A solution to the multiplexability and viewing- angle problems associated with LCDs has been proposed in the form of UVLCD technology whereby ultra-violet light from backlights is modulated by means of a liquid-crystal layer, the light which is allowed through being incident on front panel phosphors/ fluorescers which consequently emit visible light toward the viewer. For a colour display RGB light is emitted. This arrangement provides an attractive emissive colour display, but has the same disadvantage as cathode-ray tubes that in conditions of strong ambient light increased phosphor excitation power is required to prevent washout.

Under strong illumination conditions 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.

According to the invention there is provided 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.

When the activating radiation is input, 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. In order to ensure transparency the layer should be much less than 2μm thick, preferably 0.05μm-0.5μm. Although 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. The advantage of such polymers is that they can be spin-coated as thin, largely transparent layers. For colour displays, it is necessary to use three different colours of phosphor. 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.

In one preferred form of the invention 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. When using a modulating layer of this type the input radiation is most advantageously guided to the liquid-crystal layer via a Totally Internally Reflecting (TIR) backplane; the input radiation stays in the backplane until it encounters a scattering cell in the liquid-crystal layer, whereupon some of it is decoupled from the plane and from the modulating layer

and emerges as output or to activate an output means such as a phosphor as mentioned above. Emissive displays using a light guide and phosphors are known: US 4668049 (ITT/Canter) 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. 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. This results in a device which when directly viewed against a black background shows a black state and a reflective single-coloured (monochromatic) state. Such devices have been developed into commercial displays by Kent State University and Kent Display Systems. Similar devices have been developed by Advanced Display Systems Inc., which show a multiplicity of brightness states between black and brightly coloured. There are such displays on the market, which are purely ambient-light devices but are suitable for use in emissive/reflective displays in accordance with the invention.

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.

For a better understanding of the invention embodiments will now be described, by way of example, referring to the accompanying drawings, in which: 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, and

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. Alternatively the backplane could simply be the rear glass itself.

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.

Two liquid-crystal cells A and B are marked in the diagram. 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.

During operation purely under ambient light 30 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. in this case UV light, and allow other, such as visible, wavelengths to pass; hence the phosphors do not interfere with this mode of operation. If 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. Upon such switching, however, as shown at cell A, 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. This escaped UV light then hits the corresponding phosphor which then emits visible light in all directions. This operation is similar to that shown in US 4668049 mentioned above. If the phosphor emission spectrum is matched with the selective reflection band of the liquid crystal, then that part of the light emitted by the phosphor in a direction away from the viewer will be reflected forwards again by the LC cell, thus improving display brightness. Similarly, any incident light will pass through the phosphor to be selectively reflected forward again by the LC, and increase the viewed intensity. Thus a display with matched selective reflection will be brighter than a standard phosphor- emission UVLCD display, and will have a contrast which increases with increased incident illumination, i.e. there will be no "wash-out". (The dark state can be made to stay dark even as external illumination increases by virtue of the black absorbing layer behind the transparent backplane, as shown at 25).

Although the example given used UV light to

activate the phosphors the excitation light could be visible rather than UV, with phosphors tuned to match. Moreover, 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. Moreover 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) . In the example described above 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. However, if the selective reflection wavelength (for daylight viewing) is the same as the wavelength of visible light introduced for night viewing, then the strength of the scattering may be "killed" by the reflection. For many applications, though, 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. It is not, however, compatible with the TIR-scattering type of device. Figures 2 and 3 show a second embodiment, which functions in the same way as the first, although constructed slightly differently. To make this device, 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, lOμm 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 5μm 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. BL088 has a positive dielectric anisotropy (+16.0 at 20°C), and exhibits a selective reflection band which has a central wavelength λ _c =522nm at 20°C. To obtain a clear state a sine wave of δOV-^ _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.

To produce the device 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 ) . With the phosphor screen removed the cell in its clear state thus looked black under ambient illumination. When in the scattering/reflecting state, the cell appeared a vivid green. To observe the emissive states the illumination was switched on and the screen replaced. When the cell was in the scattering/reflecting state 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. With the cell in the clear state on the other hand 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. In this embodiment, as shown in Fig. 4, 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. In this embodiment 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.