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
  • G02F1/133—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
  • G02F1/1336—Illuminating devices
  • G02F1/133615—Edge-illuminating devices, i.e. illuminating from the side
• • 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
• • 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
  • G02F1/13756—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 the liquid crystal selectively assuming a light-scattering state
• • 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
  • G02F2203/00—Function characteristic
  • G02F2203/09—Function characteristic transflective

Definitions

• the invention is concerned with liquid-crystal- type displays ( CDs) .
• a reflective technology is more efficient: here a liquid crystal switches between a reflective and a transparent state, the former appearing light and the latter appearing dark. Under these conditions any increase in ambient light actually enhances the display brightness and contrast, as is the case for instance in ordinary calculator and watch displays. It is an aim of the invention to provide a display usable both in darkness and in ambient light.
• a display including a layer of material adapted to modulate both activating radiation input from one side of the layer in order to give rise to an output from the display, and also ambient light impinging on the display from the other, viewing, side, in which for modulation the said material can be switched between an "Off” state in which ambient light is not returned by, i.e. is absorbed by or penetrates, the display while the input radiation is prevented from causing output from the device; and an "On" state in which at least some of the ambient light is reflected or otherwise returned from the layer of modulating material while any input radiation causes an output from the device.
• This device will function in ambient light as a reflecting display, in that the modulating layer either reflects ambient light or not, while if further light output is required, for instance in low ambient lighting, the input, or activating, radiation can be supplied from the rear or the side of the display.
• an output will be generated from those parts of the modulating material layer which are at any moment in the On state.
• This output can be either direct, in that the input radiation is scattered and emerges itself as the output light, or, in preferred forms of the display, indirect, where for instance the scattered input radiation is visible or UV and activates photoluminescent, e.g. phosphor, material.
• the photoluminescent layer needs to be transparent so as not to interfere with the reflected light in ambient light mode.
• the photoluminescent layer may be made from a thin-film deposition of phosphor material, fluorescent glass or light-emitting polymers.
• the selective reflection band of each LC device (which depends on the cholesteric pitch) preferably matches the emission spectrum of the phosphor pixels used for that device.
• Fluorescent glass may be made by incorporating ionic species, normally rare earths, into glass.
• the ions provide a radiative recombination route for carriers excited by light (normally UV) or an electron beam. In the visible region, such glasses are normally highly transparent.
• Such glass is commercially available, supplied, for example, by Quantum Glass Ltd, UK.
• Suitable thin film phosphors may be made by depositing a very thin layer of phosphor onto a substrate.
• a typical material for a Green phosphor would be reduced Zinc Oxide, which emits green light under UVA excitation.
• the layer should be much less than 2 ⁇ m thick, preferably 0.05 ⁇ m-0.5 ⁇ m.
• depositing such thin layers reduces the screen conversion efficiency, the resulting screens are nonetheless visible in low ambient light conditions.
• the third option is to use light-emitting polymers which emit visible light when excited by UVA. Presently available materials are not very stable under UVA exposure, and so such polymers may be most useful in applications for which very long lifetimes are not critical. Encapsulating the polymer improves stability.
• Standard cathode ray tube phosphors may be used, i.e. Blue Y 2 0 2 S: Eu , green - ZnS: (Cu, Al, Au) and red - ZnS:Ag.
• the modulating layer is a liquid crystal which is transparent in the Off state, resulting from the application of a first voltage waveform, and scatters and reflects in the On state, resulting from the application of a second voltage waveform.
• the first waveform may be continuous while the second is advantageously a single pulse.
• TIR Totally Internally Reflecting
• US 4668049 discloses the use of bistable scattering cells in a TIR backplane architecture to scatter UV light selectively out of the backplane and towards an opaque photoluminescent screen made of RGB phosphors to produce coloured emission.
• Displays in accordance with this kind of embodiment of the invention thus produce the viewed image by means of two electro-optic effects.
• These effects are, for instance, firstly selective reflection in the visible range of ambient light from the modulating means, preferably a tight-pitch cholesteric liquid crystal, and secondly forward scattering of input radiation, e.g. UV light, from the TIR lightguide structure within the display on to visible-emissive phosphors.
• This can be realised using a cholesteric LC effect which in one state is transparent to visible light and in the other both reflects one handedness of a particular wavelength, or range of wavelengths, and scatters, and by using a photoluminescent means such as a phosphor material which is substantially transparent in the visible when not emitting.
• Liquid crystals of the type in question are known. Recently there has been much interest in "cholesteric gel", or polymer-stabilised cholesteric transition (PSCT), devices, which are effectively nematic- cholesteric phase-transition (NCPT) devices with cholesteric pitches selected to produce a selective reflection band in the visible spectrum.
• PSCT polymer-stabilised cholesteric transition
• NCPT nematic- cholesteric phase-transition
• Such devices can be designed to possess two stable states; a purely scattering or 'white' state and a state which both selectively reflects at one wavelength and scatters generally at the same time. The amount of scattering in the 'white' state can be reduced with proper alignment techniques and thickness optimisation until it is negligible, i.e. the device is transparent.
• the two states can be induced by the application of lower and higher voltage pulses respectively.
• the LC can also be switched by the continuous application of a waveform such as a 30V RMS sine wave to a clear, homeotropic state.
• the invention also envisages use of this effect, though it is not suitable for multiplexing.
• FIG. 1 shows a schematic drawing of a first embodiment of the device
• Fig. 2 shows views of the illuminated optical backplane used in a second embodiment
• Fig. 3 shows a section of the second embodiment of the device
• Fig. 4 shows graphs of the wavelength dependencies of (a) the transmittance of light through the NCPT device and (b) the emissivity of the green phosphor screen of the second embodiment.
• Fig. 1 shows the first embodiment in which a NCPT liquid-crystal modulating layer 1 is sandwiched in the usual way between two glass plates 5.
• the liquid- crystal layer 1 is partitioned into cells by orthogonal electrodes 3, again in a conventional manner.
• the liquid-crystal assembly consisting of the layer 1 itself and the plates is optically connected to a TIR backplane 7 on the rear side, with respect to a viewer.
• the backplane could simply be the rear glass itself.
• the liquid crystal At the front side, i.e. the viewing side, of the liquid crystal is a further glass substrate 13 on the rear side of which phosphor dots 11 are placed in correspondence to the cells in the liquid crystal.
• the phosphor material is transparent to the ambient light 30.
• Cell A is switched by the application of a suitable voltage waveform, such as a 30V rectangular pulse, to a scattering/reflecting bistable state, in which it remains after the voltage is removed, while cell B has a suitable voltage, such as a 30V RMS sine wave, continuously applied and is essentially transparent.
• a suitable voltage waveform such as a 30V rectangular pulse
• the ambient light passes through the inactivated cell B and the TIR plane and is absorbed in the system, for instance in a dark rear layer 25 optically separate from the TIR plane. It would also be possible to use the cell in a transparent mode, where the display is super-imposed on the background. Meanwhile the light striking the activated cell A is scattered, and also that portion of the light which has a wavelength and a handedness corresponding to the pitch of the cell material is reflected from the cexl. The cell therefore appears to the viewer as a bright dot tinged with a colour corresponding to the reflected wavelength. This reflected light passes through the phosphors because they absorb only the excitation light, i.e.
• UV light and allow other, such as visible, wavelengths to pass; hence the phosphors do not interfere with this mode of operation.
• input UV light 20 is supplied to the system, for instance from one or more edges of the TIR backplane 7, it is contained by total internal reflection within the combination of back-plane and liquid-crystal assembly because the refractive index of the LC itself 1 and its glass plates 5 is close to that of the backplane 7, as long as the liquid crystal is not switched to its active, scattering state.
• the UV light proceeds in various directions indicated by small arrows, some of which are sufficiently near the normal to the plane for the light to escape from the front of the cell.
• the excitation light could be visible rather than UV, with phosphors tuned to match.
• the phosphors can even be dispensed with altogether, since for monochrome displays in the reflective cholesteric mode with incident light one can use a monochrome visible input light to be scattered and there is no need for a phosphor to change the viewed light colour.
• the viewing angle is adequate for many purposes, since all light scattered at angles less than the critical angle will emerge from the front face (typically around 42° for glass to air).
• This arrangement has the characteristic that the colour of the scattered light (used in darkness) may have to be different from the reflected light (used in daylight) .
• a cell was used which had its purely scattering state made non- scattering by correct alignment, and which both scattered and selectively reflected in the other state.
• the selective reflection wavelength for daylight viewing
• the strength of the scattering may be "killed" by the reflection.
• a difference in colour will not be a drawback.
• Another variation involves having the phosphor inside rather than externally of the LC cell. This possibility, used for STN cells, is shown for instance in US-A-4830469.
• Figures 2 and 3 show a second embodiment, which functions in the same way as the first, although constructed slightly differently.
• a "Spectrasil" fused silica substrate (31) was machined to the square shape shown in Fig. 2. All machined surfaces were subsequently polished.
• Eight cold- cathode miniature mercury fluorescent lamps (33) were positioned around the edges of the substrate.
• a nematic-cholesteric phase transition (NCPT) device (35) was placed on the top surface of the illuminated back ⁇ plane, as shown in Fig. 3.
• a drop of silicon oil (37) was placed between the liquid crystal cell and the surface of the silica substrate (31) to ensure good optical contact.
• a phosphor screen (39,41) was placed above the cell, separated from the cell's surface by a small air gap.
• a filter (43) was necessary between the cell and the phosphor in order to remove the visible emissions of the Hg discharge: a 5mm thick UG5 filter from Schott was used for this purpose.
• the green-emitting phosphor screen was formed by depositing a P22G phosphor layer (41) (ZnS:Cu,Au,Al, lO ⁇ m mean diameter particles), onto a glass substrate (39 ) using a settling method with Barium Chloride and Potassium Silicate solutions.
• the screen was not transparent but was mounted removably over the liquid- crystal cell by way of spacers (45).
• a 5 ⁇ m cell gap was used for the NCPT liquid crystal device, which was assembled in the usual way and filled with the cholesteric liquid crystal mixture BL088 (Merck). The non-active regions of the cell were masked with black tape on the surface which was not in contact with the back-plane.
• ⁇ c 522nm at 20°C.
• a sine wave of ⁇ OV- ⁇ g was applied to the electrodes of the cell. Terminating the 50V rms signal abruptly by applying a zero-voltage signal caused the device to adopt an optical state which both scattered and selectively reflected incident light.
• the cell was mounted on the illuminating back-plane (31, 33) which in turn was placed, though without optical contact, on a matt black surface (47 ) .
• the phosphor screen removed the cell in its clear state thus looked black under ambient illumination.
• the illumination was switched on and the screen replaced.
• UV activating illumination was scattered from within the back-plane towards the phosphor screen and the green emissions could be easily observed under ambient illumination.
• the green emissions were very faint under ambient illumination.
• the contrast of the emitted flux between the two states was measured to be better than 52:1.
• the spectral distribution of flux emitted from the green phosphor (curve b) generally matched the wavelengths reflected by the NCPT device when in the scattering/ reflecting state - curve a in Figure 4 shows the transmitted flux in the scattering state. Therefore, when in the scattering state the NCPT device reflects some of the backward emitted visible light from the phosphor back towards the screen, enhancing brightness.
• the two functions of the display, emissive and reflective are achieved by making the phosphor screen removable. Clearly in a working display it is simpler to use a screen which is transparent to the ambient light.

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• 1996
  • 1996-04-11 GB GBGB9607531.2A patent/GB9607531D0/en active Pending
• 1997
  • 1997-04-10 EP EP97916534A patent/EP0963569A1/de not_active Withdrawn
  • 1997-04-10 WO PCT/GB1997/000997 patent/WO1997038348A1/en not_active Application Discontinuation
  • 1997-04-10 AU AU25154/97A patent/AU2515497A/en not_active Abandoned

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