DE102024118438B3 - 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), comprising a first optical element (1), in turn comprising a plurality of light-absorbing transition dipole moments, means (SE1, SE2, ...) for selectively generating at least one first electric field (EF1) or one second electric field (EF2), wherein for at least two areas on the switchable light filter (5) the first or the second electric field (EF1, EF2) can be selected separately, a liquid crystal layer (3) on which the first electric field (EF1) or the second electric field (EF2) acts and which, depending thereon, influences the polarization state of light passing through it, so that the transmission properties of the switchable light filter (5) differ between a first operating mode B1, in which the first electric field (EF1) is applied, and a second operating mode B2, in which the second electric field (EF2) is applied, wherein, if the first electric field (EF1) and at least one second region, the second electric field (EF2) is selected, for at least one transition region between the at least two regions an electric field (EF3) is present or electric fields (EF3, EF4 ...) are present, so that in this at least one transition region the transmission properties of the switchable light filter (5) differ from the transmission properties of the first operating mode B1 and the second operating mode B2.
This avoids unpleasantly perceptible edges between areas with the first operating mode B1 and the second operating mode B2.

Description

Technical field of the invention

In recent years, great strides have 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, such as banking details and other personal and sensitive data, is increasingly becoming available on mobile devices such as notebooks and tablet PCs. Accordingly, people need control over who can view this sensitive data. They need to be able to choose between a wide viewing angle – a public mode – to share information on their display with others, e.g., when viewing vacation photos or for advertising purposes. On the other hand, they need a narrow viewing angle – 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 also wants to consume these while driving. Therefore, a screen that can switch between the corresponding display modes is required.

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

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

In the US 5,993,940 A The use of a film with evenly spaced, small, strip-shaped prisms on its surface is described to achieve a private mode, i.e., a restricted viewing mode with a narrow viewing angle range. Development and production are technically quite complex.

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

The US 2012/0235891 A1 describes a very complex backlight - a background illumination - in a screen. According to 1 and 15 Not only are several light guides used, but also other complex optical elements such as microlens elements 40 and prism structures 50, which transform the light from the rear illumination on its way to the front illumination. This is expensive and technically complex to implement and also involves light loss. According to the variant according to 17 in the US 2012/0235891 A1 Both light sources 4R and 18 produce light with a narrow illumination angle, with the light from the rear light source 18 first undergoing a complex conversion process into light with a wide illumination angle. This complex conversion, as noted above, significantly reduces brightness.

According to the JP 2007-155783 A Special optical surfaces 19 are used, which are complex to calculate and manufacture, and which then deflect light into different narrow or wide areas depending on the angle of incidence. These structures resemble Fresnel lenses. Furthermore, there are interference edges that deflect light in undesirable directions. Thus, it remains unclear whether truly meaningful light distributions can be achieved.

In the US 2013/0308185 A1 describes a special light guide with steps that emits light over a large area in different directions depending on the direction from which it is illuminated from a narrow side. In conjunction with a transmissive image display device, e.g. an LC display, a screen can be created that can be switched between free and restricted view mode. A disadvantage of this is that the restricted view 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 view 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 generally have the disadvantage that light is coupled out of both large surfaces, which means that about half of the light is lost. The useful light is radiated in the wrong direction, namely toward the background lighting, where it cannot be adequately recycled due to its design. Furthermore, the polymer scattering particles distributed throughout the volume of the light guide can, under certain circumstances, especially at higher concentrations, lead to scattering effects that reduce the privacy effect in the protected operating mode.

The approach of "Electrical Birefringence (EDB)" technology is based on the idea of using the switchable liquid crystals of an additionally applied LC panel to "filter" all light rays that do not exit the imaging layer at a specific beam angle. Disadvantages of this technology include high additional energy and cost requirements, and the difficulty of changing the +/-40° sweet spot, i.e., the optimal viewing position. The absorption coefficient of the LC structures is also insufficient, as the attenuation of the light intensity increases again for viewing angles greater than the sweet spot, so that the light intensity for viewing angles greater than +/-40° is up to 3% of the maximum light intensity.

Furthermore, the DE 102023 110 269 B3 The applicant discloses a switchable light filter, a lighting device, and a screen that can be operated in at least two modes: a free view mode and a restricted view mode. However, this document does not describe in detail how visible edges between areas of different transmission ranges can be avoided when switching between the modes over a partial area.

Finally, the DE 10 2019 003 383 A1 a private liquid crystal device that enables two viewing modes. This document does not disclose how visible edges between sub-areas of different viewing modes can be avoided. A further disadvantage of this liquid crystal device is that collimated light for a free viewing mode (mode 2 there) is selectively scattered, which significantly reduces efficiency.

The aforementioned methods and arrangements generally have the disadvantage that they significantly reduce the brightness of the base 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. However, in particular, where partial switching between modes is possible, there are unpleasantly visible edges between areas in different modes.

Description of the invention

The object of the invention is therefore to describe a light filter in which light incident thereon is either transmitted, depending on the angle, or partially or (almost) completely absorbed at various points on the light filter or screen. Visible edges between areas of different transmission ranges are to be avoided. Furthermore, a corresponding screen or lighting device is to be described whose angle-dependent brightness distribution can be partially modified, while also avoiding visible edges between areas in different modes.

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, even slightly.

• - 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, wobei für mindestens zwei Bereiche auf dem schaltbaren Lichtfilter jeweils das erste oder das zweite elektrische Feld EF1, EF2 separat gewählt werden kann,
• - 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 zweite elektrische Feld EF2 anliegt, unterscheiden,
• - und so dass, wenn auf dem schaltbaren Lichtfilter für mindestens einen ersten Bereich das erste elektrische Feld EF1 und mindestens einen zweiten Bereich das zweite elektrische Feld EF2 gewählt ist, für mindestens einen Übergangsbereich zwischen diesen mindestens zwei Bereichen ein elektrisches Feld EF3 anliegt oder elektrische Felder EF3, EF4 ... anliegen, die von dem ersten und dem zweiten elektrischen Feld EF1, EF2 verschieden sind, so dass sich in diesem mindestens einen Übergangsbereich die Transmissionseigenschaften des schaltbaren Lichtfilters von den Transmissionseigenschaften der ersten Betriebsart B1 und der zweiten Betriebsart B2 unterscheiden.

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 variety of light-absorbing transition dipole moments,
  • • wherein the majority of the transition dipole moments are aligned at least in a first state (in particular also permanently) with a tolerance of a maximum of 20° (preferably a maximum of 10°) parallel to a first preferred direction that can be selected for the first optical element or vary around this,
  • • so that light 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 one first electric field EF1 or one second electric field EF2, wherein for at least two areas on the switchable light filter, the first or the second electric field EF1, EF2 can be selected separately,
• - 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 second electric field EF2 is applied,
• - and so that, if the first electric field EF1 is selected on the switchable light filter for at least a first region and the second electric field EF2 is selected for at least a second region, an electric field EF3 is present for at least one transition region between these at least two regions or electric fields EF3, EF4 ... are present which are different from the first and the second electric field EF1, EF2, so that in this at least one transition region the transmission properties of the switchable light filter differ from the transmission properties of the first operating mode B1 and the second operating mode B2.

It is advantageous in this case that if, at the same time, the first electric field EF1 is selected for at least one first region and the second electric field EF2 is selected for at least one second region on the switchable light filter, electric fields EF3, EF4 ... are present for at least one transition region between the two regions, which represent a step-wise and/or gradual transition from the first electric field EF1 to the second electric field EF2, so that around the at least one transition region between the two regions at least three, preferably at least five, particularly preferably at least ten different values of the transmission are generated for a respectively selectable, but always identical angle which deviates from the perpendicular bisector to the switchable light filter by at least 5° (e.g. by 25°, 35°, 45° or greater than 45°). This allows a viewer to experience a gradual transition of the angle-dependent transmission values between the areas, which avoids visible edges, since in said transition area there is also a transition from the first electric field EF1 to the second electric field EF2 with at least one resulting third electric field EF3 (but preferably several electric fields EF3, EF4, etc.) present therebetween, which in turn controls the angle-dependent transmission of the switchable light filter at such transition areas differently than in said first or second area, where the first or second electric field EF1, EF2 is present.

For example, it is possible for the means for selectively generating at least a first electric field EF1 or a second electric field EF2 to comprise at least two opposing flat transparent electrodes (e.g., ITO layers) between which the liquid crystal layer 3 is located, with at least one of these transparent electrodes being interrupted near the boundary between said two regions. "Proximity" here means a maximum distance of ten percent of the largest dimension of the liquid crystal layer, preferably a maximum of 5 percent. This also applies to all subsequent cases.

It is possible that the at least one interrupted transparent electrode is interrupted several times near the boundary between the two regions.

Alternatively, the two transparent electrodes can be interrupted near the boundary between the two regions. Advantageously, the respective interruptions in the transparent electrodes are not congruent when projected onto the first optical element perpendicular to the propagation direction of the first optical element.

By means of the aforementioned embodiments using one or two interrupted transparent electrodes, it is ensured that if the first electric field EF1 and the second electric field EF2 are selected simultaneously for at least a first region on the switchable light filter, an electric field EF3 is applied to at least one transition region between the at least two regions, or electric fields EF3, EF4 ... are applied that are different from the first and second electric fields EF1, EF2, which is essential for the implementation of the invention. However, the aforementioned list of embodiments is not exhaustive; further configurations can be found that achieve the aforementioned effect, for example by adding another transparent electrode near the transition region.

Furthermore, it is possible that the first electric field EF1 acts on the liquid crystal layer in such a way that linearly polarized light passing through it is shifted in its polarization orientation by at least 86°, preferably by at least 88°. is rotated, while the second electric field EF2 acts on the liquid crystal layer in such a way that linearly polarized light penetrating through it is rotated in its polarization orientation by no more than 4°, preferably not at all. It is possible, for example, that either a first electric field EF1 or a second electric field EF2 describes a field-free state for the first region, while the respective other electric field has an absolute field strength greater than zero, e.g. 0.5 MV/m, for the second region. Since the first and second electric fields EF1, EF2 differ between the two regions, an electric field EF3 is present in the transition region between the at least two regions, or electric fields EF3, EF4 ... are present which are different from the first and second electric fields EF1, EF2, so that in this at least one transition region the transmission properties of the switchable light filter differ from the transmission properties of the first operating mode B1 and the second operating mode B2.

With an aforementioned embodiment, it is generally possible for the angle-dependent transmission properties of the switchable light filter to differ between a first operating mode B1, in which the first electric field EF1 is applied, and a second operating mode B2, in which the second electric field EF2 is applied, in such a way that the transmissions for a respectively selectable but always identical angle, which deviates from the mid-perpendicular to the switchable light filter by at least 45°, differ between the two operating modes by at least a factor of ten, preferably by at least a factor of 15.

• - eine flächenartig ausgedehnte Hintergrundbeleuchtung, die Licht abstrahlt, sowie
• - einen in Betrachtungsrichtung vor der Hintergrundbeleuchtung angeordneten schaltbaren Lichtfilter 5 wie vorstehend beschrieben.

The object of the invention is also achieved by a lighting device for a screen which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising

• - a large area backlight that emits light, and
• - a switchable light filter 5 arranged in front of the background lighting in the viewing direction as described above.

• - eine Bildwiedergabeeinrichtung,
• - in Betrachtungsrichtung vor oder hinter der Bildwiedergabeeinrichtung einen schaltbaren Lichtfilter wie vorstehend beschrieben.

Finally, the object of the invention is achieved by a screen comprising

• - an image display device,
• - a switchable light filter as described above in front of or behind the image display device in the viewing direction.

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 mentioned above, depending on the design of the switchable light filter used.

The image display unit can alternatively be an OLED, an LCD, an SED, a field emission display (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 screen type is also suitable. Such a screen is advantageously used in a mobile device, a motor vehicle, aircraft, or watercraft, in a payment terminal, or in an access system. It is possible to switch between the aforementioned operating modes to protect sensitive data, i.e., to display it so that it is visible to only one viewer, or alternatively to display image content simultaneously for multiple viewers.

Said first preferred direction can advantageously enclose an angle between 0° and 45° inclusive with a surface normal of the first optical element. Furthermore, in particular embodiments, the first preferred direction can vary across the surface of the first optical element. For the purposes 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 associated with a specific transition between a first state—usually the ground state—and a second state—usually an excited state—of a system, i.e., an atom, molecule, or solid. It macroscopically corresponds to the electric dipole moment associated with this transition. The direction of the vector defines the polarization of the transition, which in turn determines how the system interacts with an electromagnetic wave of a given polarization, for example, how it absorbs 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, for which the absorption is the same for any polarization direction of the light.

Suitable 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, include, for example, dichroic dyes or dye mixtures combined with a carrier material that does not impair the properties, for example, 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, or absorption, of 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 said transition dipole moments, their strength, or the refractive index can vary within the layers of the optical element. In a passive, non-switchable optical element, the volume density of the transition dipoles can reach 100%.

For the simplified modeling of transmission, it is assumed that the transition dipole moments in the optical element are oriented parallel to the plane of incidence of the light onto the optical element. The plane of incidence refers to a plane in which the propagation direction of the light wave lies, with the surface of the first 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 electrical oscillation component, both of which are perpendicular to each other and perpendicular to the direction of propagation. Light that strikes the optical element is initially unpolarized as a whole, i.e., the oscillation directions of the transverse electrical - and accordingly also the transverse magnetic - components are statistically distributed. When it strikes the surface of the first optical element with the transition dipole moments thus aligned perpendicular to the surface of the first optical element, the light interacts with the first optical element or the transition dipole moments of the material contained therein and becomes polarized. The vibrational components that lie in the plane of incidence are absorbed. Light that is polarized parallel to the plane of incidence, i.e., 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 first optical element—i.e., transversely electric or s-polarized, is fully transmitted or absorbed to a significantly reduced extent. Unpolarized light that does not pass through the first optical element parallel to transition dipole moments is therefore at least partially s-polarized upon passing through the first optical element with transition dipole moments aligned parallel to the plane of incidence of the light.

The transition dipole moments can—as described above—be formed by one or more dichroic dyes, i.e., by such dye molecules, which form liquid crystals in a guest-host arrangement with liquid crystal molecules or are integrated into a 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 surrounding surfaces. One possible material for such surfaces is PMI.

In general, one can speak of dye-doped liquid crystal polymers. Various approaches (e.g., thermosettling LCs and epoxy LCs) are known in the art, which therefore need not 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 advantageously a transition dipole or transition dipole moment is associated with each dye molecule, ie each dye molecule corresponds to a transition dipole or transition di pole 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 of 0.2 µm to 50 µm, more preferably in the range of 0.5 µm to 20 µm, including all marginal values. The dyes or dye mixtures in the first optical element can also be formed differently in its various 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. Examples of dichroic dyes or dye mixtures that can be considered are 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 preparation of a liquid crystal dye mixture is described, for example, in US$4,695,131 A. Furthermore, the surfaces that close off the layers on the outside are preferably treated, for example brushed, in order to achieve a homogeneous surface alignment of the transition dipole moments or, if present, of the liquid crystals.

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. Alternatively, a λ/4 retardation layer can also be present to generate linearly polarized light from circularly polarized light, if necessary, depending on the properties of the light originating from a corresponding light source.

To enable the optical effect of the switchable light filter according to the invention to be switchable, in a further embodiment of the invention the transition dipole moments in each layer can be varied in their orientation and/or their magnitude 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 said liquid crystal cell with homogeneous alignment on the surfaces and 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 to achieve more than two states, e.g., three or eight states, each with different optical effects. 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 EF1 or a second electric field EF2 describes a field-free state, with the other electric field having 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.

For 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 first optical element are conceivable. Alternatively, configurations of a first optical element or each layer therein are conceivable in which the transition dipole moments are embedded in a liquid subjected to an electrowetting process. In this way, the density of the transition dipole moments can be varied, in particular, but not exclusively.

As a rule, 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—is divided into several separately switchable segments, thus enabling local switching between the respectively possible operating states, whereby the aforementioned at least two regions can be formed. In conjunction with an image display unit, this would mean that, for example, only a portion 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 an unobstructed view, while the complementary portion of the image surface is permanently in a privacy protection mode or in no privacy protection mode. The invention helps here to ensure that a viewer does not perceive a "hard" edge between the regions.

Furthermore, it is possible for the means for selectively generating at least one first electric field EF1 or one second electric field EF2 on selectable sections of the switchable light filter to selectively generate further electric Fields EF3, EF4, ... can be generated, 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, a switchable light filter can be extended by a second optical element identical to the first (or with a different preferred direction), between which the liquid crystal layer is located. This makes it possible to selectively switch angular restrictions in the transmission between one and two directions.

Ideally, the first optical element (and, if present, any further identical optical elements) or the liquid crystal layer of the switchable light filter has a non-periodic structure. This is inherently achieved when these components are manufactured as described above and is helpful for avoiding undesirable optical effects, such as moiré effects, in conjunction with image sensors, for example.

For the first optical elements, it may also be true 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 regions (A1, A2, ...) along a selectable reference line on the respective first optical element, wherein a separate region preferred direction can be selected for each region (A1, A2, ...), which applies to all transition dipole moments of the corresponding first optical element within a region (A1, A2, ...), wherein all region preferred directions are different in pairs and point in the direction of an observer up to a tolerance of maximum +/-10 degrees. Within an optical element and within each region applicable to it, all transition dipole moments are therefore aligned parallel to the preferred direction applicable there, with a tolerance of maximum +/-10°.

Advantageously, the image display unit 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. Furthermore, 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 view mode and a second operating state for a restricted view mode, because the light from the backlight is sometimes focused and sometimes unfocused in the horizontal direction due to the switchable light filter - for example, when switching in the horizontal direction. "Focusing" does not mean focusing like a lens, but rather a narrowing of the radiation range or transmission range via the angles.

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 indicated, 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 in Schnittdarstellung,
• 2 eine Prinzipskizze in Draufsicht auf die Mittel zur Erzeugung von elektrischen Feldern in einer weiteren beispielhaften Ausgestaltung,
• 3 einen beispielhaften Graphen zur Transmission eines schaltbaren Lichtfilters im Stand der Technik, sowie
• 4 einen beispielhaften Graphen zur Wirkung auf die Transmission des schaltbaren Lichtfilters.

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

• 1 the schematic diagram of an exemplary switchable light filter in sectional view,
• 2 a schematic diagram in plan view of the means for generating electric fields in a further exemplary embodiment,
• 3 an exemplary graph of the transmission of a switchable light filter in the prior art, as well as
• 4 an example graph showing the effect on the transmission of the switchable light filter.

Detailed description of the drawings

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

• - ein erstes optisches Element 1, 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 (z.B. der Mittelsenkrechten auf das erste optische Element 1) 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,
• - Mittel SE1, SE2, ... (z.B. transparente Elektroden wie ITO-Schichten mit jeweils angeschlossener Ansteuerelektronik) zur wahlweisen Erzeugung mindestens eines ersten elektrischen Feldes EF1 oder eines zweiten elektrischen Feldes EF2, wobei für mindestens zwei Bereiche auf dem schaltbaren Lichtfilter 5 jeweils das erste oder das zweite elektrische Feld EF1, EF2 separat gewählt werden kann, insbesondere auch zum gleichen Zeitpunkt,
• - Eine (aus Betrachtungsrichtung von oben) 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 zweite elektrische Feld EF2 anliegt, unterscheiden,
• - und so dass, wenn auf dem schaltbaren Lichtfilter 5 für mindestens einen ersten Bereich das erste elektrische Feld EF1 und mindestens einen zweiten Bereich das zweite elektrische Feld EF2 gewählt ist, für mindestens einen Übergangsbereich zwischen diesen mindestens zwei Bereichen ein elektrisches Feld EF3 anliegt (oder elektrische Felder EF3, EF4 ... anliegen), die von dem ersten und dem zweiten elektrischen Feld EF1, EF2 verschieden sind, so dass sich in diesem mindestens einen Übergangsbereich die Transmissionseigenschaften des schaltbaren Lichtfilters 5 von den Transmissionseigenschaften der ersten Betriebsart B1 und der zweiten Betriebsart B2 unterscheiden.

This shows 1 the schematic diagram of an exemplary switchable light filter 5 in sectional view. This includes

• - a first optical element 1, in turn comprising
  • • a variety of light-absorbing transition dipole moments,
  • • wherein the majority of the transition dipole moments are aligned at least in a first state (in particular also permanently) 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 (e.g. the bisector of the first optical element 1) or vary around this,
  • • so that light 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 SE1, SE2, ... (e.g. transparent electrodes such as ITO layers with connected control electronics) for selectively generating at least one first electric field EF1 or one second electric field EF2, wherein for at least two areas on the switchable light filter 5 the first or the second electric field EF1, EF2 can be selected separately, in particular also at the same time,
• - A liquid crystal layer 3 arranged (from the viewing direction from above) 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 second electric field EF2 is applied,
• - and so that, if the first electric field EF1 is selected on the switchable light filter 5 for at least a first region and the second electric field EF2 is selected for at least a second region, an electric field EF3 is present (or electric fields EF3, EF4 ... are present) for at least one transition region between these at least two regions, which are different from the first and the second electric field EF1, EF2, so that in this at least one transition region the transmission properties of the switchable light filter 5 differ from the transmission properties of the first operating mode B1 and the second operating mode B2.

In 1 The dashed lines indicate the first and second regions in which the first and second electric fields EF1 and EF2 are applied, respectively. Between these first and second regions lies a transition region in which a third electric field EF3 is applied.

The substrates S can be made of polymer or glass and can in principle also be arranged at other locations in the structure.

In a preferred embodiment, a polarization filter P (not shown in the drawing) is also present. Alternatively, a λ/4 retardation layer may also be present in order to generate linearly polarized light from circularly polarized light, if necessary, depending on the properties of the light originating from a corresponding light source.

Advantageous under the conditions according to 1 that if at the same time on the switchable light filter 5 the first electric field EF1 is selected for at least one first region and the second electric field EF2 is selected for at least one second region, at least one electric field EF3 is present for at least one transition region between the two regions, which represents a step-wise and/or gradual transition from the first electric field EF1 to the second electric field EF2, so that in or around the at least one transition region between the first and the second region at least three, preferably at least five, particularly preferably at least ten different values of the transmission are generated for a respectively selectable but always identical angle which deviates from the perpendicular bisector to the switchable light filter 5 by at least 5° (e.g. by 25°, 35°, 45° or greater than 45°). This allows a gradual transition of the angle-dependent transmission values between the areas for an observer, which avoids visible edges, since in the said transition area there is also a transition of the field strength from the first electric field EF1 to the second electric field EF2 with the at least one intermediate, resulting third electric field EF3 (but preferably several electric fields EF3, EF4 etc.) is present, which in turn controls the angle-dependent transmission of the switchable light filter at such transition regions differently than in the said first or second region, where the first or second electric field EF1, EF2 is present.

As in 1 For example, it is possible, as shown, for the means SE1, SE2, ... for selectively generating at least a first electric field EF1 or a second electric field EF2 to comprise at least two opposing flat transparent electrodes (e.g. ITO layers) between which the liquid crystal layer 3 is located, wherein at least one of these transparent electrodes SE1, SE2 is interrupted near the boundary between the said two regions. In this embodiment, even the two transparent electrodes SE1, SE2 are interrupted near the boundary between the said two regions (indexed with EF1 and EF2). It is advantageous in this case that the respective interruptions of the transparent electrodes SE1, SE2 are not congruent when projected parallel perpendicular to the propagation direction of the first optical element 1 onto the first optical element 1, which is the case here. In this way, a gradual transition of the field strength in the transition region with the third electric field EF3 is achieved.

"Proximity" here means a maximum distance of ten percent of the largest dimension of the liquid crystal layer 3, preferably a maximum of 5 percent. This also applies to all subsequent cases.

By means of the aforementioned embodiments using one or two interrupted transparent electrodes, it is ensured that if the first electric field EF1 is selected for at least a first region and the second electric field EF2 is selected for at least a second region on the switchable light filter 5, an electric field EF3 is applied (or electric fields EF3, EF4 ... are applied) for at least one transition region between the at least two regions, which is different from the first and second electric fields EF1, EF2, which is essential for the implementation of the invention. However, the aforementioned list of embodiments is not exhaustive; further configurations can be found that achieve the aforementioned effect, for example by adding another transparent electrode in the vicinity of the transition region.

Furthermore, it is possible for the first electric field EF1 to act on the liquid crystal layer 3 in such a way that linearly polarized light penetrating it is rotated in its polarization orientation by at least 86°, preferably by at least 88°, while the second electric field EF2 to act on the liquid crystal layer 3 in such a way that linearly polarized light penetrating it is rotated in its polarization orientation by no more than 4°, preferably not at all. It is possible, for example, for either a first electric field EF1 or a second electric field EF2 to describe a field-free state for the first region, while the respective other electric field has an absolute field strength greater than zero, e.g., 0.5 MV/m, for the second region. Since the first and second electric fields EF1, EF2 differ between the two regions, an electric field EF3 is present in the transition region between the at least two regions, or electric fields EF3, EF4 ... are present that are different from the first and second electric fields EF1, EF2, so that in this at least one transition region, the transmission properties of the switchable light filter 5 differ from the transmission properties of the first operating mode B1 and the second operating mode B2. The liquid crystal layer 3 is, for example, a TN cell or a vertical alignment LC cell, each with a corresponding control. Other configurations are possible.

With an aforementioned embodiment, it is generally possible for the angle-dependent transmission properties of the switchable light filter 5 to differ between a first operating mode B1, in which the first electric field EF1 is applied, and a second operating mode B2, in which the second electric field EF2 is applied, in such a way that the transmissions for a respectively selectable but always identical angle, which deviates from the mid-perpendicular to the switchable light filter 5 by at least 45°, differ between the two operating modes by at least a factor of ten, preferably by at least a factor of 15.

Said first preferred direction can advantageously enclose an angle between 0° and 45° inclusive with a surface normal of the first optical element. Furthermore, in particular embodiments, the first preferred direction can vary across the surface of the first optical element. For the purposes of the invention, the first preferred direction averaged across the surface then applies.

Furthermore, 2 a schematic diagram in plan view of the means for generating electric fields in a further exemplary embodiment. Here, the interruption of a transparent electrode is zigzag-shaped in order to second electric field EF1, EF2 to generate a different, third electric field EF3.

The 3 shows an example graph for the transmission of a switchable light filter in the prior art. Relative locations on the light filter are indicated on the abscissa; a second electric field EF2 acts in the left part, and a first electric field EF1 in the right part. The ordinate indicates a normalized transmission of a switchable light filter in the prior art, specifically from an angle of, for example, 5° deviating from the first preferred direction (here: the bisector of the filter) on the corresponding light filter. It can be seen that when the jump from the first electric field EF1 to the second electric field EF2 occurs, the normalized transmission also changes abruptly, for example when an observer's eye (or a camera) moves along the spatial direction. This in turn means that the transition of the areas with the first and second electric fields EF1, EF2 is associated with the perception of a prominent edge for an observer as soon as he does not look exactly from the first preferred direction (here: the perpendicular bisector to the filter).

This problem is solved by the invention: 4 represents an example graph of the effect on the transmission of the switchable light filter 5. Here, too, relative locations on the light filter 5 are indicated on the abscissa; in the left-hand part, a second electric field EF2 acts, in the right-hand part a first electric field EF1, but in between a third electric field EF3, which in this case causes a gradual transition of the field strengths from the first to the second electric field EF1 or EF2. The ordinate indicates a normalized transmission of a switchable light filter 5, specifically from an angle of, for example, 5° deviating from the first preferred direction (here: the bisector of the filter) onto the light filter 5. It can be seen that in the transition region with the third electric field EF3, the normalized transmission also changes gradually, for example when an observer's eye (or a camera) moves along the spatial direction. This in turn means that the transition of the first and second regions with the first and second electric fields EF1, EF2 is not associated with the perception of a prominent edge for an observer as long as he does not look exactly from the first preferred direction (here: the perpendicular bisector of the filter).

Materials within the meaning of the invention that contain fixed or alignable transition dipole moments include, for example, dichroic dyes or dye mixtures combined with a carrier material that does not impair the properties, such as liquid crystals or polymers. The optical element can, for example, contain only one dye. However, multiple dyes, i.e., dye mixtures, can also be contained in a single optical element. These materials relax essentially through non-radiative processes.

For 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 first optical element are conceivable. Alternatively, configurations of a first optical element or each layer therein are conceivable in which the transition dipole moments are embedded in a liquid subjected to an electrowetting process. In this way, the density of the transition dipole moments can be varied, in particular, but not exclusively.

The invention solves the stated problem: Switchable light filters and their implementations have been described, in which light incident upon them is selectively transmitted or partially or (almost) completely absorbed at various locations on the light filter or screen. This avoids visible edges between areas of different transmission ranges. Furthermore, a corresponding screen or lighting device has been described whose angle-dependent brightness distribution can be partially modified, while also avoiding visible edges between areas in different modes.

Furthermore, the switchable light filter or systems using it 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.

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

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 preferential direction selectable for the first optical element (1) or vary around this, • so that light 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 (SE1, SE2, ...) for selectively generating at least one first electric field (EF1) or a second electric field (EF2), wherein the first or the second electric field (EF1, EF2) can be selected separately for at least two areas on the switchable light filter (5), - a liquid crystal layer (3) arranged in front of or behind the first optical element (1), on which liquid crystal layer the first electric field (EF1) or the second electric field (EF2) acts and the depending on this, the polarization state of light penetrating through it is influenced, 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 present, and a second operating mode B2, in which the second electric field (EF2) is present, - characterized in that , if the first electric field (EF1) is selected for at least one first region on the switchable light filter (5) and the second electric field (EF2) is selected for at least one second region, an electric field (EF3) is applied for at least one transition region between the at least two regions, or electric fields (EF3, EF4 ...) are applied which are different from the first and the second electric field (EF1, EF2), so that in this at least one transition region the transmission properties of the switchable light filter (5) differ from the transmission properties of the first operating mode B1 and the second operating mode B2. Switchable light filter (5) according to Claim 1 , characterized in that , if the first electric field (EF1) is selected for at least a first region on the switchable light filter (5) and the second electric field (EF2) is selected for at least a second region, electric fields (EF3, EF4 ...) are applied for at least one transition region between the two regions, which represent a step-by-step and/or gradual transition from the first electric field (EF1) to the second electric field (EF2), so that around the at least one transition region between the two regions at least three, preferably at least five, particularly preferably at least ten different values of the transmission are generated for a respectively selectable but always identical angle which deviates from the perpendicular bisector to the switchable light filter (5) by at least 5°. Switchable light filter (5) according to Claim 1 or 2 , characterized in that the means (SE1, SE2, ...) for selectively generating at least a first electric field (EF1) or a second electric field (EF2) comprise at least two opposing flat transparent electrodes between which the liquid crystal layer (3) is located, at least one of these transparent electrodes being interrupted near the boundary between said two regions. Switchable light filter (5) according to Claim 3 , characterized in that the at least one interrupted transparent electrode is interrupted several times near the boundary between said two regions. Switchable light filter (5) according to Claim 3 , characterized in that the two transparent electrodes are interrupted near the boundary between said two regions. Switchable light filter (5) according to Claim 5 , characterized in that the respective interruptions of the transparent electrodes are not congruent in the case of parallel projection perpendicular to the propagation direction of the first optical element (1) onto the first optical element (1). Switchable light filter (5) according to one of the preceding claims, characterized in that the first electric field (EF1) acts on the liquid crystal layer (3) in such a way that linearly polarized light penetrating through it is rotated in its polarization orientation by at least 86°, while the second electric field (EF2) acts on the liquid crystal layer (3) in such a way that linearly polarized light penetrating through it is rotated in its polarization orientation by no more than 4°, preferably not at all. Switchable light filter (5) according to Claim 7 , characterized in that the transmission properties of the switchable light filter (5) are angle-dependent and can be adjusted 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, differ in such a way that the transmissions for a respectively selectable but always identical angle, which deviates from the mid-perpendicular to the switchable light filter (5) by at least 45°, differ between the two operating modes by at least a factor of ten, preferably by at least a factor of 15. Lighting device for a screen, which can be operated in at least two operating modes B1 for a free view mode and B2 for a restricted view mode, in which light is emitted into a viewing angle range which is restricted for a viewer compared to the free view mode, comprising - a surface-like extended background lighting which emits light, and - a switchable light filter (5) according to one of the Claims 1 until 8 . Screen, comprising - an image display device, - in the viewing direction in front of or behind the image display device a switchable light filter (5) according to one of the Claims 1 until 8 .