the optical element according to claim 11, a plate-shaped light guide which is located in front of the backlight in a viewing direction and which has out-coupling elements on at least one of the large surfaces and/or within its volume, wherein the light guide is at least 40% transparent to light emanating from the backlight, illuminants arranged laterally on at least one narrow side of the light guide, a linear polarization filter arranged in front of the backlight or in front of the light guide in the viewing direction so that light emanating from the backlight and passing through the polarization filter is limited with respect to its propagation directions, wherein the backlight is switched on and the illuminants are switched off in the second operating mode, and wherein at least the illuminants are switched on in the first operating mode.

23. The illumination device according to claim 22, wherein the light guide is at least 70% transparent to the light emanating from the backlight.

24. A display screen which can be operated in at least two operating modes, a first operating mode for a free viewing mode and a second operating mode for a limited viewing mode, comprising: a two-dimensionally extensive backlight which contains an optical element according to claim 11 and which emits light, a plate-shaped light guide which is located in front of the backlight in a viewing direction and which has out-coupling elements on at least one oflarge surface of the light guide and/or within a volume of the light guide, wherein the light guide is at least 40% transparent to light emanating from the backlight, illuminants arranged laterally on at least one narrow side of the light guide, a linear polarization filter arranged in front of the backlight or in front of the light guide in the viewing direction so that light emanating from the backlight and passing through the polarization filter is limited with respect to its propagation directions, a transmissive imaging display device which is arranged upstream of the light guide in the viewing direction and in which the linear polarization filter is arranged, wherein the backlight is switched on and the illuminants are switched off in the operating mode, and wherein at least the illuminants are switched on in the first operating mode.

25. The display screen according to claim 24, wherein the light guide is at least 70% transparent to the light emanating from the backlight.

26. The display screen according to claim 24, wherein the linear polarization filter is arranged in the imaging display device or is a part of the imaging display device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0083] The invention will be explained in more detail in the following with reference to drawings which also disclose key features of the invention. These embodiment examples are provided merely to be illustrative and should not be considered as restrictive. For example, a description of an embodiment example having a plurality of elements or components should not be interpreted to mean that all of these elements or components are necessary for its implementation. On the contrary, other embodiment examples may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different embodiment examples can be combined with one another unless otherwise stated. Modifications and alterations which are described for one of the embodiment examples may also be applicable to other embodiment examples. Like or comparable elements in the various figures are designated by the same reference numerals and not mentioned repeatedly so as to prevent repetition. The drawings show:

[0084] FIG. 1a the schematic diagram of a non-switchable optical element;

[0085] FIG. 1b the schematic diagram of a switchable optical element in a first state;

[0086] FIG. 1c the schematic diagram of the switchable optical element from FIG. 1b in a second state;

[0087] FIG. 2a the schematic diagram of a further optical element;

[0088] FIG. 2b the schematic diagram of a portion of a further switchable optical element in a first position;

[0089] FIG. 2c the schematic diagram of a portion of the further switchable optical element in a second position;

[0090] FIG. 3 the schematic diagram of a construction with a switchable optical element according to FIGS. 1b and 1c and FIGS. 2b and 2c;

[0091] FIG. 4 an exemplary diagram for comparing the transmission behavior of an optical element according to FIGS. 2a - 2c measured over different angles with that of a louver filter in the prior art;

[0092] FIG. 5 the schematic diagram of a switchable light filter in a third embodiment with a polarization filter;

[0093] FIG. 6 the schematic diagram of a switchable light filter in a third embodiment without a polarization filter;

[0094] FIGS. 7a-7c diagrams illustrating the influencing of light due to a switchable light filter according to FIG. 6;

[0095] FIG. 8 the schematic diagram of a switchable light filter in a fourth embodiment;

[0096] FIG. 9 the schematic diagram of a switchable light filter in a first embodiment;

[0097] FIG. 10 the schematic diagram of a switchable light filter in a second embodiment; and

[0098] FIGS. 11-15 optical simulations for illustrating the optical effect of an optical element.

DETAILED DESCRIPTION OF THE DRAWINGS

[0099] The drawings are not to scale and are merely schematic depictions.

[0100] FIG. 1a shows the schematic diagram of an exemplary non-switchable optical element 1. This optical element 1 comprises at least one layer S1. This layer S1 comprises material with a plurality of light-absorbing electric transition dipole moments (shown here schematically as small vertical lines) which – at least in a first state – are aligned (in this case perpendicular to the surface of the layer S1) parallel to a selectable preferential direction or fluctuate around this preferential direction, for which reason the aforementioned preferential direction forms an angle of between 0° and 45° to the surface normal of layer S1 so that light which is incident in the optical element is transmitted or partially or entirely absorbed depending on its incident direction relative to the layer S1 and depending on its polarization characteristics.

[0101] The optical element 1 can be produced, for example, by laminating a plurality of polymer film polarizers and/or by means of photoalignment of molecules or particles. The density of the aforementioned electric transition dipoles in layer S1 may vary depending on implementation. In a passive polarizer, the volume density can approach 100%.

[0102] The material of the optical element 1 which contains the transition dipole moments can also contain at least one kind of dye, particularly a kind of dye molecule, preferably at least one kind of dichroic dye or a dichroic dye mixture. A dye molecule can advantageously correspond to a transition dipole moment. A dye typically has a mass fraction of from 0.01% to 30%, preferably from 0.1% to 15% or 5%, or from 0.01% to 10% of material of the respective layers S1, S2, .... The thickness of the layers preferably ranges from 0.2 .Math.m to 50 .Math.m,preferably from 0.5 .Math.m to 20 .Math.m. The dyes or dye mixtures may vary between the different layers.

[0103] The layers S1, S2, ... which contain the transition dipole moments can also contain liquid crystals or polymers and/or be mixed with liquid crystals. Layers S1, S2, ... preferably contain a mixture of liquid crystals or polymers and at least one dye, preferably at least one dichroic dye mixture.

[0104] The optical element 1 constitutes a privacy filter for linearly polarized light. Therefore, a polarization filter P is provided here which polarizes light that is incident from below parallelly linear to the drawing plane. However, the polarization filter P does not alter the light propagation directions. Two possible light propagation directions are indicated by the two large arrows. Owing to the effect of the transition dipole moments, the light is absorbed through the optical element 1 which, for example, has an (oblique) direction of somewhat greater than 30° to the preferential direction, in this case the perpendicular bisectors to layer S1. Finally, after passing through the optical element 1, there remains substantially only the light incident along the preferential direction or, in this case, perpendicular to the optical element 1 as is shown by the solitary arrow at the top in the drawing. Depending on the case of application, each of the layers S1, S2, ... is constructed to be periodic or non-periodic in structure.

[0105] The extinction (i.e., the absorption) of the light depends on the absolute number of the transition dipole moments and therefore inherently also on the layer thicknesses of the material with the transition dipole moments. The density of the aforementioned transition dipole moments, their strength or the refractive index in the layers S1, S2, ... may vary depending on implementation. In a passive, i.e., non-switchable, optical element 1, the volume density of the transition dipoles can approach 100%.

[0106] In this regard, the drawings in FIGS. 11 to 15 show optical simulations for illustrating the optical effect of an optical element 1. FIG. 11 is a graph showing the normalized transmission of light when passing through an optical element 1 according to FIG. 1 plotted along the horizontal measurement angle from -90° to +90°. The strong absorption effect at angles over ±25° is clearly visible. The simulations are based on an assumed layer thickness of an optical element 1 of d = 0.5 mm, a refractive index of the material of n = 1.5, molarity M = 0.01 mol m.sup.-3 and an exemplary molar extinction of ε = 12,700 m.sup.2 mol.sup.-1. The thicknesses and concentration can vary in proportion to one another; accordingly, the same transmission is obtained, for example, with a ten-times smaller thickness and a ten-times higher concentration. In FIG. 12, the ratios shown in FIG. 11 are plotted on the ordinate logarithmically for the essential value range for use. It will be seen that with such parameters, the transmission is already reduced to about one percent at ±25° and at ±40° is already reduced to only around 0.001%. In FIG. 13, the assumptions made for FIG. 11 with respect to the refractive index n were calculated for values of n=1.0; 1.3; 1.5 and n=1.7. It will be seen that the smaller the refractive index of the material of layer S1, the more limited the angle of transmission. Further, FIG. 14 shows a variation of the normalized transmission through an optical element 1 according to the ratios shown in FIG. 11 with respect to the thickness of an optical element 1. Layer thicknesses of d = 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm and 0.5 mm were calculated. Consequently, as expected, a larger layer thickness ensures a sharper limiting of the transmission via the angles. Finally, FIG. 15 shows the angle-dependent normalized transmission for three selected parameter sets (1. n = 1.0 and d = 0.2 mm; 2. n = 1.5 and d = 0.5 mm; 3. n = 1.7 and d = 0.65 mm). All three parameter sets produce very similar optical effects.

[0107] The microlouver filters known from the prior art (also known as view control filters [VCF] or light control filters [LCF]) make use of geometric optics. As a result of the alternating periodic arrangement of transparent layers and absorbent layers, (virtually) all of the light propagating under large angles relative to a defined direction is absorbed. In this case, the position of the absorbers is controlled. In contrast, with the optical element 1, the transmission of light changes with different propagation directions because the absorption cross section of the molecules changes with the propagation direction. Accordingly, in this case, it is not the position but, on the contrary, the orientation of the absorbers in particular that is controlled. In other words, the effect of the optical element 1 is based on a direction-dependent absorption of the light rays as they pass through it, specifically, independent from the position of the light rays. This applies to non-switchable optical elements as described above and to switchable embodiments described in the following.

[0108] To enable switching of the optical effect of the optical element 1, i.e., for a switchable optical element 1, the electric light-absorbing transition dipole moments in every layer S1, S2, ... can be varied with respect to their orientation and/or their amount so that the respective layer S1, S2, ... can be brought into at least two different states. Possible embodiments of a switchable optical element 1 or of each layer S1, S2, ... therein are based, for example, on liquid crystals or fluorophores which can be arranged in a so-called vertical alignment cell or in a liquid crystal cell with homogeneous alignment at the surfaces and can be rotated therein between at least two states. In so doing, the light-absorbing electric transition dipole moments are also rotated and can accordingly take on at least two effect states. It is conceivable particularly in such embodiments that more than two states, e.g., three or eight states, with different respective optical effects are achieved. Other embodiments of liquid crystal cells are also possible.

[0109] In this connection, FIG. 1b shows the schematic diagram of a switchable optical element in a first state and FIG. 1c shows the schematic diagram of a switchable optical element in the second state.

[0110] For this exemplary case, it shall be assumed that the incident light is P-polarized, i.e., polarized parallel to the plane of incidence (see FIG. 1b). If the transparent electrodes E1, E2 are uncharged, i.e., they generate the electric field EF1 with 0 V/m (field-free), the liquid crystal molecules and dye molecules (which are formed in this instance, for example, as layers S1, S2 and S3) shown here as dots are aligned along the surface of electrodes E1, E2. This can be achieved through suitable combination of surface functionalization and liquid crystals and is known in the art. For light which propagates in the drawing plane and is S-polarized, the polarization of the light and transition dipole moments of the liquid crystals are always oriented perpendicular to one another. Therefore, no absorption takes place so that the light propagation directions with S-polarization shown above and below the substrate S pass freely through the switchable optical element 1.

[0111] If the electrodes E1 and E2 are charged as shown in FIG. 1c, i.e., they generate the electric field EF2 > 0 V/m, the ICH liquid crystals in layers S1,S2 and S3 rotate. If the voltage and therefore the field strength EF2 exceeds a certain threshold, liquid crystal molecules and therefore also dye molecules, if any, are accordingly oriented virtually parallel to the field lines of the electric field EF2. Accordingly, light is absorbed depending on angle α, the angle between the propagation direction of light and the surface normal of the surface of layer S1. The absorption increases with the angle α. The extinction of the electric field of the light is proportional to sine(α). In principle, a control of the orientation of the dye molecules is advantageous in order to control the axis of the protected view or in order to influence the light in a defined manner. In the state shown in FIG. 1b, the transition dipole moments are oriented perpendicular to the polarization of the incident light and, in the state shown in FIG. 1c, parallel to the perpendicular incidence of light. In such switchable optical elements, possible volume densities of the dye range between 0.1% and 20%, or possibly more up to 90% based on liquid crystals.

[0112] FIG. 2a shows the schematic diagram of a further non-switchable optical element 1. This non-switchable optical element 1 comprises (in this case only illustratively) a layer S1 which comprises material with a plurality of light-absorbing transition dipole moments (indicated in FIG. 2a by the small lines of layer S1). At least in a first state, each transition dipole moment is oriented, with a tolerance of 10° at the maximum, parallel to a respective preferential direction (indicated here by the thick arrows) or fluctuates around it, this preferential direction being selectable for the transition dipole depending on the position of such a transition dipole within the layer S1. At least two such preferential directions in a selectable plane (in this case, the drawing plane) differ by more than 10°. Accordingly, it is brought about that light which is incident in the optical element 1 is transmitted or partially or entirely absorbed depending on its incident direction relative to layer S1 and its polarization characteristics. The highest transmittance for each transition dipole is in the preferential direction selected for its position within layer S1. A tolerance of 10° maximum is allowed.

[0113] In contrast to FIG. 1, the optical element 1 shown in FIG. 2 is configured such that layer S1 is divided into different regions A1, A2, A3, A4, A5 along a selectable reference line (in this case, its lower edge), and every region A1, A2, ... has its own selected regional preferential direction which applies to all of the transition dipole moments of layer S1 located within a region A1, A2, ... . All of the regional preferential directions differ from one another pairwise and face in direction of an observer 6 to within a tolerance of ±10° at the maximum. Accordingly, all of the transition dipole moments within the layer S1 and within each region A1, A2, ... applicable to this layer S1 are oriented in each instance parallel to the applicable regional preferential direction with a tolerance of ±10° at the maximum. Because of the definable transmittance on the optical element 1, the latter can be used in a particularly advantageous manner to produce privacy viewing solutions.

[0114] To enable switching of the optical effect of the optical element 1, i.e., for a switchable optical element 1, the light-absorbing transition dipole moments in every layer S1, S2, ... can be varied with respect to their orientation and/or their absolute value and/or their density so that the respective layer S1, S2, ... can be brought into at least two different states.

[0115] In this regard, FIG. 2b shows the schematic diagram of a first portion of a switchable optical element in a first state corresponding to A1 in FIG. 2a. FIG. 2c shows the schematic diagram of a second portion of a switchable optical element in a first state. This corresponds to A5 in FIG. 2a. Here, three layers S1, S2, S3 are shown by way of example. The liquid crystals which are mixed with the dichroic dye mixture are indicated by the elliptic shapes in layers S1, S2, S3, where in a highly schematic manner the black ellipses represent the dye molecules and the white ellipses represent the liquid crystals. Further, the tilt of the ellipses indicates the spatial orientation. The substrates S can be glass or a polymer or another transparent material.

[0116] The respective transparent electrodes E1, E2, e.g., layers of indium tin oxide (ITO layers), are used to control the orientation of the liquid crystals mixed with the dye mixtures. However, the different orientation of the liquid crystal molecules and, therefore, of the dyes is preferably achieved by means of different surface functionalizations. Mechanical and optical methods are contemplated for this purpose. The orientation of the liquid crystals, including dye mixture, shown in FIG. 2b corresponds approximately to the orientation according to the preferential direction of portion A1 in FIG. 2a. The orientation of the liquid crystals, including dye mixture, shown in FIG. 2c corresponds approximately to the orientation according to preferential direction of portion A5 in FIG. 2a. Light which is incident on the optical element 1 from below is transmitted to the maximum extent given the conditions shown in FIG. 2b in direction of the corresponding preferential direction of the transition dipoles; other directions are partially or entirely absorbed. This applies analogously to the conditions shown in FIG. 2c.

[0117] As regards the behavior of P-polarized and S-polarized light when entering the optical element in connection with applied electric fields EF1 and EF2, reference is made to the statements referring to FIG. 1b and FIG. 1c which are applicable here in an analogous manner, although the transition dipole moments are oriented differently and although O-polarization instead of S-polarization and E-polarization instead of P-polarization are essential in the medium.

[0118] FIG. 3 shows the schematic diagram of a construction with a switchable optical element 1 according to FIGS. 1b, 1c and FIGS. 2b, 2c. Two linear polarization filters P, one of which is optional, are provided at the outer surfaces, the polarization directions thereof being oriented substantially (that is, within a few degrees tolerance) parallel to one another. Following on the inside - that is, on the sides of the polarization filters P facing one another - is a transparent substrate S followed again inwardly by electrodes E1 and E2. The inwardly facing alignment layers 4 thereof serve for the orientation of liquid crystals which, in a mixture with at least one dichroic dye, form the inner layers S1, S2, .... The transition dipole moments are formed in this instance by the at least one dichroic dye. In principle, additional retarder films can also be used in the construction of the optical element 1 in all of the variants described herein in order to further adapt the polarization states.

[0119] The electric square wave voltage which is applied between electrodes E1 and E2 preferably has root mean square values between 0 V and 20 V. The orientation layers 4 are, for example, treated surfaces (e.g., brushed glasses or polymers) in order to achieve a uniform surface orientation of the transition dipole moments or liquid crystals. For example, the states shown in FIGS. 2b, 2c can be produced in this way.

[0120] Finally, FIG. 4 shows an exemplary graph for comparing the normalized transmission behavior of an optical element 1 (solid line) measured over various angles according to FIG. 2 and FIG. 3 with that of a louver filter from the prior art (dashed line). The respective measurement angle is plotted on the abscissa, and the normalized transmission is plotted on the ordinate. It will be seen that the continuous curve shows an approximate “top hat” distribution for the transmission behavior of an exemplary optical element 1, i.e., the transmission remains stable over a wide angular range of approximately -17° to +17° with at least 80% stability. The half-power width amounts to 40° total in this case. As a result, the observer 6 discerns a good homogeneity of transmission for visual angle changes of ± 15% so that, in turn, a good discerned homogeneity is also achieved in the illumination or imaging in combination with an imaging display unit. In contrast, the exemplary louver filter of the prior art used for comparison whose normalized transmission behavior is shown in dashes in FIG. 4 has a reduced half-power width of only approximately 35° and, further, does not have a top-hat-like distribution and also provides an inferior protected view in the angular range of -30° to -25° and +25° to +30° because the transmission is greater than in the optical element 1.

[0121] The optical element can be used in particular in a switchable light filter 5. FIG. 5 shows a third embodiment of a switchable light filter 5 of this kind. The designation “third” being used prior to “first” and “second” was chosen so as to be consistent with the general description of the invention. This switchable light filter 5 comprises a first optical element 1 and a second optical element 2. The two optical elements 1, 2 per se are not switchable, i.e., they are static. The preferential directions of the transition dipole moments of the two optical elements differ from one another by less than 40°, preferably less than 20° and particularly preferably less than 10°. A first preferential direction is selectable for the first optical element 1, and a second preferential direction is selectable for the second optical element 2. In the present case, the two preferential directions are parallel to one another, for example, and correspond to the perpendicular bisector of the switchable light filter 5 which lies in the drawing plane. A switchable liquid crystal layer 3 which influences or does not influence the polarization characteristics of the light passing through it depending on an electric field EF1 or EF2 acting on it is arranged between the optical elements 1, 2. A polarization filter P is arranged below the optical elements 1, 2 in the drawing and as seen by an observer 6. It could also be arranged above the optical elements 1, 2. Means for selectively generating the first electric field EF1 and the second electric field EF2, for example, electrodes arranged above and below the liquid crystal layer 3, are not shown in the drawing.

[0122] In a first operating mode B1 in which the first electric field EF1 is applied and which has a first sub-operating mode B 1H and a second sub-operating mode B 1V, at least 24% of unpolarized light which is incident in the switchable light filter 5 parallel to the first preferential direction or to the second preferential direction is transmitted on the one hand, and 85% of unpolarized light which is incident in the switchable light filter 5 at an angle of more than 30° relative to the corresponding preferential direction is absorbed on the other hand. This absorption is carried out exclusively in a first direction in the first sub-operating mode B1H and exclusively in a second direction perpendicular to the first direction in the second sub-operating mode B1V. Either the first direction or the second direction is perpendicular to a polarization direction of the polarization filter P. When used in a display screen, the first direction can correspond to the horizontal direction and the second direction can correspond to the vertical direction. By “horizontal” is meant a line extending parallel to the distance line between the eyes of an observer. The horizontal line then extends parallel to the lower edge, for example, and the vertical line extends parallel to the lefthand or right-hand lateral edge of the layer S1 or display screen.

[0123] In a second operating mode B2 in which the second electric field EF2 is applied, at least 24% of unpolarized light which is incident in the switchable light filter 5 parallel to the first preferential direction or to the second preferential direction is transmitted on the one hand, and at least 85% of unpolarized light which is incident in the switchable light filter 5 at an angle of greater than 30° to the corresponding preferential direction is absorbed on the other hand.

[0124] FIG. 6 shows a modification of the third embodiment of the switchable light filter 5 without polarization filter P. While the behavior in this modification in operating mode B2 is identical to the switchable light filter 5 described with reference to FIG. 5, at least 24% of unpolarized light which is incident in the switchable light filter 5 at any angle is transmitted in the first operating mode B1 in which the first electric field EF1 is applied.

[0125] Accordingly, the switchable light filter 5 of the third embodiment in combination with an imaging display unit allows toggling either between a two-sided protected view and a four-sided protected view (e.g., up/down-protected in B1V versus up/down/left/right-protected in B2) when there is a polarization filter P or toggling between a free view in all directions and a four-sided protected view (e.g., free view in B1 versus up/down/left/right-protected in B2) when there is no polarization filter P.

[0126] It is possible, for example, that either electric field EF1 or electric field EF2 describes a field-free state, the other respective electric field EF2 or EF1 having an absolute field strength of greater than zero, e.g., 0.5 MV/m. Depending on the configuration of the optical elements 1 and 2, the field-free state may mean that operating mode B2 is in effect. However, it is also possible that operating mode B1 is in effect in the field-free state when a polarization filter P is provided with one of the two sub-operating modes B1H, B1V.

[0127] FIGS. 7a to 7c show diagrams of the polarization state of the light due to a switchable light filter according to FIG. 6. FIG. 7a shows the polarization state of the public mode corresponding to operating mode B1 with unrestricted visual angle range in which the light for visual angles greater than 30° is tangentially polarized and light with perpendicular incidence is linearly polarized. The preferential directions correspond to the respective perpendicular bisector. FIG. 7c shows the polarization state in the limited viewing mode. In this case, light for an angle of incidence greater than 30°, for example, is very strongly attenuated, while light with perpendicular incidence of the polarization state is transmitted with no change.

[0128] The light for incident angles is polarized through the alignment or orientation of the transition dipole moments, the linear polarization of the light is always oriented perpendicular to the direction of the origin in the visual angle space; in the described case, the origin is the perpendicular bisector. This state is shown in FIG. 7a and is applicable after the light passes through the non-switchable optical element 1. The light with small angles is only slightly polarized. Ideally, the light which is incident perpendicularly is not absorbed, i.e., the polarization state stays the same.

[0129] If the light passes out of the optical element 2 into the liquid crystal layer 3, the state of the liquid crystal layer 3 determines whether and, if so, how the polarization state of the light is changed or remains unchanged. For example, if the polarization rotation in the liquid crystal layer 3 is switched off in a field-free state EF1, nothing changes in the above-described state. After passing through the liquid crystal layer 3 and then through the non-switchable optical element 1, the transmission remains substantially unchanged. If there is a polarization filter P above optical element 1 or below optical element 2, one of the sub-operating modes B1H or B1V is achieved depending on the embodiment. If this polarizer is not present, operating mode B1 is realized.

[0130] On the other hand, if the polarization rotation in the liquid crystal layer 3 is switched on, for example, by applying an electric field EF2 > 0 V/m, the polarization is rotated by 90° overall. FIG. 7b shows the polarization state after passage through the optical element 2 and the polarization-rotating liquid crystal layer 3. Accordingly, in the course of the further propagation of light, all light that propagates under an angle greater than 25° or approximately 30° is extinguished because of the optical element 1. This corresponds to operating mode B2 and is shown in FIG. 4c.

[0131] FIG. 8 shows the schematic diagram of a switchable light filter 5 in a fourth embodiment. This switchable light filter 5 comprises two switchable optical elements 1, 2, each of the two optical elements comprising a first layer S1 or a first layer S1 and a plurality of further layers S2, ... . Similar to the optical elements of the third embodiment, each of the layers S1, S2, ... comprises a material with a plurality of light-absorbing transition dipole moments. At least in a first state, each transition dipole moment is oriented, with a tolerance of 10° at the maximum, parallel to a first preferential direction selectable for the first optical element 1 and second preferential direction selectable for the second optical element 2 or fluctuates around this so that light which is incident in the first optical element 1 or second optical element 2 is transmitted or at least partially absorbed depending on its incident direction relative to the layers S1, S2, ... and depending on its polarization state.

[0132] However, in contrast to the third embodiment, the optical elements 1, 2 are switchable in this case, i.e., the transition dipole moments in each of the layers S1, S2 ... can be varied with respect to their orientation and/or their amount between the first state and at least a second state in order to bring the respective layer S1, S2, ... alternatively into at least two different states. In this embodiment, the switchable light filter 5 comprises means for selectively producing a first electric field EF1 or a second electric field EF2. For each of the two optical elements 1, 2, the first state is produced by applying the first electric field EF1 and the second state is produced by applying the second electric field. In the embodiment example according to the fourth embodiment, the first electric field EF1 is applied to the first optical element 1 and the second electric field EF2 is applied to the second optical element 2.

[0133] Optionally, a polarization filter can be arranged above or below the optical element 1 combined as unit. This is not necessary but can improve the performance capability of the switchable light filter 5. The polarization of the polarization filter P and that of the incident light must be congruent. An optically anisotropic layer 7 is arranged between the two optical elements 1, 2 for the 90 degree rotation of a polarization direction of light that passes through the optically anisotropic layer 7. For example, the optically anisotropic layer 7 can be a layer with liquid crystals or a half-wave plate. Means for generating the two electric fields EF 1 and EF2 are not shown in the drawing.

[0134] When the first electric field EF 1 is applied in a first operating mode B1 for a free viewing mode, at least 24% of unpolarized light which is incident in the switchable light filter 5 at any angle thereto is transmitted. In the first operating mode B 1, the transition dipole moments of the two optical elements 1, 2 are oriented perpendicular to one another and, in case a polarization filter P is provided, polarization filter transition dipole elements of the polarization filter P are aligned parallel to the transition dipole moments of the switchable optical element 1, 2 located closest to the polarization filter P.

[0135] In contrast, when the second electric field is applied in a second operating mode B2 for a limited viewing mode, on the one hand, at least 24% of unpolarized light which is incident in the switchable light guide 5 parallel to the first preferential direction or second preferential direction is transmitted and, on the other hand, at least 85% of unpolarized light which is incident in the switchable light guide 5 at an angle of greater than 30° to the corresponding preferential direction is absorbed. In operating mode B2, the transition dipole moments of the polarization filter P, if present, and the transition dipole moments of the switchable optical element 1, 2 located closest to the polarization filter P are oriented perpendicular to one another, and the transition dipole moments of the two optical elements 1, 2 are oriented parallel to one another. In this case also, the preferential directions preferably correspond to the perpendicular bisector of the switchable light filter 5 which lies in the drawing plane in FIG. 8. With corresponding orientation of the switchable light filter 5 as already mentioned with reference to the third configuration, this applies particularly at the same time in both horizontal and vertical direction when the horizontal is oriented parallel to the lower edge of the layer S1 and the vertical is oriented parallel to the left or right lateral edge of layer S1.

[0136] Accordingly, in combination with an imaging display unit, the switchable light filter 5 in this fourth configuration allows toggling between a free view in all directions and a four-sided private view (free view B1 versus up/down/left/right-protected B2).

[0137] A first configuration of the switchable light filter 5 is shown as schematic diagram in FIG. 9. It comprises a non-switchable first optical element 1 again comprising a first layer S1 or a first layer S1 and a plurality of further layers S2, ... , each layer S1, S2, ... comprising a material with a plurality of light-absorbing transition dipole moments. At least in a first state, each transition dipole moment is oriented, with a tolerance of 10° at the maximum, parallel to a first preferential direction selectable for the first optical element 1 or fluctuates around this so that light which is incident in the first optical element 1 is transmitted or at least partially absorbed depending on its incident direction relative to the layers S1, S2, ... and depending on its polarization state.

[0138] A polarization filter P is arranged upstream or downstream of the first optical element 1. The means for selectively generating a first electric field EF1 or a second electric field EF2 are again not shown in the drawing. A liquid crystal layer 3 which is acted upon by the first electric field EF1 or the second electric field EF2 and which, depending on this, influences the polarization state of light passing through it is arranged between the first optical element 1 and the polarization filter P.

[0139] In this first configuration, in a first operating mode B1 in which the first electric field EF 1 is applied and which has a first sub-operating mode B1H and a second sub-operating mode B1V, at least 24% of unpolarized light which is incident in the switchable light filter 5 parallel to the first preferential direction is transmitted, and 85% of unpolarized light which is incident in the switchable light filter 5 at an angle of more than 30° relative to the first preferential direction is absorbed on the other hand. This absorption is carried out exclusively in a first direction in the first sub-operating mode B 1H and exclusively in a second direction perpendicular to the first direction in the second sub-operating mode B1V. The first preferential direction is again preferably parallel to the perpendicular bisector of the switchable light filter 5 in the drawing plane. With respect to the position of the first direction and second direction, the statements referring to the third embodiment can be applied in an analogous manner.

[0140] In a second operating mode B2 in which the second electric field EF2 is applied and with the first sub-operating mode B1H and the second sub-operating mode B1V, at least 24% of unpolarized light which is incident in the switchable light filter 5 parallel to the first preferential direction is transmitted on the one hand, and at least 85% of unpolarized light which is incident in the switchable light filter 5 at an angle of greater than 30° to the first preferential direction is absorbed on the other hand. This absorption is carried out exclusively in the second direction in the first sub-operating mode B1H and exclusively in the second direction perpendicular to the first direction in the second sub-operating mode B1V so that the directions of absorption differ from one another by 90° in each instance for each of the two sub- operating modes B1H, B1V for the first operating mode B1 and the second operating mode B2.

[0141] Unlike in the preceding, the two operating modes in this case are operating modes with limited viewing modes, and the protected view can be changed between two directions – for example, horizontal and vertical - which are perpendicular to one another. The configuration of this first embodiment of a switchable light filter 5 can be changed between sub-operating modes B1V and B1H by rotating the polarization filter P 90 degrees.

[0142] It is also possible in this case, for example, that either the first electric field EF1 or the second electric field EF2 describes a field-free state and the other respective electric field EF2 or EF1, respectively, has an absolute field strength of greater than zero, e.g., 0.5 MV/m. Depending on the embodiment of the optical element 1 and of the polarization filter P, the field-free state can mean that sub-operating mode B1H is in effect. However, it is also possible that sub-operating mode B1V is in effect in the field-free state.

[0143] Accordingly, the switchable light filter 5 of this first embodiment in combination with an imaging display unit allows toggling between a protected view in vertical direction and a protected view in horizontal direction (e.g., up/down-protected in B1V versus left/right-protected in B1H). In a laptop, for example, this would mean that a user can look at the contents in sub-operating mode B1V together with other people located next to the user and at substantially the same eye level, whereas in sub-operating mode B1H the persons positioned to the side cannot see the image contents. The switchable light filter 5 of this third configuration can be varied with respect to its construction as has already been described.

[0144] Finally, FIG. 10 schematically shows a second embodiment of a switchable light filter 5. The first optical element 1 is constructed in a manner similar to the optical element 1 of the first embodiment but, in contrast to the latter, so as to be switchable, i.e., the transition dipole moments in each of the layers S1, S2 can be varied with respect to their orientation and/or their amount between the first state and at least a second state in order to alternatively put the respective layer S1, S2, ... in at least two different states. A polarization filter P is arranged upstream or downstream of the first optical element 1. Means for selectively generating a first electric field EF1 or a second electric field EF2 are not shown. For the first optical element 1, the first state is produced by applying the first electric field EF1, and the second state is produced by applying the second electric field EF2.

[0145] In a first operating mode B1 in which the first electric field EF1 is applied and the transition dipole moments of the layers S1, S2, ... of the first optical element 1 are oriented along the first preferential direction and which has a first sub-operating mode B1H and a second sub-operating mode B1V, at least 24% of unpolarized light which is incident in the switchable light filter 5 parallel to the first preferential direction is transmitted on the one hand and at least 85% of unpolarized light which is incident in the switchable light filter 5 at an angle of greater than 30° to the first preferential direction is absorbed on the other hand, this absorption taking place exclusively in a first direction in the first sub-operating mode B1H and exclusively in a second direction perpendicular to the first direction in the second sub-operating mode B1V. The first preferential direction is again preferably parallel to the perpendicular bisector of the switchable light filter 5 in the drawing plane. With respect to the position of the first direction and second direction, the statements referring to the third embodiment can be applied in an analogous manner.

[0146] In a second operating mode B2 in which the second electric field EF2 is applied, the transition dipole moments of layers S1, S2, ... of the first optical element 1 are oriented parallel to a surface of the polarization filter P – also referred to in this connection as substrate - and perpendicular to a transmission direction of the polarization filter P. At least 24% of unpolarized light which is incident in the switchable light filter 5 at any angle to the first preferential direction is then transmitted.

[0147] Here also, it is again possible, for example, that either electric field EF1 or electric field EF2 describes a field-free state and the other respective electric field EF2 or EF1, respectively, has an absolute field strength of greater than zero, e.g., 0.5 MV/m. Depending on the embodiment of the optical element 1 and of the polarization filter P, the field-free state can mean that operating mode B2 is in effect. However, it is also possible that operating mode B1 with one of the sub-operating modes B 1H or B1V is in effect in the field-free state.

[0148] Accordingly, the switchable light filter 5 of this second embodiment in combination with an imaging display unit allows toggling between a protected view in vertical direction or in horizontal direction and no protected view effect (up/down-protected in B1V or left/right-protected in B 1H versus no protected view B3).

[0149] For certain cases of application, a switchable light filter 5 in any of the configurations mentioned above can be divided into a plurality of separately switchable segments so that localized toggling between the respective possible operating states is made possible. In combination with an imaging display unit, this would mean that, for example, only a portion of the image area can be toggled between a protected view and no protected view effect for free viewing, while the complementary portion of the image area is permanently in a privacy mode or not in a privacy mode. There can even be a plurality of such segments geometrically separate from one another which can be toggled between the operating modes separately or jointly.

[0150] As has already been stated, the above-described switchable light filter 5 can be combined with an imaging display unit to form a display screen. Such a display screen which can be operated in at least a first operating state B1V and/or B3 for a free viewing mode in horizontal direction and in at least a second operating state B1H and/or B2 for a viewing mode restricted in horizontal direction comprises a switchable light filter 5 as described above in one of the four configurations mentioned above and an imaging display unit arranged downstream or upstream of the switchable light filter 5 from the perspective of an observer 6.

[0151] The imaging display unit advantageously corresponds to an LCD panel whose one polarization filter corresponds to the polarization filter P. This can be the front or back polarizer in the LCD construction. Moreover, the switchable light filter can advantageously be arranged between the LCD panel and the backlight thereof in order to toggle between a first operating state B3 (or B1V) for a free viewing mode and a second operating state B1H or B2, respectively, for a limited viewing mode because the light of the backlight is sometimes focused (B2 or B1H) and sometimes not focused (B3 or B1V) in horizontal direction because of the switchable light filter. “Focusing” does not mean, in this instance, focusing in the manner of lenses but rather a narrowing of the emission area or transmission area depending on the incidence angle.

[0152] Such a display screen is advantageously used in a mobile device, a motor vehicle, aircraft or watercraft, in a pay terminal or in an access system. Toggling between the aforementioned operating modes can then be carried out in order to protect sensitive data, i.e., to make it visible to only one observer or, alternatively, to display image contents to a plurality of observers simultaneously.

[0153] In the optical element described above, light which is incident in and passes through this optical element is transmitted or partially or entirely absorbed depending on its incident direction and its polarization characteristics. A switchable light filter which uses an optical element of this kind influences the transmission of light in an angle-dependent manner (optionally perpendicularly). Toggling can be effected between at least two operating states for a free viewing mode – with respect to the visual angle range for an observer – and a limited viewing mode. In so doing, angle limits particularly in transmission are switchable in particular directions. The optical element or systems based thereon can be implemented at low cost and, in particular, are universally useable with various types of display screen to enable toggling between a protected view existing at least in the horizontal direction and a free viewing mode. In so doing, the resolution of such a display screen is not fundamentally reduced.

[0154] The invention described above can advantageously be used in combination with an imaging display device anywhere that confidential data are displayed and/or entered, such as when entering a PIN number or displaying data in automatic teller machines or payment terminals or for entering passwords or when reading emails on mobile devices. As was described above, the invention can also be applied in passenger cars to selectively withhold distracting or unwanted image contents from the driver or passenger.

REFERENCE CHARACTERS

[0155] 1 first optical element [0156] 2 second optical element [0157] 3 liquid crystal layer [0158] 4 alignment layer [0159] 5 switchable light filter [0160] 6 observer [0161] 7 optically anisotropic layer [0162] A1...A5 regions [0163] E1, E2 electrodes [0164] E2 second electric field [0165] P polarization filter [0166] S transparent substrate [0167] S1...S3 layer