LIQUID-CRYSTAL DISPLAY USING UV LIGHT SOURCE

Abstract: A particularly useful form of light source is disclosed for UV/phosphor-type liquid-crystal displays. The source is a discharge lamp (1) lined with a first phosphor (3) that emits UVA light (5) which is then modulated by a liquid-crystal device (9) before reaching a second phosphor (11) acting as a display surface and emitting visible light when excited by the modulated narrow-band UVA light. To provide the narrow bandwidth the first phosphor is preferably Eu<2+>-doped CaSO4. The CaSO4:Eu<2+> phosphor is prepared by heating a mixture of CaSO4, (NH4)2SO4 and EU2O3 in an inert atmosphere, cooling the resultant mixture, adding a reducing agent, in particular NH4Cl, heating again in an inert atmosphere and grinding or otherwise processing to the required particle size.

Description: Licruid-Crystal Display using UV light source The invention relates to liquid-crystal displays, particularly those having a source of narrow-band UVA light to activate phosphors. Photoluminescent liquid-crystal display (PLLCD) architectures, as described, for example, in the patent applications GB 2154355 (Ricoh Ltd) , WO 95/27920 (Crossland et al) and GB 2291734 (Samsung) , utilise a combination of a light source, liquid-crystal cells and a phosphor material to produce an image. This combines the viewing-angle characteristics of a CRT with the compactness and cost of a passive LCD. The specific components of such a system can include cholesteric polarisers and compensating retarders as well as the liquid crystal cells. These components tend to be optimised for a specific wavelength and to work less well at other wavelengths. As a consequence, if the bandwidth of the excitation light is large, the performance of the components generally deteriorates. Superior display performance can be obtained by directing the excitation light as closely as possible to the optimum direction and in a narrow spectral range . It has been found advisable when constructing such displays to use UVA light (a wavelength of about 350 - 400 nm) rather than shorter wavelengths. Such light is transmitted through conventional glasses and polymers better than shorter-wavelength UV light, and yet still has enough energy to excite red, green and blue phosphors. Also, such longer UV wavelengths cause less degradation of the liquid-crystal materials themselves. It is therefore desirable to find a suitable source of light for PLLCDs . SUMMARY OF THE INVENTION According to the invention there is provided a liquid-crystal-type display comprising an ultra-violet light source, a first phosphor that emits UVA light when struck by light from this source, a liquid-crystal device for modulating the light emitted by the first phosphor and a second phosphor acting as a display surface and emitting visible light when excited by the modulated narrow-band UVA light; the first phosphor is preferably Eu 2+ -doped CaS0 4 . This phosphor produces narrow-band light in the UVA band from 350-400 nm with a full width at half maximum bandwidth of about 15 nm or less. Its spectrum is shown in Fig. 1. The narrow emission characteristics of phosphors used in the present invention also largely eliminate another disadvantage of the prior art. In the prior art, a significant amount of visible light is emitted and this must be filtered out, to prevent it adding to the visible emissions of the phosphors in the PLLCD architecture and desaturating the observed colours. The considerable amount of blue light emitted by conventional long-wave UV phosphors needs a very sharp transmission cut-off filter to avoid significant loss of the useful UVA light whilst blocking visible light. In contrast, the phosphor of the present invention emits much less in the visible spectrum. The narrowest practicable UVA emissions are the lines of, for example, low-pressure mercury cold- cathode fluorescent lamps (CCFL) . Such lines are typically no wider than about 2 nm, but those in the desired near-visible region are unsuitable because of their low relative intensity. The primary emission from low-pressure mercury CCFLs is at 254 nm and this is used to excite phosphor powders coated onto the inside of the fluorescent tubes. Unfortunately, phosphor emission bandwidths are generally rather large and in most cases the spectra extend over several tens of nanometres. Commercially available UVA-emitting phosphors typically have full width at half-maximum (FWHM) bandwidths of 18 nm or more. Hence the phosphor of the invention ideally comprises a transition metal or rare earth activator incorporated into a host lattice. Transitions involving transition metal or rare earth species as an activator generally have lower bandwidths than ' defect' -type phosphors. It is of course also essential that the activator fits properly into the host lattice, since otherwise increased disorder would be created that would lead to a broader emission spectrum. The design of the phosphors is thus also important . In the phosphor the europium should be incorporated into the calcium sulphate lattice in the form of doubly charged Eu 2+ ions . The Eu 2+ ions use the 5d-4f transition and emit in a narrow band (13-14 nm FWHM) centred at 385 nm. There is very little visible light emission. In the method according to the invention CaS0 4 :Eu 2+ phophor is prepared by heating a mixture of CaS0 4 , (NH 4 ) 2 S0 4 and Eu 2 0 3 in an inert atmosphere, cooling the resultant mixture, adding a reducing agent, in particular NH 4 C1 , heating again in an inert atmosphere and grinding or otherwise processing to the required particle size. The powder can then be applied to the inside or outside of a cold-cathode mercury-vapour tube, or it can be supported on a separate substrate. DESCRIPTION OF EMBODIMENTS An embodiment of the invention will now be described, purely by way of example, with reference to the accompanying Fig. 2 which is a highly schematic diagram of a liquid-crystal display in accordance with the invention. In Fig. 2, a low-pressure mercury cold-cathode discharge tube 1 has electrodes, not shown, and a phosphor 3 coated on the inside of the tube 1. The phosphor is made from a calcium sulphate host lattice incorporating europium. The low-pressure mercury emits light mostly at a wavelength around 254 nm; this light is incident on the phosphor which in turn emits narrowband UV radiation 5 centred at 388 nm and having a bandwidth of 13nm FWHM. The quantum efficiency was measured to be 85% which is comparable to conventional phosphors. The light 5 emitted by the phosphor is then passed through a collimator 7 and a modulator, here a liquid- crystal modulator 9, and is then incident on a display phosphor 11 upon which images can be displayed. The liquid crystal is of a type suited to the 388 nm wavelength, and is equipped with orthogonal arrays of ITO electrodes, alignment layers and polarisers in the usual way. The output phosphor 11 can be pixellated to correspond with the modulator pixels, or it can be a uniform layer. Here the polarisers can be cholesteric, since such polarisers are well adapted to work with a narrow range of wavelengths. The liquid crystal can then either modulate circularly polarised light or have the light converted to linearly polarised light by a quarter-wave plate. The fact that the input-light phosphor 3 following the invention is a narrow-band phosphor means that the cholesteric polariser can operate at a narrower band of wavelengths for a given collimation and contrast ratio, and the compensation films can be simpler or even omitted for a given collimation and contrast ratio. Also the fact that the input light is only just sub- visible means that the transmission and lifetime of components are improved. A further advantage of the narrow-band source light is that it is easier to collimate the light, in particular by adding a collimating dielectric stack as the collimator 7, as described in PCT/GB 98/01203. The CaS0 4 : Eu 2+ phosphor can be produced from a range of synthesis routes and has previously been reported in Dhopte et al, Journal of Luminescence 50 (1991) pages 187-195 (North-Holland) or in US-A-3669897 (Wachtel) . For the present embodiment the following process can be employed to produce the phosphor: For bulk amounts : CaS0 4 : • Dissolve 500g Ca(N0 3 ) 2 in 31 water; • Add 10ml concentrated H 2 S0 4 , filter and discard residue; • Add 250ml concentrated H 2 S0 4 to filtrate to precipitate CaS0 4 and filter again; • Wash with water until pH of washings is about 6, followed by rinsing in IPA. • Dry precipitate in oven overnight . • This gives a yield of about 80%. Phosphor: • Mix 120g CaS0 4 with 1. lg Eu 2 0 3 . • Add 16g (NH 4 ) 2 S0 4 and mix; • Place the mixture in a large silica firing tube and purge for 20 mins with Argon; • Place the tube in a tube furnace at 950°C for 45 mins, keeping the Argon flowing; • Take the tube out of the furnace and cool while still having Ar flowing through it; • When cooled, mix 20g NH 4 C1 with the sample and then put back into the tube, purge with Ar for 20 mins and fire for 30 mins at 920°C. • Upon cooling, grind the sample and sieve through a fine mesh. The amount of NH 4 C1 was in excess and the sample is over-reduced to produce the blue-emitting CaS:Eu 2+ which oxidises rapidly to CaS0 4 :Eu 2+ . This ensures a more complete conversion of Eu 3+ to Eu 2+ . For small amounts: • Mix 16.8g CaS0 4 with 0.88g EU 2 0 3 • Same method as above but slightly lower ratios of (NH 4 ) 2 S0 4 and NH 4 C1. • Add lOg CaS0 4 . • Add 2g mixture of CaS0 4 + EU 2 0 3 . • Add 1.6g (NH 4 ) 2 S0 4 for first firing, 30 mins, • Add 1. Og NH 4 C1 for second firing, 30 mins. Or (for very small amounts) : • Prepare 2.5g CaS0 4 • 0.5g mixture • 0.6g (NH 4 ) 2 S0 4 • 0.5g NH 4 C1 NOTE : If instead of excess (NH 4 ) 2 S0 4 and NH 4 C1 a reducing atmosphere, e.g. Ar/H 2 , is used, a sulphide is produced, which emits red. The powder thus prepared can be coated on to the inside of a glass tube in any known way, and electrodes and a gas filling introduced to produce a lamp. Several linear tubes, or a single serpentine tube, can be used as a backlight for the display, or a TIR lightguide with tubes along its sides can be constructed as a diffuse light source. CLAIMS : 1. A display comprising an ultra-violet light source, a phosphor (3) that emits UVA light (5) when struck by light from this source, a device (9) for modulating the light emitted by the phosphor and a display output material (11) acting as a display surface and emitting visible light when excited by the UVA light; in which the phosphor has a narrow-band emission spectrum with a FWHM of less than about 14 nm. 2. A display as claimed in Claim 1, in which the bandwidth of the narrow-band phosphor is in the region of 13-14 nm FWHM. 3. A display as claimed in claim 2 , in which the phosphor is Eu 2+ -doped CaS0 4 and emits wavelengths around 388 nm. 4. A display as claimed in any preceding claim, in which the modulator (9) is a liquid-crystal cell. 5. A display as claimed in any preceding claim, in which the display output material is itself a phosphor or a mixture of phosphors. 6. A display according to any preceding claim, in which the UV light source is a gas discharge in a vessel (1) lined by the first phosphor. 7. A display according to claim 6, in which the gas discharge is a mercury discharge. 8. A display according to any preceding claim, in which the modulator (9) includes cholesteric polarisers optimised to work over the narrow emission

Claims: band of the source phosphor. 9. A display according to any preceding claim and further including a collimator (7) between the UVA source and the modulator. 10. A method of preparing a lamp, in which CaS0 4 :Eu 2+ phosphor is prepared by heating a mixture of CaS0 4 , (NH 4 )2S0 4 and EU 2 0 3 in an inert atmosphere, cooling the resultant mixture, adding a reducing agent, in particular NH 4 C1, heating again in an inert atmosphere and grinding or otherwise processing to the required particle size. 11. A method according to claim 9, in which the resulting powder is applied to a discharge tube and the tube is assembled with a liquid-crystal device.

Inventors: BAYLEY PAUL ANDREW, NEWPORT ANNE CHRISTINE, VECHT ARON, BAYLEY, PAUL, ANDREW, NEWPORT, ANNE, CHRISTINE, VECHT, ARON
Applicants: BAYLEY PAUL ANDREW, NEWPORT ANNE CHRISTINE, SECR DEFENCE, VECHT ARON, BAYLEY, PAUL, ANDREW, NEWPORT, ANNE, CHRISTINE, THE SECRETARY OF STATE FOR DEFENCE, VECHT, ARON
Application year: 1998
Main classification: G02F1
IPC codes: G02F1