DE102023005411A1 - Switchable light filter, lighting device and screen with such a switchable light filter - Google Patents

Abstract

The invention relates to a switchable light filter (5, 5a) comprising a first optical element (1), in turn comprising a plurality of light-absorbing transition dipole moments, so that light which is incident on the first optical element (1) is transmitted or at least partially absorbed depending on its polarization state and its direction of incidence relative to the first optical element (1), means for selectively generating a first electric field (EF1) or a second electric field (EF2), a liquid crystal layer (3) arranged in front of or behind the first optical element (1), on which the first electric field (EF1) or the second electric field (EF2) acts and which, depending thereon, influences the polarization state of light penetrating through it, so that the transmission properties of the switchable light filter (5, 5a) differ between a first operating mode B1, in which the first electric field (EF1) is present, and a second operating mode B2, in which the first electric field (EF2) is present, wherein
Improvements for the liquid crystal layer (3) or the optical stack are disclosed in order to improve the transmission properties.

Description

Technical field of the invention

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

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

Additional films based on micro-louvres have already been used for mobile displays to achieve visual data protection. However, these films were not switchable or reversible; they always had to be applied by hand and then removed again. They also have to be transported separately to the display when they are not needed. A major disadvantage of using such louvre films is the associated loss of light.

The US 6,765,550 B2 describes such a privacy screen using micro-louvres. The biggest disadvantage here is the mechanical removal or mechanical installation of the filter as well as the loss of light in protected mode.

In the US 5,993,940 A describes the use of a film that has small, strip-shaped prisms evenly arranged on its surface to achieve a private mode, ie a restricted viewing mode with a small viewing angle range. Development and production are technically quite complex.

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

The US 2012/0235891 A1 describes a very complex backlight - a background lighting - in a screen. According to 1 and 15 Not only several light guides are used, but also other complex optical elements such as microlens elements 40 and prism structures 50, which transform the light from the rear lighting on the way to the front lighting. This is expensive and technically complex to implement and also involves light loss. According to the variant according to 17 in the US 2012/0235891 A1 Both light sources 4R and 18 produce light with a narrow illumination angle, whereby the light from the rear light source 18 is first converted at great expense into light with a large illumination angle. This complex conversion has - as already mentioned above - a significant reduction in brightness.

According to the JP 2007-155783 A Special optical surfaces 19 are used, which are complex to calculate and produce and then deflect light into different narrow or wide areas depending on the angle of incidence. These structures are similar to Fresnel lenses. There are also interference edges that deflect light in undesirable directions. It therefore remains unclear whether really useful light distributions can be achieved.

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

The WO 2015/121398 A1 The applicant describes a screen with two operating modes, in which scattering particles are present in the volume of the corresponding light guide for switching between the operating modes. However, the scattering particles made of a polymer used there generally have the disadvantage that light is coupled out of both large surfaces, which means that about half of the of the useful light is emitted in the wrong direction, namely towards the background lighting, and cannot be recycled there to a sufficient extent due to the structure. In addition, the scattering particles made of polymer distributed in the volume of the light guide can, under certain circumstances, particularly at higher concentrations, lead to scattering effects that reduce the visual protection effect in the protected operating mode.

The approach of the "Electrical Birefringence (EDB)" technology is based on the idea of using the switchable liquid crystals of an additional LC panel to "filter" all light rays that do not emerge from the imaging layer at a specific beam angle. Disadvantages of this technology are high additional energy and cost expenditure and the difficulty of changing the +/-40° sweet spot, i.e. the best possible viewing position. The absorption level of the LC structures is also inadequate, since the attenuation of the light intensity increases again for viewing angles larger than the sweet spot, so that the light intensity for viewing angles larger than +/-40° is up to 3% of the maximum light intensity.

The aforementioned methods and arrangements generally have the disadvantage that they significantly reduce the brightness of the basic screen and/or require a complex and expensive optical element for mode switching and/or reduce the resolution in the freely viewable, public mode and/or exhibit visual artifacts on very high-resolution displays.

Description of the Invention

The object of the invention is therefore to describe a switchable light filter with an optical element in which light which is incident on the optical element is transmitted or partially or completely absorbed depending on its direction of incidence and its polarization properties - but not depending on its position. The switchable light filters which use the optical element are intended to influence the transmission of light depending on the angle - optionally perpendicular to a seated or standing observer - whereby it is possible to switch between at least two operating states. The transmission should be switchable for certain directions of light propagation.

The switchable light filter or systems using it should be inexpensive to implement and, in particular, universally usable with different types of screens in order to enable switching between a visual protection - at least in the horizontal direction with respect to a standing or sitting viewer - i.e. a restricted viewing mode, and a free viewing mode, whereby the resolution of such a screen should not be reduced, not even slightly. In particular, a high-quality angle restriction should also be achieved for viewing positions in which a viewer does not only look horizontally (from his point of view) at an angle to the switchable light filter, but also at an angle from above, and good image quality should be achieved overall in the event that a switchable light filter is arranged in front of an image display device.

• - ein erstes optisches Element, seinerseits umfassend
  • - eine Vielzahl an Licht absorbierenden Übergangsdipolmomenten,
  • - wobei die Mehrzahl der Übergangsdipolmomente mindestens in einem ersten Zustand (insbesondere auch permanent) mit einer Toleranz von maximal 20° (bevorzugt maximal 10°) parallel zu einer für das erste optische Element wählbaren ersten Vorzugsrichtung ausgerichtet ist oder um diese herum variiert,
  • - so dass Licht, welches in das erste optische Element einfällt, in Abhängigkeit von seinem Polarisationszustand und von seiner Einfallsrichtung gegenüber dem ersten optischen Element transmittiert oder mindestens teilweise absorbiert wird,
• - Mittel zur wahlweisen Erzeugung mindestens eines ersten elektrischen Feldes EF1 oder eines zweiten elektrischen Feldes EF2,
• - eine vor oder hinter dem ersten optischen Element angeordnete Flüssigkristallschicht, auf welche das erste elektrische Feld EF1 oder das zweite elektrische Feld EF2 wirkt und die in Abhängigkeit davon den Polarisationszustand von durch sie hindurchdringendem Licht beeinflusst, so dass
• - sich die Transmissionseigenschaften des schaltbaren Lichtfilters zwischen einer ersten Betriebsart B1, in welcher das erste elektrische Feld EF1 anliegt, und einer zweiten Betriebsart B2, in welcher das erste elektrische Feld EF2 anliegt, unterscheiden,
• - wobei erfindungsgemäß mindestens eine der folgenden Maßnahmen im schaltbaren Lichtfilter umgesetzt ist:
  • - die Flüssigkristallschicht umfasst mindestens eine TN-Zelle sowie einen Verzögerungsfilm, wobei der besagte Verzögerungsfilm die in der ersten Betriebsart B1 durch die TN-Zelle durchgeführte Polarisationsänderung für den schaltbaren Lichtfilter durchdringendes Licht zu mindestens 80% aufhebt (ideal sogar zu 90%, 95% oder mehr; dies kann z.B. mittels eines discotischen Films realisiert werden), oder
  • - die Flüssigkristallschicht 3, wenn sie beispielweise mittels PA, FFS-, IPS-, VA-, ADS- (oder vergleichbarer) Technologie realisiert wird, weist in der ersten Betriebsart B1 ihr Transmissionsmaximum in einem Winkel, der zwischen 20° und 80° zu Flächennormalen der Flüssigkristallschicht liegt, auf, so dass die Beeinflussung der Polarisation des den schaltbaren Lichtfilter durchdringenden Lichtes aufgrund der Flüssigkristallschicht für unter dem besagten Winkel, der zwischen 20° und 80° zu Flächennormalen der Flüssigkristallschicht liegt, einfallende Lichtstrahlen maximal ist, wodurch die Beeinflussung der winkelabhängigen Transmission durch das optische Element verstärkt wird, und/oder
  • - der schaltbare Lichtfilter umfasst weiterhin ein negativ dispersives optisches Element (z.B. einen negativ dispersiven Verzögerungsfilm), oder mehrere optische Elemente, die in der Zusammenwirkung wie ein negativ dispersives optisches Element wirken (dies kann erreicht werden durch die Kombination von verschiedenen Elementen mit unterschiedlich hoher Dispersion), so dass Farbverschiebungen des den schaltbaren Lichtfilter durchdringenden Lichtes (insbesondere im Fall von weißem oder polychrome Licht) mindestens teilweise kompensiert werden.

This object is achieved in a first embodiment according to the invention by a switchable light filter comprising

• - a first optical element, itself comprising
  • - a multitude of light-absorbing transition dipole moments,
  • - wherein the majority of the transition dipole moments are aligned parallel to a first preferred direction selectable for the first optical element or vary around it, at least in a first state (in particular also permanently) with a tolerance of a maximum of 20° (preferably a maximum of 10°),
  • - so that light which is incident on the first optical element is transmitted or at least partially absorbed depending on its polarization state and its direction of incidence relative to the first optical element,
• - means for selectively generating at least a first electric field EF1 or a second electric field EF2,
• - a liquid crystal layer arranged in front of or behind the first optical element, on which the first electric field EF1 or the second electric field EF2 acts and which, depending thereon, influences the polarisation state of light passing through it, so that
• - the transmission properties of the switchable light filter differ between a first operating mode B1, in which the first electric field EF1 is applied, and a second operating mode B2, in which the first electric field EF2 is applied,
• - wherein according to the invention at least one of the following measures is implemented in the switchable light filter:
  • - the liquid crystal layer comprises at least one TN cell and a retarder film, wherein said retarder film has the the polarization change carried out by the TN cell in the first operating mode B1 cancels out light penetrating the switchable light filter by at least 80% (ideally even by 90%, 95% or more; this can be achieved by means of a discotic film, for example), or
  • - the liquid crystal layer 3, if it is realized for example by means of PA, FFS, IPS, VA, ADS (or comparable) technology, has its transmission maximum in the first operating mode B1 at an angle that lies between 20° and 80° to the surface normal of the liquid crystal layer, so that the influence of the polarization of the light penetrating the switchable light filter due to the liquid crystal layer is maximum for light rays incident at the said angle, which lies between 20° and 80° to the surface normal of the liquid crystal layer, whereby the influence of the angle-dependent transmission by the optical element is increased, and/or
  • - the switchable light filter further comprises a negatively dispersive optical element (e.g. a negatively dispersive retardation film), or several optical elements which act together like a negatively dispersive optical element (this can be achieved by combining different elements with different levels of dispersion), so that color shifts of the light penetrating the switchable light filter (in particular in the case of white or polychrome light) are at least partially compensated.

The mode of action of the measures according to the invention is further explained below:

In order to achieve the best possible privacy protection, the electric field in the restricted-view mode must oscillate as much as possible in the plane of incidence of the light. One way to make deviations from linear polarization visible and measurable is to align a polarizer perpendicular to the desired polarization. The lower the transmission after the vertical polarizer, the better the privacy protection with the optical element.

Nematically twisted liquid crystal cells located between two crossed polarizers have the disadvantage that they have a high transmission when viewed non-perpendicularly in the black state, i.e. in the non-transmitting state between two crossed polarizers. The lower the transmission, the greater the degree of linear polarization. This high black transmission and thus the deviation from linear polarization is caused by incomplete alignment of the liquid crystals by the electric field near the interfaces. The polarization deviations can be minimized using discotic liquid crystal layers, among other things. The layers mentioned are polymerized and the LC molecules do not change their orientation. The reduction of the polarization change follows the idea that in the field-affected state, the change in polarization by the LC cell is reversed by means of the discotic LC film. The LC cell in the field-affected state and the discotic LC film have the same amount of birefringence with different signs. The light filter described requires the purest possible polarization states for optimal function.

The liquid crystal cell types ADS, FFS and IPS have significantly lower (i.e. better) transmissions compared to TN displays, which means that the polarization state of the light after the LC cell deviates only slightly from the linear polarization. In terms of concept, FFS, IPS and ADS liquid crystal cell geometries are essentially the same. In the field-free state, the linearly polarized light is transmitted essentially without any change in polarization. In the field-affected state, the liquid crystal molecules are rotated as far as possible exclusively in planes parallel to the surface. Due to the arrangement of the electrodes, the rotation of the LC molecules is different. This changes the polarization of the light to different degrees. Typically, the LC cells are optimized for maximum transmission under perpendicular incidence. For the present case, however, it is advantageous if the maximum polarization change is achieved for non-perpendicular cases. This can be achieved, for example, by increasing the cell thickness, the birefringence of the LC molecules or by increasing the electric fields.

It is known in the art that performance can be improved by using passive delay filters. In vertically aligned LC cells, performance is improved by using delay plates with negative birefringence to reverse the birefringence of the LC cell. The delay plate ensures that the light is transmitted as linearly as possible.

The main challenge in implementing the described privacy filters in screens is that numerous components are structured and located close to each other, which often leads to moiré artifacts. To achieve multi-color, three subpixels usually form a full pixel. It is important that the individual Subpixels are electrically and optically isolated from each other to prevent color distortions and to be able to cover as large a part of the color space as possible. Technically, this is achieved by a black mask between neighboring subpixels. In the case of the LC cell for controlling the privacy screen, i.e. for the liquid crystal layer in the switchable light filter, the strong isolation of neighboring pixels is not necessary. The black mask can be made smaller.

In a preferred embodiment, a polarization filter is also present, which can be arranged upstream or downstream of the first optical element in the direction of incidence.

• - in einer ersten Betriebsart B1, in welcher das erste elektrische Feld EF1 anliegt, einerseits linear oder elliptisch polarisiertes Licht (im Falle von elliptisch polarisiertem Licht sollten die Verhältnisse der Beträge Halbachsen mindestens 1:4 betragen, besser mindestens 1:10 oder größer), welches parallel zur ersten Vorzugsrichtung in den schaltbaren Lichtfilter einfällt, zu mindestens 24% transmittiert und andererseits linear oder elliptisch polarisiertes Licht, welches in mindestens einem ersten Winkel von mehr als 35° zu der ersten Vorzugsrichtung, der in einer ersten Ebene liegt, in den schaltbaren Lichtfilter einfällt, zu mindestens 85% absorbiert wird, und dass
• - in einer zweiten Betriebsart B2, in welcher das zweite elektrische Feld EF2 anliegt, einerseits linear oder elliptisch polarisiertes Licht, welches parallel zu der ersten Vorzugsrichtung in den schaltbaren Lichtfilters einfällt, zu mindestens 24% transmittiert und andererseits linear oder elliptisch polarisiertes Licht, welches in mindestens einem zweiten Winkel von mehr als 35° zu der ersten Vorzugsrichtung, der in einer zweiten Ebene liegt, in den schaltbaren Lichtfilter einfällt, zu mindestens 85% absorbiert wird, wobei die erste und die zweite Ebene sich in einem Winkel von 80° bis 100° schneiden, so dass sich die Richtungen der Absorption für die erste Betriebsart B1 und die zweite Betriebsart B2 um jeweils 80° bis 100° unterscheiden.

Furthermore, it is advantageous to use the switchable light filter described above in such a way that

• - in a first operating mode B1, in which the first electric field EF1 is applied, on the one hand linearly or elliptically polarized light (in the case of elliptically polarized light, the ratios of the semi-axes should be at least 1:4, better at least 1:10 or greater), which is incident on the switchable light filter parallel to the first preferred direction, is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident on the switchable light filter at at least a first angle of more than 35° to the first preferred direction, which lies in a first plane, is absorbed to at least 85%, and that
• - in a second operating mode B2, in which the second electric field EF2 is applied, on the one hand linearly or elliptically polarized light which is incident on the switchable light filter parallel to the first preferred direction is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident on the switchable light filter at at least a second angle of more than 35° to the first preferred direction, which lies in a second plane, is absorbed to at least 85%, wherein the first and the second plane intersect at an angle of 80° to 100°, so that the directions of absorption for the first operating mode B1 and the second operating mode B2 differ by 80° to 100° in each case.

The said first preferred direction can advantageously enclose an angle between 0° and 45° inclusive to a surface normal of the optical element. Furthermore, in special embodiments, the first preferred direction can vary across the surface of the optical element. In the sense of the invention, the first preferred direction averaged across the surface then applies.

The transition dipole moment - also known as the transition matrix element - is a quantum mechanical vector quantity and is assigned to a specific transition between an initial state - usually the ground state - and a final state - usually an excited state - of a system, i.e. an atom, molecule or solid, and corresponds macroscopically to the electric dipole moment associated with this transition. The direction of the vector defines the polarization of the transition, which in turn determines how the system interacts with an electromagnetic wave with a given polarization, for example absorbing light of the corresponding polarization direction during the transition from the ground state to the excited state. The magnitude of the vector corresponds to the strength of the interaction or the transition probability.

The first preferred direction corresponds to the orientation of the transition dipole moments for a given direction of propagation of light, where the absorption is the same for any polarization direction of the light.

Materials within the meaning of the invention which contain fixed or - for the second embodiment of switchable light filters described below - alignable transition dipole moments are, for example, dichroic dyes or dye mixtures which are combined with a carrier material that does not impair the properties, for example with liquid crystals or polymers. The optical element can, for example, contain only one dye. However, several dyes, i.e. dye mixtures, can also be contained in a single optical element. The materials mentioned relax essentially through non-radiative processes.

The extinction, i.e. the absorption, of the light depends on the absolute number of transition dipole moments - and thus inherently also on the layer thickness in which the transition dipole moments are located - and the alignment between the transition dipole moment and the polarization of the incident light. Depending on the implementation, the density of the said transition dipole moments, their strength or the refractive index in the layers of the optical element can vary. In a passive, i.e. non-switchable optical element, the volume density of the transition dipoles can approach 100%.

For the simplified modeling of the transmission, it is assumed that the transition dipole moments in the optical element are parallel to the plane of incidence of the light on the optical element are oriented. The plane of incidence does not refer to the surface of the optical element, but rather a plane in which the direction of propagation of the light wave lies, with the surface of the optical element and the plane of incidence forming a right angle. As a transverse wave, the light wave has a transverse-magnetic oscillation component and a transverse-electric oscillation component, both of which are perpendicular to one another and perpendicular to the direction of propagation. Light that hits the optical element is initially unpolarized in its entirety as light waves, i.e. the oscillation directions of the transverse-electric - and accordingly also the transverse-magnetic - components are statistically distributed. When it hits the surface of the optical element with the transition dipole moments aligned perpendicular to the surface of the optical element, the light interacts with the optical element or the transition dipole moments of the material contained therein and is polarized. The oscillation components that lie in the plane of incidence are absorbed. Light that is polarized parallel to the plane of incidence, ie transversely magnetic or p-polarized, is absorbed, whereas light that is polarized perpendicular to the plane of incidence - i.e. parallel to the surface of the optical element - ie transversely electric or s-polarized, is completely transmitted or only absorbed to a significantly reduced extent. Unpolarized light that does not pass through the optical element parallel to transition dipole moments is therefore at least partially s-polarized when it passes through the optical element with transition dipole moments that are aligned parallel to the plane of incidence of the light.

This property is essential in all embodiments of the invention. The transmission of p-polarized light with an incident intensity I _o (a) is modeled below. The transmission of light through an absorbing layer is described by the Lambert-Beer law:
$$\text{I}\left(\mathrm{\alpha }\right)={\text{I}}_0\left(\mathrm{\alpha }\right){\text{e}}^{-\text{d}\left(\mathrm{\alpha }\right)N{\mathrm{\sigma }}_{\text{abs}}\left(\mathrm{\alpha }\right)}.$$

Here, α is the angle between the propagation direction and the surface normal, d(α) is the optical path length depending on the direction of propagation, N is the number of absorbing molecules per volume and α _abs (α) is the absorption cross section depending on the angle of incidence. Using Snell's law of refraction, the propagation angle in the medium α can be calculated from the angle of incidence β. This then results in the equation of the transition dipole moment and the change in the optical path:
$$\text{I}\left(\mathrm{\alpha }\right)={\text{I}}_0\left(\mathrm{\alpha }\right){\text{e}}^{-\text{d}/\text{N }{\mathrm{\sigma }}_{\text{abs}}\text{ sin}{\left(\mathrm{\alpha }\right)}^2}.$$

A first optical element can be formed, for example, as a laminate of layers of polymer film polarizers. Other manufacturing variants and material designs are of course possible. Alternatively or in combination, the optical element can also be produced by photo-alignment of molecules or particles.

• - ein erstes optisches Element, welches
  • - eine Vielzahl an Licht absorbierenden Übergangsdipolmomenten, die in Form von Molekülen einer und/oder in Form eines oder mehrerer dichroitischer Farbstoffe ausgebildet sind, umfasst,
  • - wobei die Mehrzahl der Übergangsdipolmomente mindestens in einem ersten Zustand mit einer Toleranz von maximal 20° (bevorzugt maximal 10°) parallel zu einer für das erste optische Element wählbaren ersten Vorzugsrichtung ausgerichtet ist oder um diese herum variiert,
  • - so dass Licht, welches in das erste optische Element einfällt, in Abhängigkeit von seinem Polarisationszustand und von seiner Einfallsrichtung gegenüber dem ersten optischen Element und transmittiert oder mindestens teilweise absorbiert wird,
  • - wobei die Übergangsdipolmomente in eine Flüssigkristallschicht eingebettet sind, so dass die Übergangsdipolmomente in ihrer Ausrichtung und/oder ihrem Betrag zwischen dem ersten und mindestens einem zweiten Zustand variiert werden können, um das erste optische Element alternativ in mindestens zwei verschiedene Zustände versetzen zu können,
• - Mittel zur wahlweisen Erzeugung mindestens eines ersten elektrischen Feldes EF1 oder eines zweiten elektrischen Feldes EF2, wobei für das erste optische Element der erste Zustand durch Anlegen des ersten elektrischen Feldes EF1 und der zweite Zustand durch Anlegen des zweiten elektrischen Feldes EF2 erzeugt wird, so dass sich die Transmissionseigenschaften des schaltbaren Lichtfilters 5 zwischen einer ersten Betriebsart B1, in welcher das erste elektrische Feld EF1 anliegt, und einer zweiten Betriebsart B2, in welcher das erste elektrische Feld EF2 anliegt, unterscheiden,
• - wobei erfindungsgemäß mindestens eine der folgenden Maßnahmen im schaltbaren Lichtfilter der zweiten Ausgestaltung umgesetzt ist:
  • - die Flüssigkristallschicht umfasst mindestens eine TN-Zelle sowie einen Verzögerungsfilm, wobei der besagte Verzögerungsfilm die in der ersten Betriebsart B1 durch die TN-Zelle durchgeführte Polarisationsänderung für den schaltbaren Lichtfilter 5a durchdringendes Licht zu mindestens 80% aufhebt (bevorzugt sogar zu 90%, 95% oder mehr; dies kann z.B. mittels eines discotischen Films realisiert werden), oder
  • - die Flüssigkristallschicht 3a, wenn sie beispielweise als mittels PA-, FFS-, IPS-, VA-, ADS- (oder vergleichbarer) Technologie realisiert wird, weist in der ersten Betriebsart B1 ihr Transmissionsmaximum in einem Winkel, der zwischen 20° und 80° zu Flächennormalen der Flüssigkristallschicht liegt, auf, so dass die Beeinflussung der Polarisation des den schaltbaren Lichtfilter durchdringenden Lichtes aufgrund der Flüssigkristallschicht für unter dem besagten Winkel, der zwischen 20° und 80° zu Flächennormalen der Flüssigkristallschicht liegt, einfallende Lichtstrahlen maximal ist, wodurch die Beeinflussung der winkelabhängigen Transmission durch das optische Element verstärkt wird, und/oder
  • - der schaltbare Lichtfilter umfasst weiterhin ein negativ dispersives optisches Element (z.B. einen negativ dispersiven Verzögerungsfilm), oder mehrere optische Elemente, die in der Zusammenwirkung wie ein negativ dispersives optisches Element wirken (dies kann erreicht werden durch die Kombination von verschiedenen Elementen mit unterschiedlich hoher Dispersion), so dass Farbverschiebungen des den schaltbaren Lichtfilter durchdringenden Lichtes (insbesondere im Fall von weißem oder buntem Licht) mindestens teilweise kompensiert werden.

The object of the invention is further achieved by a second embodiment of a switchable light filter, comprising

• - a first optical element which
  • - a plurality of light-absorbing transition dipole moments which are formed in the form of molecules of one and/or in the form of one or more dichroic dyes,
  • - wherein the majority of the transition dipole moments are aligned parallel to or vary around a first preferred direction selectable for the first optical element, at least in a first state, with a tolerance of a maximum of 20° (preferably a maximum of 10°),
  • - so that light which is incident on the first optical element is transmitted or at least partially absorbed depending on its polarization state and its direction of incidence relative to the first optical element,
  • - wherein the transition dipole moments are embedded in a liquid crystal layer, so that the transition dipole moments can be varied in their orientation and/or their amount between the first and at least one second state in order to be able to alternatively place the first optical element into at least two different states,
• - Means for selectively generating at least a first electric field EF1 or a second electric field EF2, wherein for the first optical element the first state is generated by applying the first electric field EF1 and the second state is generated by applying the second electric field EF2, so that the transmission properties of the switchable light filter 5 can be switched between a first operating mode B1, in which the first electric field EF1 is applied, and a second Operating mode B2, in which the first electric field EF2 is applied,
• - wherein according to the invention at least one of the following measures is implemented in the switchable light filter of the second embodiment:
  • - the liquid crystal layer comprises at least one TN cell and a retarder film, wherein said retarder film cancels the polarization change carried out by the TN cell in the first operating mode B1 for light penetrating the switchable light filter 5a by at least 80% (preferably even by 90%, 95% or more; this can be realized e.g. by means of a discotic film), or
  • - the liquid crystal layer 3a, if it is realized, for example, using PA, FFS, IPS, VA, ADS (or comparable) technology, has its transmission maximum in the first operating mode B1 at an angle that lies between 20° and 80° to the surface normal of the liquid crystal layer, so that the influence of the polarization of the light penetrating the switchable light filter due to the liquid crystal layer is maximum for light rays incident at the said angle, which lies between 20° and 80° to the surface normal of the liquid crystal layer, whereby the influence of the angle-dependent transmission by the optical element is increased, and/or
  • - the switchable light filter further comprises a negatively dispersive optical element (e.g. a negatively dispersive retardation film), or several optical elements which, when combined, act like a negatively dispersive optical element (this can be achieved by combining different elements with different levels of dispersion), so that color shifts of the light penetrating the switchable light filter (in particular in the case of white or colored light) are at least partially compensated.

The transition dipole moments can - as described above - be formed as one or more dichroic dye(s), i.e. by such dye molecules, which are integrated in a guest-host arrangement in liquid crystals of the liquid crystal layer. The dye molecules in turn then align themselves parallel to the liquid crystal molecules of the liquid crystal layer. The alignment of the liquid crystal molecules in the voltage-free electric field is in turn induced by the surfaces embedding them. One possible material for such surfaces is PMI.

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• C. Carfagna, E. Amendola, and M. Giamberini, „Liquid crystalline epoxy based thermosetting polymers,“ Progress in Polymer Science, vol. 22, pp. 1607-1647, 1997 .
• V. P. Shibaev and A. Yu. Bobrovsky, „Liquid crystalline polymers: development trends and photocontrollable materials,“ Russian Chemical Reviews, vol. 86, pp. 1024-1072, 2017 .
• T. Ikeda, J.-i. Mamiya, and Y. Yu, „Photomechanics of liquid-crystalline elastomers and other polymers,“ Angew. Chem. Int. Ed., vol. 46, pp. 506-528, 2007 .

In general, this can be referred to as liquid crystal polymers doped with dye(s). Various approaches are known in the art (e.g. thermosettling LCs and epoxy LC), which therefore do not need to be discussed in detail. The following publications are examples in this context:

• N. Saba, M. Jawaid, OY Alothman, MT Paridah, and A. Hassan, "Recent advances in epoxy resin, natural fiber-reinforced epoxy composites and their applications," Journal of Reinforced Plasticsand Composites, vol. 35, no. 6, 2015 .
• C. Carfagna, E. Amendola, and M. Giamberini, “Liquid crystalline epoxy based thermosetting polymers,” Progress in Polymer Science, vol. 22, pp. 1607-1647, 1997 .
• VP Shibaev and A. Yu. Bobrovsky, “Liquid crystalline polymers: development trends and photocontrollable materials,” Russian Chemical Reviews, vol. 86, pp. 1024-1072, 2017 .
• T. Ikeda, J.-i. Mamiya, and Y. Yu, “Photomechanics of liquid-crystalline elastomers and other polymers,” Angew. Chem. Int. Ed., vol. 46, pp. 506-528, 2007 .

The at least one dye consists of dye molecules, wherein a transition dipole or transition dipole moment is advantageously associated with each dye molecule, i.e. each dye molecule corresponds to a transition dipole or transition dipole moment. Typically, a dye has a mass fraction of 0.01% to 10% (or possibly significantly more), preferably 0.1% to 5% of the material of the respective layer(s) in the first optical element. The thickness of the layers is preferably in the range from 0.2 µm to 50 µm, preferably in the range from 0.5 µm to 20 µm, all marginal values included. The dyes or dye mixtures in the first optical element can also be formed differently in its different layers, if present.

A preferred embodiment for each layer is a mixture of liquid crystals with at least one dye, especially with at least one dichroic dye mixture. Possible dichroic dyes or dye mixtures are, for example, azomethine dyes, indigoid and thioindigoid dyes, merocyanines, azulene, quinophthalone dyes, perylene dyes, phthaloperine dyes, dioxazine dyes, triphenodioxazine dyes, quinoxaline dyes, triazine dyes, tartrazine, azo dyes and anthraquinone dyes. The production of a liquid crystal dye mixture is described, for example, in the US 4,695,131 A. Furthermore, the surfaces that close the layers on the outside preferably treated, for example brushed, to achieve a homogeneous surface alignment of the transition dipole moments or, if present, the liquid crystals.

In a preferred embodiment, a polarization filter P is also present, which can be arranged upstream or downstream of the first optical element in the direction of incidence. Alternatively, a λ/4 retardation layer can also be present in order to generate linearly polarized light from circularly polarized light, depending on the properties of the light originating from a corresponding light source.

In order to switch the optical effect of the switchable light filter according to the invention, in this second embodiment of the invention the transition dipole moments in each layer are varied in their orientation and/or their amount between the first and at least one second state in order to be able to alternatively place the respective layer into at least two different states. Possible embodiments of a switchable optical element or each layer therein are based, for example, on liquid crystals and/or dyes or dye mixtures, which can be arranged in the liquid crystal cell with a homogeneous orientation on the surfaces and can be rotated therein between at least two states. The light-absorbing transition dipole moments are also rotated and can therefore assume at least two states of action. In particular in such embodiments it is conceivable that more than two states, e.g. three or eight states, each with different optical effects, are achieved. Other embodiments of the liquid crystal cells are also conceivable. For this purpose, electric fields are used in particular to rotate the liquid crystals. It is possible, for example, that either a first electric field or a second electric field describes a field-free state, whereby the other electric field has an absolute field strength greater than zero, e.g. 0.5 MV/m.

A first such state corresponds to the conditions described above and at least a second state is different from this, i.e. has at least one other preferred direction.

In such active, i.e. switchable, light filters, volume densities of the transition dipole moments between 0.1% and 90% based on liquid crystals in the optical element are conceivable. Alternatively, designs of an optical element or each layer therein are conceivable in which the transition dipole moments are embedded in a liquid that is subjected to an electro-wetting process. In this way, in particular, but not only, the density of the transition dipole moments can be varied.

• - in einer ersten Betriebsart B1, in welcher das erste elektrische Feld EF1 anliegt, einerseits linear oder elliptisch polarisiertes Licht (im Falle von elliptisch polarisiertem Licht sollten die Verhältnisse der Beträge der Halbachsen mindestens 1:4 betragen, besser mindestens 1:10 oder größer), welches parallel zur ersten Vorzugsrichtung in den schaltbaren Lichtfilter einfällt, zu mindestens 24% transmittiert und andererseits linear oder elliptisch polarisiertes Licht, welches in mindestens einem ersten Winkel von mehr als 35° zu der ersten Vorzugsrichtung, der in einer ersten Ebene liegt, in den schaltbaren Lichtfilter einfällt, zu mindestens 85% absorbiert wird, und dass
• - in einer zweiten Betriebsart B2, in welcher das zweite elektrische Feld EF2 anliegt, einerseits linear oder elliptisch polarisiertes Licht, welches parallel zu der ersten Vorzugsrichtung in den schaltbaren Lichtfilters einfällt, zu mindestens 24% transmittiert und andererseits linear oder elliptisch polarisiertes Licht, welches in mindestens einem zweiten Winkel von mehr als 35° zu der ersten Vorzugsrichtung, der in einer zweiten Ebene liegt, in den schaltbaren Lichtfilter einfällt, zu mindestens 85% absorbiert wird, wobei die erste und die zweite Ebene sich in einem Winkel von 80° bis 100° schneiden, so dass sich die Richtungen der Absorption für die erste Betriebsart B1 und die zweite Betriebsart B2 um jeweils 80° bis 100° unterscheiden.

Furthermore, in this second embodiment of the switchable light filter it is also advantageous to use it in such a way that

• - in a first operating mode B1, in which the first electric field EF1 is applied, on the one hand linearly or elliptically polarized light (in the case of elliptically polarized light, the ratios of the amounts of the semi-axes should be at least 1:4, better at least 1:10 or greater), which is incident on the switchable light filter parallel to the first preferred direction, is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident on the switchable light filter at at least a first angle of more than 35° to the first preferred direction, which lies in a first plane, is absorbed to at least 85%, and that
• - in a second operating mode B2, in which the second electric field EF2 is applied, on the one hand linearly or elliptically polarized light which is incident on the switchable light filter parallel to the first preferred direction is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident on the switchable light filter at at least a second angle of more than 35° to the first preferred direction, which lies in a second plane, is absorbed to at least 85%, wherein the first and the second plane intersect at an angle of 80° to 100°, so that the directions of absorption for the first operating mode B1 and the second operating mode B2 differ by 80° to 100° in each case.

For special applications, the switchable light filter, i.e. in particular the liquid crystal layer and/or the means for selectively generating at least a first electric field EF1 or a second electric field EF2 - regardless of which of the aforementioned embodiments - can be divided into several separately switchable segments, so that local switching between the respective possible operating states is possible. In conjunction with an image display unit, this would mean that, for example, only part of the image surface can be switched between a private mode for privacy protection and a public mode with no privacy protection effect, i.e. for a clear view, while the complementary part of the image surface is permanently in a privacy protection mode or in no privacy protection mode.

Furthermore, it is possible that the means for selectively generating at least a first electric field EF1 or a second electric field EF2 can selectively generate further electric fields EF3, EF4, ..., so that third, fourth and possibly further states can be generated for the liquid crystal layer, which then in turn cause third, fourth and possibly further absorption or transmission characteristics of the switchable light filter.

Furthermore, the first design of a switchable light filter can be expanded to include a second optical element that is identical to the first optical element (or has a different preferred direction) and between which the liquid crystal layer is located. This makes it possible to optionally switch angle restrictions in the transmission between one and two directions.

In an analogous manner, the second design of a switchable light filter can also be supplemented by a second liquid crystal layer that is identical in construction to the (first) liquid crystal layer. This makes it possible, for example, to optionally switch angle restrictions in the transmission between no, one and two directions.

Ideally, the first optical element (and if present, every other identical optical element) in the first design of the switchable light filter or the liquid crystal layer in the second design of the switchable light filter is non-periodic in its structure. This is inherently achieved when these components are produced as described above and is helpful for avoiding undesirable optical effects, such as moiré effects, in interaction with image sensors, for example.

For the first optical elements of the first and second embodiments, it may further apply that the respective preferred direction of a transition dipole moment can be selected depending on its position on the respective optical element.

In this context, it can be particularly useful for each first optical element to be divided into different areas (A1, A2, ...) along a selectable reference line on the respective first optical element, whereby a separate area preferred direction can be selected for each area (A1, A2, ...), which applies to all transition dipole moments of the corresponding first optical element within an area (A1, A2, ...), whereby all area preferred directions are different in pairs and point in the direction of an observer up to a tolerance of a maximum of +/-10 degrees. Within an optical element and within each area applicable to it, all transition dipole moments are therefore aligned parallel to the preferred direction applicable there with a tolerance of a maximum of +/-10°.

• - eine flächenartig ausgedehnte Hintergrundbeleuchtung, die einen schaltbaren Lichtfilter der ersten oder zweiten Ausgestaltung enthält, und Licht abstrahlt,
• - einen in Betrachtungsrichtung vor der Hintergrundbeleuchtung gelegenen, plattenförmigen Lichtleiter, welcher auf mindestens einer der Großflächen und / oder innerhalb seines Volumens Auskoppelelemente aufweist,
• - seitlich an mindestens einer Schmalseite des Lichtleiters angeordnete Leuchtmittel, und
• - einen in Betrachtungsrichtung vor der Hintergrundbeleuchtung oder vor dem Lichtleiter angeordneten linearen Polarisationsfilter, wodurch Licht, welches von der Hintergrundbeleuchtung ausgeht und den linearen Polarisationsfilter durchdringt, in seinen Ausbreitungsrichtungen eingeschränkt wird,
• - wobei in der Betriebsart BE2 die Hintergrundbeleuchtung ein- und die Leuchtmittel ausgeschaltet sind, und wobei in der Betriebsart BE1 mindestens die Leuchtmittel eingeschaltet sind.

The invention can also be advantageously used in a lighting device for a screen, which can be operated in at least two operating modes BE1 for a free viewing mode and BE2 for a restricted viewing mode, in which light is emitted in an angular range that is restricted compared to the free viewing mode. This comprises

• - a surface-like backlight which contains a switchable light filter of the first or second embodiment and emits light,
• - a plate-shaped light guide located in front of the background lighting in the viewing direction, which has output coupling elements on at least one of the large surfaces and/or within its volume,
• - illuminants arranged laterally on at least one narrow side of the light guide, and
• - a linear polarisation filter arranged in front of the backlight or in front of the light guide in the viewing direction, whereby light emanating from the backlight and penetrating the linear polarisation filter is restricted in its propagation directions,
• - whereby in operating mode BE2 the backlight is on and the lamps are off, and whereby in operating mode BE1 at least the lamps are switched on.

In this lighting device, the operating modes BE1 and BE2 are then correlated accordingly to the operating modes B2 and B1 of the switchable light filter installed therein.

The invention is particularly important when a switchable light filter as described above is combined with an image display unit to form a screen. Such a screen comprises, in addition to a switchable light filter as described above, an image display unit arranged upstream or downstream of the switchable light filter as seen by a viewer. The operating modes of the switchable light filter described above can be easily transferred to the screen, so that the screen can also be operated in the various operating modes already mentioned above, depending on the design of the switchable light filter used, for example at least in a first operating mode for a horizontally free, public viewing mode with unrestricted radiation or viewing angle range and a horizontally restricted private viewing mode with a horizontally restricted radiation or viewing angle range compared to the free viewing mode, so that viewers positioned outside this restricted viewing angle range can only perceive image content displayed on the screen in the free viewing mode.

The image display unit advantageously corresponds to an LCD panel, one of whose polarization filters corresponds to the polarization filter of the switchable light filter. This can be the front or rear polarizer in the LCD structure. In addition, the switchable light filter can advantageously be arranged between the LCD panel and its backlight in order to switch between a first operating state for a free viewing mode and a second operating state for a restricted viewing mode, because the light from the backlight is sometimes focused and sometimes not focused in the horizontal direction due to the switchable light filter - for example when switching in the horizontal direction. "Focusing" does not mean focusing in the manner of lenses, but rather a narrowing of the radiation range or transmission range across the angles.

• - eine flächenartig ausgedehnte Hintergrundbeleuchtung, die einen schaltbaren Lichtfilter der ersten oder der zweiten Ausgestaltung enthält, und Licht abstrahlt, und die optional direkt leuchtend aufgebaut ist,
• - einen in Betrachtungsrichtung vor der Hintergrundbeleuchtung angeordneten linearen Polarisationsfilter, wodurch Licht, welches von der Hintergrundbeleuchtung ausgeht und den linearen Polarisationsfilter durchdringt, in seinen Ausbreitungsrichtungen eingeschränkt wird, und
• - eine transmissive Bildwiedergabeeinrichtung (z.B. ein LCD-Panel), welche in Betrachtungsrichtung vor dem Lichtleiter angeordnet ist, und nahe derer oder bevorzugt in welcher der lineare Polarisationsfilter angeordnet ist,
• - wobei in der Betriebsart BB2 das zweite elektrische Feld EF2 und wobei in der Betriebsart BB1 das erste elektrische Feld EF1 anliegt.

In this sense, the invention can be used in a first screen that can be operated in at least two operating modes BB1 for a free viewing mode and BB2 for a restricted viewing mode, in which light is emitted into a viewing angle range that is restricted for a viewer compared to the free viewing mode. This comprises

• - a surface-like extended background lighting, which contains a switchable light filter of the first or second embodiment and emits light, and which is optionally designed to be directly luminous,
• - a linear polarization filter arranged in front of the backlight in the viewing direction, whereby light emanating from the backlight and penetrating the linear polarization filter is restricted in its propagation directions, and
• - a transmissive image display device (e.g. an LCD panel) which is arranged in front of the light guide in the viewing direction and near which or preferably in which the linear polarization filter is arranged,
• - where in the operating mode BB2 the second electric field EF2 is applied and where in the operating mode BB1 the first electric field EF1 is applied.

Even with such a screen, it is possible that the linear polarization filter is arranged in the transmissive image display device or is a part of it.

• - eine Bildwiedergabeeinrichtung,
• - in Betrachtungsrichtung vor der Bildwiedergabeeinrichtung einen schaltbaren Lichtfilter der ersten oder der zweiten Ausgestaltung,
• - wobei in der Betriebsart BB2 das zweite elektrische Feld EF2 und wobei in der Betriebsart BB1 das erste elektrische Feld EF1 anliegt.

The invention also finds application in a second screen which can be operated in at least two operating modes BB1 for a free viewing mode and BB2 for a restricted viewing mode in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free viewing mode. This comprises

• - an image display device,
• - in the viewing direction in front of the image display device, a switchable light filter of the first or second embodiment,
• - where in the operating mode BB2 the second electric field EF2 is applied and where in the operating mode BB1 the first electric field EF1 is applied.

The image display unit can alternatively be an OLED, an SED screen, a field emission screen (FED), a microLED panel or a vacuum fluorescent display (VFD) in front of which a switchable light filter is arranged. Since the switchable light filter is effective regardless of the type of image display unit, any other type of screen can also be used. Such a screen is advantageously used in a mobile device, a motor vehicle, aircraft or water vehicle, in a payment terminal or in an access system. It is possible to switch between the above-mentioned operating modes in order to protect sensitive data, i.e. to display it so that it is perceptible to only one viewer, or alternatively to display image content simultaneously for several viewers.

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

It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or on their own, without departing from the scope of the present invention.

Short description of the drawings

• 1 die Prinzipskizze eines beispielhaften schaltbaren Lichtfilters,
• 2a und 2b die Simulationen einer beispielhaften, gedrehten nematischen Flüssigkristallzelle, sowie
• 3a und 3b die beispielhafte (simulierte) Dispersion von Halbwellenplatten.

The invention is described below using embodiments with reference to the accompanying drawings, which also essential features are explained in more detail. These embodiments are for illustrative purposes only and are not to be interpreted as restrictive. For example, a description of an embodiment with a large number of elements or components is not to be interpreted as meaning that all of these elements or components are necessary for implementation. Rather, other embodiments can also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different embodiments can be combined with one another unless otherwise stated. Modifications and variations described for one of the embodiments can also be applicable to other embodiments. To avoid repetition, identical or corresponding elements in different figures are designated with the same reference numerals and are not explained more than once. They show:

• 1 the schematic diagram of an exemplary switchable light filter,
• 2a and 2b the simulations of an exemplary twisted nematic liquid crystal cell, as well as
• 3a and 3b the exemplary (simulated) dispersion of half-wave plates.

Detailed description of the drawings

The drawings are not to scale and merely represent schematic diagrams.

• - ein erstes optisches Element 1, seinerseits umfassend
  • - eine Vielzahl an Licht absorbierenden Übergangsdipolmomenten,
  • - wobei die Mehrzahl (bevorzugt sogar mehr als 95%) der Übergangsdipolmomente (permanent) mit einer Toleranz von maximal 20° (bevorzugt maximal 10°) parallel zu einer für das erste optische Element 1 wählbaren ersten Vorzugsrichtung ausgerichtet ist oder um diese herum variiert,
  • - so dass Licht, welches in das erste optische Element 1 einfällt, in Abhängigkeit von seinem Polarisationszustand und von seiner Einfallsrichtung gegenüber dem ersten optischen Element 1 transmittiert oder mindestens teilweise absorbiert wird,
• - (zeichnerisch nicht dargestellte) Mittel zur wahlweisen Erzeugung eines ersten elektrischen Feldes EF1 oder eines zweiten elektrischen Feldes EF2, z.B. ITO-Schichten, welche mit einem Signalgenerator verbunden sind,
• - eine vor oder hinter dem ersten optischen Element 1 angeordnete Flüssigkristallschicht 3, auf welche das erste elektrische Feld EF1 oder das zweite elektrische Feld EF2 wirkt und die in Abhängigkeit davon den Polarisationszustand von durch sie hindurchdringendem Licht beeinflusst, so dass
• - sich die Transmissionseigenschaften des schaltbaren Lichtfilters 5 zwischen einer ersten Betriebsart B1, in welcher das erste elektrische Feld EF1 anliegt, und einer zweiten Betriebsart B2, in welcher das erste elektrische Feld EF2 anliegt, unterscheiden,
• - wobei mindestens eine der folgenden Maßnahmen im schaltbaren Lichtfilter 5 umgesetzt ist:
  • - die Flüssigkristallschicht 3 umfasst mindestens eine TN-Zelle sowie einen (zeichnerisch nicht dargestellten) Verzögerungsfilm, wobei der besagte Verzögerungsfilm die in der ersten Betriebsart B1 durch die TN-Zelle durchgeführte Polarisationsänderung für den schaltbaren Lichtfilter 5 durchdringendes Licht zu mindestens 90% aufhebt (ideal sogar zu 95%, 99% oder mehr; dies kann z.B. mittels eines discotischen Films realisiert werden), oder
  • - die Flüssigkristallschicht 3, wenn sie beispielweise mittels PA, FFS-, IPS-, VA-ADS- (oder vergleichbarer) Technologie realisiert wird, weist in der ersten Betriebsart B1 ihr Transmissionsmaximum in einem Winkel, der zwischen 20° und 80° zu Flächennormalen der Flüssigkristallschicht 3 liegt, auf, so dass die Beeinflussung der Polarisation des den schaltbaren Lichtfilter 5 durchdringenden Lichtes aufgrund der Flüssigkristallschicht 3 für unter dem besagten Winkel, der zwischen 20° und 80° zu Flächennormalen der Flüssigkristallschicht 3 liegt, einfallende Lichtstrahlen maximal ist, wodurch die Beeinflussung der winkelabhängigen Transmission durch das optische Element 1 verstärkt wird, und/oder
  • - der schaltbare Lichtfilter 5 umfasst weiterhin ein negativ dispersives optisches Element (z.B. einen negativ dispersiven Verzögerungsfilm), oder mehrere optische Elemente, die in der Zusammenwirkung wie ein negativ dispersives optisches Element wirken (dies kann erreicht werden durch die Kombination von verschiedenen Elementen mit unterschiedlich hoher Dispersion), so dass Farbverschiebungen des den schaltbaren Lichtfilter 5 durchdringenden Lichtes (insbesondere im Fall von weißem oder polychrome Licht) mindestens teilweise kompensiert werden.

1a shows the schematic diagram of an exemplary switchable light filter 5. In a first embodiment, this switchable light filter 5 comprises

• - a first optical element 1, itself comprising
  • - a multitude of light-absorbing transition dipole moments,
  • - wherein the majority (preferably even more than 95%) of the transition dipole moments are (permanently) aligned with a tolerance of a maximum of 20° (preferably a maximum of 10°) parallel to a first preferred direction selectable for the first optical element 1 or vary around this,
  • - so that light which is incident on the first optical element 1 is transmitted or at least partially absorbed depending on its polarization state and its direction of incidence relative to the first optical element 1,
• - (not shown in the drawing) means for selectively generating a first electric field EF1 or a second electric field EF2, e.g. ITO layers, which are connected to a signal generator,
• - a liquid crystal layer 3 arranged in front of or behind the first optical element 1, on which the first electric field EF1 or the second electric field EF2 acts and which, depending thereon, influences the polarization state of light passing through it, so that
• - the transmission properties of the switchable light filter 5 differ between a first operating mode B1, in which the first electric field EF1 is applied, and a second operating mode B2, in which the first electric field EF2 is applied,
• - wherein at least one of the following measures is implemented in the switchable light filter 5:
  • - the liquid crystal layer 3 comprises at least one TN cell and a retarder film (not shown in the drawing), wherein the said retarder film cancels the polarization change carried out by the TN cell in the first operating mode B1 for light penetrating the switchable light filter 5 by at least 90% (ideally even by 95%, 99% or more; this can be realized e.g. by means of a discotic film), or
  • - the liquid crystal layer 3, if it is realized for example by means of PA, FFS, IPS, VA-ADS (or comparable) technology, has its transmission maximum in the first operating mode B1 at an angle that lies between 20° and 80° to the surface normal of the liquid crystal layer 3, so that the influence of the polarization of the light penetrating the switchable light filter 5 due to the liquid crystal layer 3 is maximum for light rays incident at the said angle, which lies between 20° and 80° to the surface normal of the liquid crystal layer 3, whereby the influence of the angle-dependent transmission by the optical element 1 is increased, and/or
  • - the switchable light filter 5 further comprises a negatively dispersive optical element (eg a negatively dispersive retardation film), or several optical elements which act together like a negatively dispersive optical element (this can be achieved by combining different elements with different levels of dispersion), so that color shifts of the light penetrating the switchable light filter 5 (in particular in the case of white or polychrome light) can be at least partially compensated.

In a preferred embodiment, a polarization filter is also present, which can be arranged upstream or downstream of the first optical element 1 in the direction of incidence.

The way the aforementioned measures work is explained in more detail below: In order to achieve the best possible privacy protection, the electric field in the restricted-view mode must oscillate as much as possible in the plane of incidence of the light. One way to make deviations from linear polarization visible and measurable is to align a polarizer perpendicular to the desired polarization. The lower the transmission after the vertical polarizer, the better the privacy protection with the optical element.

Nematically twisted liquid crystal cells located between two crossed polarizers have the disadvantage that they have a high transmission in the black state when viewed from a non-perpendicular angle. The lower the transmission, the stronger the degree of linear polarization. This high black transmission and thus the deviation from linear polarization is caused by incomplete alignment of the liquid crystals by the electric field near the interfaces. The polarization deviations can be minimized using discotic liquid crystal layers, among other things. The layers mentioned are polymerized and the LC molecules do not change their orientation. The reduction of the polarization change follows the idea that in the field-affected state, the change in polarization by the LC cell is reversed by means of the discotic LC film. The LC cell in the field-affected state and the discotic LC film have the same amount of birefringence with different signs. The switchable light filter described requires polarization states that are as pure as possible to function optimally.

The drawings provide 2a and 2b the simulations of a twisted nematic liquid crystal cell. For these simulations, a parallel cell geometry with a thickness of 5 µm was assumed, ie the liquid crystal molecules are limited in their extension by two plates. For the simulations, it was also assumed that the LC molecules have positive birefringence and that the polarity of the LC molecules is oriented parallel to their long axis.

The two drawings 2a and 2b show the orientation of the liquid crystal molecules of a 90° exemplary rotated nematic liquid crystal cell in the field-free (solid line) and field-affected state (dashed line), where 2a the angle of rotation and 2b represents the angle of inclination. The orientation of the liquid crystal molecules is determined in the field-free state (solid line) only by the interfaces. The orientation of the LC molecules is shown in spherical coordinates. The azimuthal angle describes the orientation in the projection onto the interfaces and the polar angle the inclination of the long molecular axis to the interfaces. The molecules are aligned almost parallel to the surface with a so-called "pretilt" angle of 2°, for example, i.e. the molecules are inclined 2° to the surface. The orientation in the plane differs by 90° between the two interfaces, which is achieved by targeted surface modifications of the interfaces. The pretilt angle of the molecules is determined, for example, by the processing of the orientation layer and its surface properties, e.g. brushing. The pretilt angle ensures that all molecules rotate in the same direction as soon as an electric field is applied. The angle of rotation describes the angle of the liquid crystal cell (see 2a) in the plane of the interfaces. The inclination angle (see 2b) describes the angle of the liquid crystal molecules to the first interface.

In the field-free state, the angle of rotation changes proportionally with the distance from the lower interface. The polar angle is equal to the pretilt angle and constant over the entire layer thickness. From a physical point of view, the order is induced by the interaction between the interface and the LC molecules located in its vicinity. Molecule-molecule interactions align the molecules throughout the layer.

If an electric field is applied (e.g. the second electric field EF2), the high polarity of the LC molecules along the long axis creates a force that is directed in the direction of the field. The orientation of the molecules is determined by the balance of the intramolecular interaction and the electric field. For the calculation, the free energy is minimized. In the example shown, the maximum angle of inclination, which is in the middle of the cell, is 88°. The angle of inclination increases from the interfaces to the middle and has the maximum value there. The angle of rotation (see 2a) initially changes slightly and then abruptly rotates from 0° to 90°.

• „ Optics of Liquid Crystal Displays“ von Pochi Yeh und Claire Gu, John Wiley & Sons, 2009 und
• „ Deformation of Nematic Liquid Crystals in an Electric Field“ von Heinz J. Deuling, Pages 123-131 | Received 02 Feb 1972, Published online: 28 Mar 2007 sowie
• „ Deformation Pattern of Twisted Nematic Liquid Crystal Layers in an Electric Field“ von H. J. Deuling, Pages 81-93 | Received 30 Aug 1973, Published online: 21 Mar 2007 .

For local orientation of LC molecules, please refer to the following references:

• " Optics of Liquid Crystal Displays” by Pochi Yeh and Claire Gu, John Wiley & Sons, 2009 and
• " Deformation of Nematic Liquid Crystals in an Electric Field” by Heinz J. Deuling, Pages 123-131 | Received 02 Feb 1972, Published online: 28 Mar 2007 as well as
• " Deformation Pattern of Twisted Nematic Liquid Crystal Layers in an Electric Field” by HJ Deuling, Pages 81-93 | Received 30 Aug 1973, Published online: 21 Mar 2007 .

In contrast, the liquid crystal cell types ADS, FFS and IPS have significantly lower transmissions (i.e. better black values) compared to TN displays, which means that the polarization state of the light after the LC cell deviates only slightly from linear polarization. In terms of concept, FFS, IPS and ADS liquid crystal cell geometries are essentially the same. In the field-free state, the linearly polarized light is transmitted essentially without any change in polarization. In the field-affected state, the liquid crystal molecules are rotated as exclusively as possible in planes parallel to the surface. Due to the arrangement of the electrodes, the rotation of the LC molecules is of varying strength. This changes the polarization of the light to varying degrees. Typically, such LC cells are optimized for maximum transmission under perpendicular incidence. For the present case, however, it is advantageous if the maximum polarization change is achieved for non-perpendicular cases. This can be achieved, for example, by increasing the cell thickness, the birefringence of the LC molecules or by increasing the electric fields. It is known in the state of the art that performance can be improved by using passive retardation plates. In vertically aligned LC cells, performance is improved by retardation plates with negative birefringence reversing the birefringence of the LC cell. The retardation plate ensures that the light is transmitted as linearly as possible.

The illustrations 3a and 3b serve to explain the case where the switchable light filter further comprises a negatively dispersive optical element (e.g. a negatively dispersive retardation film), or several optical elements which, when combined, act like a negatively dispersive optical element (this can be achieved by combining different elements with different levels of dispersion), so that color shifts of the light penetrating the switchable light filter (in particular in the case of white or polychrome light) are at least partially compensated.

• • Keine Dispersion: Die Brechzahldifferenz zwischen ordentlicher und außerordentlicher Brechzahl ist unabhängig von der Wellenlänge,
• • Positive Dispersion: Die Brechzahldifferenz zwischen ordentlicher und außerordentlicher Brechzahl wird größer mit kleiner werdender Wellenlänge,
• • Negative Dispersion: Die Brechzahldifferenz zwischen ordentlicher und außerordentlicher Brechzahl wird kleiner mit kleiner werdender Wellenlänge,
• • Ideale negative Dispersion: Die Brechzahldifferenz ordentlicher und außerordentlicher Brechzahl ist so angepasst, dass die Phasendifferenz zwischen den zwei linearen Polarisationen konstant für alle Wellenlängen konstant ist.

Said images 3a and 3b show the (simulated) dispersion of a half-wave plate in units of phase delay ( 3a) and the optical path length difference ( 3b) The four possible cases are shown.

• • No dispersion: The difference in refractive index between ordinary and extraordinary refractive index is independent of the wavelength,
• • Positive dispersion: The difference in refractive index between ordinary and extraordinary refractive index increases with decreasing wavelength,
• • Negative dispersion: The difference in refractive index between ordinary and extraordinary refractive index becomes smaller with decreasing wavelength,
• • Ideal negative dispersion: The difference in refractive index between ordinary and extraordinary refractive indices is adjusted so that the phase difference between the two linear polarizations is constant for all wavelengths.

For those in 3a shown phase delay applies $\frac{2\cdot \mathrm{\pi }\cdot \text{d}\cdot \left({\text{n}}_{\text{e}}-{\text{n}}_{\text{o}}\right)}{\mathrm{\lambda }}$

For those in 3b shown path length difference applies: $\text{d}\cdot \left({\text{n}}_{\text{e}}-{\text{n}}_{\text{o}}\right)$

In both cases, d stands for the respective thickness of the half-wave plate, n _e for the extraordinary refractive index and n _o for the ordinary refractive index, and λ for the respective wavelength.

• - in einer ersten Betriebsart B1, in welcher das erste elektrische Feld EF1 anliegt, einerseits linear oder elliptisch polarisiertes Licht (im Falle von elliptisch polarisiertem Licht sollten die Verhältnisse der Beträge Halbachsen mindestens 1:4 betragen, besser mindestens 1:10 oder größer), welches parallel zur ersten Vorzugsrichtung in den schaltbaren Lichtfilter 5 einfällt, zu mindestens 24% transmittiert und andererseits linear oder elliptisch polarisiertes Licht, welches in mindestens einem ersten Winkel von mehr als 35° zu der ersten Vorzugsrichtung, der in einer ersten Ebene liegt, in den schaltbaren Lichtfilter 5 einfällt, zu mindestens 85% absorbiert wird, und dass
• - in einer zweiten Betriebsart B2, in welcher das zweite elektrische Feld EF2 anliegt, einerseits linear oder elliptisch polarisiertes Licht, welches parallel zu der ersten Vorzugsrichtung in den schaltbaren Lichtfilters 5 einfällt, zu mindestens 24% transmittiert und andererseits linear oder elliptisch polarisiertes Licht, welches in mindestens einem zweiten Winkel von mehr als 35° zu der ersten Vorzugsrichtung, der in einer zweiten Ebene liegt, in den schaltbaren Lichtfilter 5 einfällt, zu mindestens 85% absorbiert wird, wobei die erste und die zweite Ebene sich in einem Winkel von 80° bis 100° schneiden, so dass sich die Richtungen der

Absorption für die erste Betriebsart B1 und die zweite Betriebsart B2 um jeweils 80° bis 100° unterscheiden.Furthermore, it is advantageous to use the switchable light filter 5 in such a way that

• - in a first operating mode B1, in which the first electric field EF1 is applied, on the one hand linearly or elliptically polarized light (in the case of elliptically polarized light, the ratios of the semi-axes should be at least 1:4, better at least 1:10 or greater), which is incident parallel to the first preferred direction into the switchable light filter 5, is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident at at least a first angle of more than 35° to the first preferred direction, which lies in a first plane, into the switchable light filter 5 is absorbed to at least 85%, and that
• - in a second operating mode B2, in which the second electric field EF2 is applied, on the one hand linearly or elliptically polarized light, which is incident parallel to the first preferred direction in the switchable light filter 5, is transmitted to at least 24% and on the other hand linearly or elliptically polarized light, which is incident in at least at least a second angle of more than 35° to the first preferred direction, which lies in a second plane, is absorbed to at least 85% into the switchable light filter 5, wherein the first and the second plane intersect at an angle of 80° to 100°, so that the directions of the

Absorption for the first operating mode B1 and the second operating mode B2 differ by 80° to 100° each.

The said first preferred direction can advantageously enclose an angle between 0° and 45° to a surface normal of the optical element 1. Furthermore, in special embodiments, the first preferred direction can vary across the surface of the optical element 1. In the sense of the invention, the first preferred direction averaged over the surface then applies.

Materials within the meaning of the invention which contain fixed or - for the second embodiment of switchable light filters 5 described below - alignable transition dipole moments are, for example, dichroic dyes or dye mixtures which are combined with a carrier material which does not impair the properties, for example with liquid crystals or polymers. The optical element 1 can, for example, contain only one dye. However, several dyes, i.e. dye mixtures, can also be contained in a single optical element 1.

The extinction, i.e. the absorption, of the light depends on the absolute number of transition dipole moments - and thus inherently also on the layer thickness in which the transition dipole moments are located - and the alignment between the transition dipole moment and the polarization of the incident light. Depending on the implementation, the density of the said transition dipole moments, their strength or the refractive index in the layers of the optical element can vary. In a passive, i.e. non-switchable optical element, the volume density of the transition dipoles can approach 100%.

• - ein erstes optisches Element 1a, welches
  • - eine Vielzahl an Licht absorbierenden Übergangsdipolmomenten, die in Form von Molekülen einer und/oder in Form eines oder mehrerer dichroitischer Farbstoffe ausgebildet sind, umfasst,
  • - wobei die Mehrzahl der Übergangsdipolmomente mindestens in einem ersten Zustand mit einer Toleranz von maximal 20° (bevorzugt maximal 10°) parallel zu einer für das erste optische Element 1 wählbaren ersten Vorzugsrichtung ausgerichtet ist oder um diese herum variiert,
  • - so dass Licht, welches in das erste optische Element 1a einfällt, in Abhängigkeit von seinem Polarisationszustand und von seiner Einfallsrichtung gegenüber dem ersten optischen Element 1a und transmittiert oder mindestens teilweise absorbiert wird,
  • - wobei die Übergangsdipolmomente in eine Flüssigkristallschicht 3a eingebettet sind, so dass die Übergangsdipolmomente in ihrer Ausrichtung und/oder ihrem Betrag zwischen dem ersten und mindestens einem zweiten Zustand variiert werden können, um das erste optische Element 1a alternativ in mindestens zwei verschiedene Zustände versetzen zu können,
• - (zeichnerisch nicht dargestellte) Mittel zur wahlweisen Erzeugung mindestens eines ersten elektrischen Feldes EF1 oder eines zweiten elektrischen Feldes EF2, wobei für das erste optische Element 1a der erste Zustand durch Anlegen des ersten elektrischen Feldes EF1 und der zweite Zustand durch Anlegen des zweiten elektrischen Feldes EF2 erzeugt wird, so dass sich die Transmissionseigenschaften des schaltbaren Lichtfilters 5a zwischen einer ersten Betriebsart B1, in welcher das erste elektrische Feld EF1 anliegt, und einer zweiten Betriebsart B2, in welcher das erste elektrische Feld EF2 anliegt, unterscheiden,
• - wobei mindestens eine der folgenden Maßnahmen im schaltbaren Lichtfilter 5a der zweiten Ausgestaltung umgesetzt ist:
  • - die Flüssigkristallschicht 3a umfasst mindestens eine TN-Zelle sowie einen (zeichnerisch nicht dargestellten) Verzögerungsfilm, wobei der besagte Verzögerungsfilm die in der ersten Betriebsart B1 durch die TN-Zelle durchgeführte Polarisationsänderung für den schaltbaren Lichtfilter 5a durchdringendes Licht zu mindestens 80% aufhebt (bevorzugt sogar zu 90%, 95% oder mehr; dies kann z.B. mittels eines discotischen Films realisiert werden), oder
  • - die Flüssigkristallschicht 3a, wenn sie beispielweise als mittels PA-, FFS-, IPS-, VA-, ADS- (oder vergleichbarer) Technologie realisiert wird, weist in der ersten Betriebsart B1 ihr Transmissionsmaximum in einem Winkel, der zwischen 20° und 80° zu Flächennormalen der Flüssigkristallschicht 3a liegt, auf, so dass die Beeinflussung der Polarisation des den schaltbaren Lichtfilter 5a durchdringenden Lichtes aufgrund der Flüssigkristallschicht 3a für unter dem besagten Winkel, der zwischen 20° und 80° zu Flächennormalen der Flüssigkristallschicht 3a liegt, einfallende Lichtstrahlen maximal ist, wodurch die Beeinflussung der winkelabhängigen Transmission durch das optische Element 1a verstärkt wird, und/oder
  • - der schaltbare Lichtfilter 5a umfasst weiterhin ein negativ dispersives optisches Element (z.B. einen negativ dispersiven Verzögerungsfilm), oder mehrere optische Elemente, die in der Zusammenwirkung wie ein negativ dispersives optisches Element wirken (dies kann erreicht werden durch die Kombination von verschiedenen Elementen mit unterschiedlich hoher Dispersion), so dass Farbverschiebungen des den schaltbaren Lichtfilter 5a durchdringenden Lichtes (insbesondere im Fall von weißem oder buntem Licht) mindestens teilweise kompensiert werden

The 1a can also be used to explain a second embodiment of an exemplary switchable light filter 5a. This comprises

• - a first optical element 1a, which
  • - a plurality of light-absorbing transition dipole moments which are formed in the form of molecules of one and/or in the form of one or more dichroic dyes,
  • - wherein the majority of the transition dipole moments are aligned parallel to a first preferred direction selectable for the first optical element 1 or vary around it, at least in a first state, with a tolerance of a maximum of 20° (preferably a maximum of 10°),
  • - so that light which is incident on the first optical element 1a is transmitted or at least partially absorbed depending on its polarization state and its direction of incidence relative to the first optical element 1a,
  • - wherein the transition dipole moments are embedded in a liquid crystal layer 3a, so that the transition dipole moments can be varied in their orientation and/or their amount between the first and at least one second state in order to be able to alternatively place the first optical element 1a into at least two different states,
• - means (not shown in the drawing) for selectively generating at least a first electric field EF1 or a second electric field EF2, wherein for the first optical element 1a the first state is generated by applying the first electric field EF1 and the second state is generated by applying the second electric field EF2, so that the transmission properties of the switchable light filter 5a differ between a first operating mode B1, in which the first electric field EF1 is applied, and a second operating mode B2, in which the first electric field EF2 is applied,
• - wherein at least one of the following measures is implemented in the switchable light filter 5a of the second embodiment:
  • - the liquid crystal layer 3a comprises at least one TN cell and a retarder film (not shown in the drawing), wherein said retarder film cancels the polarization change carried out by the TN cell in the first operating mode B1 for light penetrating the switchable light filter 5a by at least 80% (preferably even by 90%, 95% or more; this can be realized e.g. by means of a discotic film), or
  • - the liquid crystal layer 3a, if it is realized, for example, using PA, FFS, IPS, VA, ADS (or comparable) technology, has its transmission maximum in the first operating mode B1 at an angle that lies between 20° and 80° to the surface normal of the liquid crystal layer 3a, so that the influence on the polarization of the light penetrating the switchable light filter 5a due to the liquid crystal layer 3a, the incident light rays are at a maximum for the angle in question, which is between 20° and 80° to the surface normal of the liquid crystal layer 3a, whereby the influence of the angle-dependent transmission by the optical element 1a is increased, and/or
  • - the switchable light filter 5a further comprises a negatively dispersive optical element (e.g. a negatively dispersive retardation film), or several optical elements which, when combined, act like a negatively dispersive optical element (this can be achieved by combining different elements with different levels of dispersion), so that color shifts of the light penetrating the switchable light filter 5a (in particular in the case of white or colored light) are at least partially compensated

The transition dipole moments can - as described above - be formed as one or more dichroic dye(s), thus by such dye molecules, which are integrated in a guest-host arrangement in liquid crystals of the liquid crystal layer 3a. The dye molecules in turn then align themselves parallel to the liquid crystal molecules of the liquid crystal layer 3a. The alignment of the liquid crystal molecules in the voltage-free electric field is in turn induced by the surfaces embedding them. One possible material for such surfaces is PMI.

In general, we can speak of liquid crystal polymers doped with dye(s). Various approaches are known for this (e.g. thermosettling LCs and epoxy LC), which therefore do not need to be discussed in detail.

The at least one dye consists of dye molecules, wherein a transition dipole or transition dipole moment is advantageously associated with each dye molecule, i.e. each dye molecule corresponds to a transition dipole or transition dipole moment. Typically, a dye has a mass fraction of 0.01% to 10% (or possibly significantly more), preferably 0.1% to 5% of the material of the respective layer(s) in the optical element 1 or 1a. The thickness of the layers is preferably in the range from 0.2 µm to 50 µm, preferably in the range from 0.5 µm to 20 µm, all marginal values included. The dyes or dye mixtures in the optical element 1, 1a can also be formed differently in its different layers, if present.

In a preferred embodiment, a polarization filter P is also present, which can be arranged upstream or downstream of the first optical element 1a or 1 in the direction of incidence. Alternatively, a λ/4 retardation layer can also be present in order to generate linearly polarized light from circularly polarized light, depending on the properties of the light originating from a corresponding light source.

In order to switch the optical effect of the switchable light filter 5a according to the invention, in this second embodiment of the invention the transition dipole moments in each layer are varied in their orientation and/or their amount between the first and at least one second state in order to be able to alternatively place the respective layer into at least two different states. Possible embodiments of a switchable optical element 1a or each layer therein are based, for example, on liquid crystals and/or dyes or dye mixtures, which can be arranged in the liquid crystal cell with homogeneous orientation on the surfaces and can be rotated therein between at least two states. The light-absorbing transition dipole moments are also rotated and can thus assume at least two states of action. In particular in such embodiments it is conceivable that more than two states, e.g. three or eight states, each with different optical effects, are achieved. Other embodiments of the liquid crystal cells are also conceivable. For this purpose, electric fields are used in particular to rotate the liquid crystals. It is possible, for example, that either a first electric field or a second electric field describes a field-free state, whereby the other electric field has an absolute field strength greater than zero, e.g. 0.5 MV/m.

A first such state corresponds to the conditions described above and at least a second state is different from this, i.e. has at least one other preferred direction.

• - in einer ersten Betriebsart B1, in welcher das erste elektrische Feld EF1 anliegt, einerseits linear oder elliptisch polarisiertes Licht (im Falle von elliptisch polarisiertem Licht sollten die Verhältnisse der Beträge der Halbachsen mindestens 1:4 betragen, besser mindestens 1:10 oder größer), welches parallel zur ersten Vorzugsrichtung in den schaltbaren Lichtfilter 5a einfällt, zu mindestens 24% transmittiert und andererseits linear oder elliptisch polarisiertes Licht, welches in mindestens einem ersten Winkel von mehr als 35° zu der ersten Vorzugsrichtung, der in einer ersten Ebene liegt, in den schaltbaren Lichtfilter 5a einfällt, zu mindestens 85% absorbiert wird, und dass
• - in einer zweiten Betriebsart B2, in welcher das zweite elektrische Feld EF2 anliegt, einerseits linear oder elliptisch polarisiertes Licht, welches parallel zu der ersten Vorzugsrichtung in den schaltbaren Lichtfilters 5a einfällt, zu mindestens 24% transmittiert und andererseits linear oder elliptisch polarisiertes Licht, welches in mindestens einem zweiten Winkel von mehr als 35° zu der ersten Vorzugsrichtung, der in einer zweiten Ebene liegt, in den schaltbaren Lichtfilter 5a einfällt, zu mindestens 85% absorbiert wird, wobei die erste und die zweite Ebene sich in einem Winkel von 80° bis 100° schneiden, so dass sich die Richtungen der Absorption für die erste Betriebsart B1 und die zweite Betriebsart B2 um jeweils 80° bis 100° unterscheiden.

Furthermore, in this second embodiment of the switchable light filter 5a, it is also advantageous to use it in such a way that

• - in a first operating mode B1, in which the first electric field EF1 is applied, on the one hand linearly or elliptically polarized light (in the case of elliptically polarized light, the ratios of the amounts of the semi-axes should be at least 1:4, better at least 1:10 or greater), which is incident parallel to the first preferred direction into the switchable light filter 5a, is transmitted to at least 24% and on the other hand, linearly or elliptically polarized light which is incident on the switchable light filter 5a at at least a first angle of more than 35° to the first preferred direction, which lies in a first plane, is absorbed to at least 85%, and that
• - in a second operating mode B2, in which the second electric field EF2 is applied, on the one hand linearly or elliptically polarized light which is incident on the switchable light filter 5a parallel to the first preferred direction is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident on the switchable light filter 5a at at least a second angle of more than 35° to the first preferred direction, which lies in a second plane, is absorbed to at least 85%, wherein the first and the second plane intersect at an angle of 80° to 100°, so that the directions of absorption for the first operating mode B1 and the second operating mode B2 differ by 80° to 100° in each case.

For special applications, the switchable light filter 5, 5a, i.e. the liquid crystal layer 3, 3a and/or the means for selectively generating at least a first electric field (EF1) or a second electric field (EF2) - regardless of which of the aforementioned embodiments - can be divided into several separately switchable segments, corresponding to pixels P _LC , so that local switching between the respectively possible operating states is possible. In conjunction with an image display unit, this would mean that, for example, only part of the image surface can be switched between a private mode for privacy protection and a public mode with no privacy protection effect, i.e. for a clear view, while the complementary part of the image surface is permanently in a privacy protection mode or in no privacy protection mode.

Furthermore, it is possible that the means for selectively generating at least a first electric field (EF1) or a second electric field (EF2) can selectively generate further electric fields (EF3, EF4, ...), so that third, fourth and possibly further states can be generated for the liquid crystal layer (3, 3a).

The invention is particularly important when a switchable light filter 5, 5a of the first or second embodiment described above is combined with an image display unit (e.g. OLED, microLED, LCD) to form a screen. In addition to a switchable light filter 5, 5a as described above, such a screen comprises an image display unit arranged upstream or downstream of the switchable light filter as seen by an observer. The operating modes of the switchable light filter described above can be easily transferred to the screen, so that this too can be operated in the various operating modes already mentioned above, depending on the design of the switchable light filter used, for example at least in a first operating mode for a horizontally free, public viewing mode with an unrestricted radiation or viewing angle range and a horizontally restricted, private viewing mode with a horizontally restricted radiation or viewing angle range compared to the free viewing mode, so that viewers who are positioned outside of this restricted viewing angle range can only perceive image content displayed on the screen in the free viewing mode.

The image display unit advantageously corresponds to an LCD panel, one of whose polarization filters corresponds to the polarization filter of the switchable light filter 5, 5a. This can be the front or rear polarizer in the LCD structure. In addition, the switchable light filter 5, 5a can advantageously be arranged between the LCD panel and its backlight in order to switch between a first operating state for a free viewing mode and a second operating state for a restricted viewing mode, because the light from the backlight is sometimes focused and sometimes not focused in the horizontal direction due to the switchable light filter 5, 5a - for example when switching in the horizontal direction. "Focusing" does not mean focusing in the manner of lenses, but rather a narrowing of the radiation range or transmission range across the angles.

The invention solves the problem: Switchable light filters have been described (or systems using such filters) which can be implemented inexpensively and in particular can be used universally with different types of screens in order to enable switching between a visual protection - at least in the horizontal direction with respect to a standing or sitting viewer - i.e. a restricted viewing mode, and a free viewing mode, whereby the resolution of such a screen is not reduced, not even slightly. In particular, even for viewing positions in which a viewer does not only look horizontally (from his point of view) at an angle to the switchable light filter, but also at an angle from above, a high-quality angle restriction is achieved and overall good image quality is achieved in the event that a switchable light filter is arranged in front of an image display device.

The invention described above can be used in conjunction with an image display device to advantage wherever confidential data is displayed and/or entered, such as when entering a PIN or displaying data at cash machines or payment terminals or when entering a password or reading emails on mobile devices. The invention can - as described above - also be used in cars to selectively withhold disturbing image content from the driver or front passenger.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

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• US 2013/0308185 A1 [0009]
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Cited non-patent literature

• N. Saba, M. Jawaid, O. Y. Alothman, M. T. Paridah, and A. Hassan, "Recent advances in epoxy resin, natural fiber-reinforced epoxy composites and their applications," Journal of Reinforced Plasticsand Composites, vol. 35, no. 6, 2015 [0035]
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• Deformation Pattern of Twisted Nematic Liquid Crystal Layers in an Electric Field” by H. J. Deuling, Pages 81-93 | Received 30 Aug 1973, Published online: 21 Mar 2007 [0070]

Claims (10)

Switchable light filter (5), comprising - a first optical element (1), in turn comprising • a plurality of light-absorbing transition dipole moments, • wherein the majority of the transition dipole moments are aligned with a tolerance of a maximum of 20° parallel to a first preferred direction that can be selected for the first optical element (1) or vary around this, • so that light that is incident on the first optical element (1) is transmitted or at least partially absorbed depending on its polarization state and its direction of incidence relative to the first optical element (1), - means 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 first optical element (1), on which the first electric field (EF1) or the second electric field (EF2) acts and which, depending thereon, influences the polarization state of light passing through it, so that - the transmission properties of the switchable light filter (5) can be switched between a first operating mode B1, in which the first electric field (EF1) is applied, and a second operating mode B2, in which the second electric field (EF2) is applied, - characterized in that the liquid crystal layer (3) in the first operating mode B1 has its transmission maximum at an angle which lies between 20° and 80° to the surface normal of the liquid crystal layer (3), so that the influence of the polarization of the light penetrating the switchable light filter (5) due to the liquid crystal layer (3) is maximum for light rays incident at the said angle which lies between 20° and 80° to the surface normal of the liquid crystal layer (3). Switchable light filter (5), after claim 1 , characterized in that - in a first operating mode B1, in which the first electric field (EF1) is applied, on the one hand linearly or elliptically polarized light which is incident on the switchable light filter (5) parallel to the first preferred direction is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident on the switchable light filter (5) at at least a first angle of more than 35° to the first preferred direction, which lies in a first plane, is absorbed to at least 85%, - in a second operating mode B2, in which the second electric field (EF2) is applied, on the one hand linearly or elliptically polarized light which is incident on the switchable light filter (5) parallel to the first preferred direction is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident on the switchable light filter (5) at at least a second angle of more than 35° to the first preferred direction, which lies in a second plane, is absorbed to at least 85%, switchable light filter (5) is absorbed to at least 85%, wherein the first and the second plane intersect at an angle of 80° to 100°, so that the directions of absorption for the first operating mode B1 and the second operating mode B2 differ by 80° to 100° each. Switchable light filter (5a), comprising - a first optical element (1a), which • comprises a plurality of light-absorbing transition dipole moments, which are in the form of molecules and/or in the form of one or more dichroic dyes, • wherein the majority of the transition dipole moments are aligned parallel to or vary around a first preferred direction that can be selected for the first optical element (1a) with a tolerance of a maximum of 20°, at least in a first state, • so that light that is incident on the first optical element (1a) is transmitted or at least partially absorbed depending on its polarization state and its direction of incidence relative to the first optical element (1a), • wherein the transition dipole moments are embedded in a liquid crystal layer (3a), so that the transition dipole moments can be varied in their orientation and/or their amount between the first and at least one second state in order to be able to alternatively put the first optical element (1a) into at least two different states, - means for selectively generating at least one first electric field (EF1) or a second electric field (EF2), wherein for the first optical element (1a) the first state is generated by applying the first electric field (EF1) and the second state is generated by applying the second electric field (EF2), so that the transmission properties of the switchable light filter (5a) differ between a first operating mode B1, in which the first electric field (EF1) is applied, and a second operating mode B2, in which the first electric field (EF2) is applied, - characterized in that the liquid crystal layer (3a) in the first operating mode B1 has its transmission maximum at an angle which lies between 20° and 80° to the surface normal of the liquid crystal layer (3a), so that the influence on the polarization of the light penetrating the switchable light filter (5a) due to the liquid crystal layer (3a) is maximum for light rays incident at said angle, which lies between 20° and 80° to the surface normal of the liquid crystal layer (3a). Switchable light filter (5a) according to claim 3 , characterized in that - in a first operating mode B1, in which the first electric field (EF1) is applied, on the one hand linearly or elliptically polarized light which is incident on the switchable light filter (5a) parallel to the first preferred direction is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident on the switchable light filter (5a) at at least a first angle of more than 35° to the first preferred direction, which lies in a first plane, is absorbed to at least 85%, - in a second operating mode B2, in which the second electric field (EF2) is applied, on the one hand linearly or elliptically polarized light which is incident on the switchable light filter (5a) parallel to the first preferred direction is transmitted to at least 24% and on the other hand linearly or elliptically polarized light which is incident on the switchable light filter (5a) at at least a second angle of more than 35° to the first preferred direction, which lies in a second plane, incident on the switchable light filter (5a) is absorbed to at least 85%, wherein the first and the second plane intersect at an angle of 80° to 100°, so that the directions of absorption for the first operating mode B1 and the second operating mode B2 differ by 80° to 100° respectively. Switchable light filter (5, 5a) according to one of the preceding claims, characterized in that the respective liquid crystal layer (3, 3a) and/or the means for selectively generating at least a first electric field (EF1) or a second electric field (EF2) is divided into several separately switchable segments, so that local switchability between the respectively possible operating modes is made possible. Switchable light filter (5, 5a) according to one of the preceding claims, characterized in that the means for selectively generating at least a first electric field (EF1) or a second electric field (EF2) can selectively generate further electric fields (EF3, EF4, ...), so that third, fourth and optionally further states can be generated for the liquid crystal layer (3, 3a). Illumination device for a screen, which can be operated in at least two operating modes BE1 for a free viewing mode and BE2 for a restricted viewing mode, in which light is emitted in an angular range that is restricted compared to the free viewing mode, comprising - a surface-like extended background lighting, which has a light filter (5) according to claim 1 or 2 , or a light filter (5, 5a) according to one of the Claims 3 until 6 contains and emits light, - a plate-shaped light guide located in front of the background lighting in the viewing direction, which has output elements on at least one of the large surfaces and/or within its volume, - lighting means arranged laterally on at least one narrow side of the light guide, and - a linear polarization filter arranged in front of the background lighting or in front of the light guide in the viewing direction, whereby light which emanates from the background lighting and penetrates the linear polarization filter is restricted in its propagation directions, - wherein in the operating mode BE2 the back lighting is switched on and the lighting means are switched off, and wherein in the operating mode BE1 at least the lighting means are switched on. Screen which can be operated in at least two operating modes BB1 for a free viewing mode and BB2 for a restricted viewing mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free viewing mode, comprising - a surface-like extended backlight which has a light filter (5) according to claim 1 or 2 or a light filter (5, 5a) according to one of the Claims 3 until 6 contains and emits light, and which is optionally constructed to be directly luminous, - a linear polarization filter arranged in front of the background lighting in the viewing direction, whereby light which emanates from the background lighting and penetrates the linear polarization filter is restricted in its propagation directions, and - a transmissive image display device which is arranged in front of the light guide in the viewing direction and close to which or preferably in which the linear polarization filter is arranged, - wherein in the operating mode BB2 the second electric field (EF2) and wherein in the operating mode BB1 the first electric field (EF1) is applied. screen after claim 8 , characterized in that the linear polarization filter is arranged in the transmissive image display device or is a part of it. Screen which can be operated in at least two modes BB1 for a free view mode and BB2 for a restricted view mode in which light is projected into a viewing angle which is restricted for a viewer compared to the free view mode. richly radiated, comprising - an image display device, - in the viewing direction in front of the image display device a light filter (5) according to claim 1 or 2 or a light filter (5, 5a) according to a Claims 3 until 6 , - wherein in the operating mode BB2 the second electric field (EF2) is applied and in the operating mode BB1 the first electric field (EF1) is applied.

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