WO2024251453A1 - Filtre de lumière commutable, dispositif d'éclairage et écran équipé d'un tel filtre de lumière commutable - Google Patents

Filtre de lumière commutable, dispositif d'éclairage et écran équipé d'un tel filtre de lumière commutable Download PDF

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Publication number
WO2024251453A1
WO2024251453A1 PCT/EP2024/062680 EP2024062680W WO2024251453A1 WO 2024251453 A1 WO2024251453 A1 WO 2024251453A1 EP 2024062680 W EP2024062680 W EP 2024062680W WO 2024251453 A1 WO2024251453 A1 WO 2024251453A1
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Prior art keywords
light
light filter
electric field
switchable
optical element
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German (de)
English (en)
Inventor
André HEBER
Markus Klippstein
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SiOptica GmbH
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SiOptica GmbH
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Priority to TW113118262A priority Critical patent/TW202501042A/zh
Publication of WO2024251453A1 publication Critical patent/WO2024251453A1/fr
Anticipated expiration legal-status Critical
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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
    • G02F1/13725—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on guest-host interaction
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/1323—Arrangements for providing a switchable viewing angle
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
    • G02F1/13363—Birefringent elements, e.g. for optical compensation
    • G02F1/133637—Birefringent elements, e.g. for optical compensation characterised by the wavelength dispersion
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09F—DISPLAYING; ADVERTISING; SIGNS; LABELS OR NAME-PLATES; SEALS
    • G09F9/00—Indicating arrangements for variable information in which the information is built-up on a support by selection or combination of individual elements
    • G09F9/30—Indicating arrangements for variable information in which the information is built-up on a support by selection or combination of individual elements in which the desired character or characters are formed by combining individual elements
    • G09F9/35—Indicating arrangements for variable information in which the information is built-up on a support by selection or combination of individual elements in which the desired character or characters are formed by combining individual elements being liquid crystals

Definitions

  • US 5,993,940 A describes the use of a film that has small, strip-shaped prisms evenly arranged on its surface in order to achieve a private mode, i.e. a restricted viewing mode with a small viewing angle range. Development and production are technically quite complex.
  • US 2012/0235891 A1 describes a very complex backlight - a background lighting - in a screen.
  • Fig.1 and 15 not only several light guides are used there, 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.
  • 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 - as already mentioned above - greatly reduces brightness.
  • 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.
  • a transmissive image display device eg an LC display
  • a screen can be created that can be switched between free and restricted viewing mode.
  • the 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.
  • residual light is still visible from blocked viewing angles.
  • WO 2015/121398 A1 of 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.
  • the scattering particles made of a polymer selected there generally have the disadvantage that light is coupled out of both large surfaces, whereby about half 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.
  • 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 privacy effect in the protected operating mode.
  • 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 propagation of the light.
  • 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.
  • a high-quality angle restriction should also be achieved for viewing positions in which a viewer not only looks 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.
  • first optical element in turn comprising a plurality 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 or vary around this, 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,
  • 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,
  • the liquid crystal layer comprises at least one TN cell and a retarder film, wherein said retarder film cancels out the polarization change carried out by the TN cell in the first operating mode B1 for light penetrating the switchable light filter by at least 80% (ideally even by 90%, 95% or more; this can be achieved e.g. by means of a discotic film), or the liquid crystal layer 3, if it is realized e.g.
  • the switchable light filter further comprises a negatively dispersive optical element (eg a negatively dispersive retardation film), or several optical elements which in interaction 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 (especially in the case of white or polychrome light) are at least partially compensated.
  • a negatively dispersive optical element eg a negatively dispersive retardation film
  • several optical elements which in interaction act like a negatively dispersive optical element this can be achieved by combining different elements with different levels of dispersion
  • 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 LC cells are optimized for maximum transmission under normal incidence.
  • 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 increasing the electric fields.
  • the main challenge in implementing the described privacy filters in screens is that numerous components are structured and located close to one another, which often leads to moiré artifacts.
  • three subpixels usually form a full pixel. It is important that the individual subpixels are electrically and optically isolated from one another in order to prevent color distortions and to be able to cover as large a part of the color space as possible.
  • this is achieved by a black mask between neighboring subpixels.
  • the LC cell for controlling the privacy protection 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.
  • a polarization filter is also present, which can be arranged upstream or downstream of the first optical element in the direction of incidence.
  • 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
  • the transition dipole moment - also referred to as transition matrix element - is a quantum mechanical vector quantity and is associated with a specific transition between an initial state - usually the ground state - and a final state - usually an excited state - of a system, ie 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, 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, in which 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 which 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.
  • the density of the said transition dipole moments, their strength or the refractive index in the layers of the optical element can vary.
  • the volume density of the transition dipoles can approach 100%.
  • the transition dipole moments in the optical element are oriented parallel to the plane of incidence of the light on the optical element.
  • the plane of incidence does not refer to the surface of the optical element, but rather to 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.
  • the light wave as a transverse 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 which strikes the optical element is initially unpolarized in its entirety of the light waves, i.e.
  • the directions of oscillation of the transverse-electrical - and correspondingly also the transverse-magnetic - components are statistically distributed.
  • the light When it strikes 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 which lie in the plane of incidence are absorbed.
  • Light which is polarized parallel to the plane of incidence i.e. transverse-magnetic or p-polarized, is absorbed, whereas light which is polarized perpendicular to the plane of incidence - i.e. parallel to the surface of the optical element - i.e.
  • transverse-electrical or s-polarized is fully 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 passing through the optical element with transition dipole moments that are aligned parallel to the plane of incidence of the light.
  • a is the angle between the propagation direction and the surface normal
  • d(a) is the optical path length as a function of the propagation direction
  • N is the number of absorbing molecules per volume
  • o abs (a) is the absorption cross section as a function of the angle of incidence.
  • 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.
  • a switchable light filter comprising
  • the liquid crystal layer comprises at least one TN cell and a retarder film, wherein the said retarder film has the Operating mode B1 by the TN cell cancels out the change in polarization for the light penetrating the switchable light filter 5a by at least 80% (preferably even by 90%, 95% or more; this can be achieved by means of a discotic film, for example), 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
  • 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 (particularly in the case of white or colored light) are at least partially compensated.
  • 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.
  • 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.
  • thermosettling LCs and epoxy LC 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 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.
  • 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 pm to 50 pm, preferably in the range from 0.5 pm to 20 pm, 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 is a mixture of liquid crystals with at least one dye, especially with at least one dichroic dye mixture, for each layer.
  • dichroic dyes or dye mixtures that can be used are azomethine dyes, indigoid and thioindigoid dyes, merocyanines, azulene, quinophthalone dyes, perylene dyes, phthaloperine dyes, dioxazine dyes, triphenodioxazine dyes,
  • a polarization filter P is also present, which can be arranged upstream or downstream of the first optical element in the direction of incidence.
  • a ⁇ /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.
  • 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 put 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 thus assume at least two states of action. In particular in such embodiments it is conceivable that more than two states, e.g.
  • liquid crystal cells are also conceivable.
  • 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.
  • volume densities of the transition dipole moments between 0.1% and 90% based on liquid crystals in the optical element are conceivable.
  • designs of an optical element or each layer therein are conceivable in which the transition dipole moments are embedded in a liquid which 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.
  • 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
  • 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.
  • 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.
  • the means for selectively generating at least one first electric field EF1 or one 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.
  • the first embodiment of a switchable light filter can be expanded by a second optical element that is identical in construction to the first optical element (or also varies with respect to its preferred direction), 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.
  • the second embodiment 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.
  • the first optical element (and if present, any other identical optical element) in the first embodiment of the switchable light filter or the liquid crystal layer in the second embodiment of the switchable light filter is constructed in a non-periodic manner. 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.
  • the respective preferred direction of a transition dipole moment can be selected depending on its position on the respective optical element.
  • each first optical element is divided into different regions (A1, A2, ...) along a selectable reference line on the respective first optical element, wherein for each region (A1, A2, ...) a separate region preferred direction can be selected, which for all transition dipole moments of the corresponding first optical element, whereby all preferred directions of the ranges 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 applicable range, all transition dipole moments are therefore aligned parallel to the preferred direction applicable there with a tolerance of maximum +/-10 degrees .
  • 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.
  • 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 an observer.
  • the operating modes of the The properties 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 public viewing mode that is free in the horizontal direction with an unrestricted radiation or viewing angle range and a private viewing mode that is restricted in the horizontal direction with a radiation or viewing angle range that is restricted in the horizontal direction compared to the free viewing mode, so that viewers who are positioned outside of this restricted viewing angle range can only perceive image content that is 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.
  • 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 via the angles.
  • 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.
  • 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
  • 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,
  • the linear polarization filter is arranged in the transmissive image display device or is a part of it.
  • 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.
  • 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 is also possible. 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 operating modes mentioned. to protect sensitive data, ie to display it so that it is visible to only one viewer, or alternatively to display image content simultaneously for several viewers.
  • Fig.1 shows the schematic diagram of an exemplary switchable light filter
  • Fig.3a and 3b show the exemplary (simulated) dispersion of half-wave plates. Detailed description of the drawings
  • Fig. 1 shows the schematic diagram of an exemplary switchable light filter 5.
  • this switchable light filter 5 comprises
  • first optical element 1 in turn comprising a plurality 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,
  • 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,
  • the liquid crystal layer 3 comprises at least one TN cell and a delay film (not shown in the drawing), wherein the said delay 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 achieved, for example, by means of a discotic film), or the liquid crystal layer 3, if it is achieved, 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
  • a negatively dispersive retardation film or several optical elements that act together like a negatively dispersive optical element (this can achieved by combining different elements with different levels of dispersion), so that color shifts of the light penetrating the switchable light filter 5 (particularly in the case of white or polychrome light) are at least partially compensated.
  • the mode of operation of the aforementioned measures is explained in more detail below:
  • the electric field in the visibility-restricted mode must oscillate to the greatest possible extent in the plane of incidence of the light.
  • One way of making 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 which are 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 an 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 the purest possible polarization states to function optimally.
  • FIG. 2a and 2b show the simulations of a twisted nematic liquid crystal cell.
  • a parallel cell geometry with a thickness of 5 pm was assumed, i.e. the liquid crystal molecules are limited in their extension by two plates.
  • the LC molecules have positive birefringence and that the polarity of the LC molecules is oriented parallel to their long axis.
  • FIG. 2a and 2b show the orientation of the liquid crystal molecules of a 90° exemplary, rotated nematic liquid crystal cell in the field-free state (solid line) and the field-affected state (dashed line), where Fig. 2a represents the angle of rotation and Fig. 2b the angle of inclination.
  • the orientation of the liquid crystal molecules in the field-free state (solid line) is determined 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 Fig.2a) in the plane of the interfaces.
  • the angle of inclination (see Fig.2b) describes the angle of the liquid crystal molecules to the first interface.
  • 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. The molecules are aligned throughout the layer through molecule-molecule interactions.
  • 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 equilibrium of the intramolecular interaction and the electric field. For the calculation, the free energy is minimized.
  • 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 Fig. 2a) initially changes slightly, before abruptly rotating from 0° to 90°.
  • liquid crystal cell types ADS, FFS and IPS have significantly lower transmissions (ie better black values), which means that the polarization state of the light after the LC cell deviates only slightly from linear polarization.
  • FFS, IPS and ADS liquid crystal cell geometries are essentially the same.
  • the linearly polarized light is transmitted essentially without any change in polarization.
  • 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 of varying strength. This changes the polarization of the light to varying degrees.
  • such LC cells are optimized for maximum transmission under normal incidence.
  • 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.
  • performance can be improved by using passive retardation plates.
  • performance is improved by using retardation plates with negative birefringence to reverse the birefringence of the LC cell.
  • the retardation plate ensures that the light is transmitted as linearly as possible.
  • Figures 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.
  • a negatively dispersive optical element e.g. a negatively dispersive retardation film
  • 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.
  • Fig. 3a and 3b show the (simulated) dispersion of a half-wave plate in units of the phase delay (Fig. 3a) and the optical path length difference (Fig. 3b). The four possible cases are shown in each case.
  • Negative dispersion The difference in refractive index between ordinary and extraordinary refractive index becomes smaller with decreasing wavelength
  • phase delay shown in Fig.3a is: - - -
  • d stands for the respective thickness of the half-wave plate
  • n e for the extraordinary refractive index and n 0 for the ordinary refractive index
  • X for the respective wavelength
  • 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
  • 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.
  • Suitable 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.
  • the density of the said transition dipole moments, their strength or the refractive index in the layers of the optical element can vary.
  • the volume density of the transition dipoles can approach 100%.
  • Fig.1 can also be used to explain a second embodiment of an exemplary switchable light filter 5a. This comprises
  • first optical element 1 a which comprises a plurality of light-absorbing transition dipole moments, which are 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 parallel at least in a first state with a tolerance of a maximum of 20° (preferably a maximum of 10°) is aligned with a first preferred direction that can be selected for the first optical element 1 or varies around this, so that light that is incident on the first optical element 1 a is transmitted or at least partially absorbed depending on its polarization state and its direction of incidence relative to the first optical element 1 a, 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 1 a 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 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,
  • the liquid crystal layer 3a comprises at least one TN cell and a delay film (not shown in the drawing), wherein the said delay film cancels out the polarization change carried out by the TN cell in the first operating mode B1 for the 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 e.g.
  • the switchable light filter 5a further comprises a negatively dispersive optical element (e.g.
  • 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.
  • thermosettling LCs and epoxy LCs 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.
  • 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 pm to 50 pm, preferably in the range from 0.5 pm to 20 pm, all boundary values included.
  • the dyes or dye mixtures in the optical element 1, 1a can also be formed differently in its different layers, if present.
  • liquid crystal cells are also conceivable.
  • 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 therefrom, i.e. has at least one other preferred direction.
  • linearly or elliptically polarized light in the case of elliptically polarized light, the ratios of the magnitudes of the semi-axes at least 1:4, better at least 1:10 or greater
  • linearly or elliptically polarized light 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 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 5a, is absorbed to at least 85%, and that
  • 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 PLC, so that local switching between the respective possible operating states is possible.
  • EF1 first electric field
  • EF2 second electric field
  • 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).
  • EF1 first electric field
  • EF2 second electric field
  • EF3 further electric fields
  • 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.
  • an image display unit e.g. OLED, microLED, LCD
  • 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 polarization filter of which corresponds to the polarization filter of the switchable light filter 5, 5a.
  • This can be the front or rear polarizer in the LCD structure.
  • 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 via the angles.
  • Switchable light filters have been described (or systems using such filters) which can be implemented inexpensively and can be used universally with various types of screens in order to switch 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.
  • 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.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Dispersion Chemistry (AREA)
  • Mathematical Physics (AREA)
  • Liquid Crystal (AREA)

Abstract

L'invention concerne un filtre de lumière (5, 5a) commutable, comprenant un premier élément (1) optique, comprenant lui-même une pluralité de moments dipolaires de transition absorbant la lumière, de sorte que la lumière incidente sur le premier élément (1) optique soit transmise ou au moins partiellement absorbée en fonction de son état de polarisation et de sa direction d'incidence par rapport au premier élément (1) optique, des moyens pour générer sélectivement un premier champ (EF1) électrique ou un deuxième champ (EF2) électrique, une couche de cristaux liquides (3) disposée devant ou derrière le premier élément (1) optique, sur laquelle le premier champ (EF1) électrique ou le deuxième champ (EF2) électrique agit et qui, en fonction de celui-ci, influe sur l'état de polarisation de la lumière qui la traverse, de sorte que les propriétés de transmission du filtre de lumière (5) commutable diffèrent entre un premier mode de fonctionnement (B1), dans lequel le premier champ (EF1) électrique est appliqué, et un deuxième mode de fonctionnement (B2), dans lequel le premier champ (EF2) électrique est appliqué, des améliorations étant apportées à la couche de cristaux liquides (3) ou à l'empilement optique, respectivement, afin d'améliorer les propriétés de transmission.
PCT/EP2024/062680 2023-06-07 2024-05-08 Filtre de lumière commutable, dispositif d'éclairage et écran équipé d'un tel filtre de lumière commutable Ceased WO2024251453A1 (fr)

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