IMAGE-FORMING DEVICE

Abstract: An image-forming device includes a source of radiation (20), e.g. UV, a means such as a liquid-crystal cell (1) for modulating this radiation according to the image to be formed, and an output means (11) towards which the modulated radiation is directed and whose reflective or absorptive properties are altered when it receives the radiation. A suitable material for the output means is a photochromic material which changes colour under the influence of UV. Such a display can be made easily visible in strong ambient light. The output means can also be made removable, the device then acting as a printer.

Description: IMAGE-FORMING DEVICE The invention is concerned with displays and related devices where input light is modulated to affect a secondary output. One such type of device is described in WO 95/27920 (Crossland et al) , which describes the use of LC panels to modulate continuous UV illumination incident on phosphors which consequently emit visible light. These displays have excellent viewing angle characteristics by comparison with standard LC displays where the light passing through the liquid crystal is the light reaching the viewer. Moreover, since they are believed to be more efficient than TFT colour displays, they can produce greater visible light power than TFTs making them viewable in higher levels of ambient light. However, they still suffer from the problem intrinsic to emissive displays that the output light power must be increased as ambient light increases, and hence the power consumption increases. Existing displays for use under high ambient light frequently use reflective or "transflective" displays (i.e. reflective displays whose reflecting plane behind the active elements is in fact partly transparent and has a light source behind it which makes the display visible in low ambient light) whose contrast and brightness actually improves with increased ambient light without a corresponding increase in power consumption. However such displays are only available as monochrome displays and also suffer from poor viewing angle characteristics. It would be desirable to combine the excellent viewing angle properties of emissive UVLCDs with the bright ambient display capabilities of reflective displays. According to the invention there is provided an image-forming device including a source of radiation, a means of modulating this radiation according to the image to be formed, and an output means towards which the modulated radiation is directed and whose reflective or light-absorptive properties are altered when it receives the radiation. the alteration may be permanent, giving a function analogous to a printer, or temporary, giving a display function. In an alternative aspect the invention is concerned with a method of displaying data, in which the data are represented by a material whose reflective or absorptive characteristics are altered by the selective application of incident radiation. The invention is also directed to a display unit having a screen which can be removed from the unit and stores the latest displayed image . The output means is generally a sheet of material, or a substrate coated with such a material, which reflects a different amount or portion of the incident light according to whether it has been exposed to suitable radiation, which for most practical purposes will be light of shorter wavelengths, such as UV light. The effect is that the material changes for instance from transparent to dark or coloured, or vice versa, or from one colour to a different colour. Various materials, known as photochromic dyes, exist whose absorption characteristics in the visible spectrum can be changed by exposure to specific narrow¬ band activation light. This narrow-band light need not be within the spectrum generally referred to as ultra¬ violet, but in many cases this is so; hence the term UV will be used here to describe the light used to activate the photochromic materials. Pilkingtons manufacture a range of such materials; see for instance US 4913544 and 4851530. They are sold by James Robinson Ltd. of Huddersfield, UK under the name Reversacols, and are described as switching between a nearly transparent and a coloured state. A well known usage of photochromic materials is in sunglasses which change their degree of tint according to the intensity of ambient sunlight. These materials can be tailored to have various 'on' and 'off colours, though sunglasses are generally brown or green when on and transparent when off . The modulating means is most naturally an addressable liquid-crystal layer, and for a display the reflecting substance can be applied in the form of dots on the glass cover of the LC layer. These dots can be made of a photochromic dye in a suitable carrier, such as a polymeric film. Since photochromic materials are usually absorptive dyes rather than reflective it will normally be desirable to provide a white background; this can be afforded by a scattering means below or incorporated with the dye. In one particularly simple embodiment, for instance, the scattering means is represented by a roughening of the top surface of the glass cover of the liquid crystal. Alternatively the scatterer could be below the entire assembly as in a known transflective display, though here the usual parallax problems are presen . Different types of unit could be designed using this principle, such as a display itself, secondly a printer, thirdly a hybrid unit for which the concept does not yet exist - a display which can be converted instantly to hard copy, rather than having to be printed separately. For a better understanding of the invention, embodiments of it will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of an embodiment of the invention as a display; Fig. 2 shows an embodiment incorporating a printer; Fig. 3 shows a variation of Fig. 1; and Fig. 4 shows a further display embodiment. The display shown in Fig. 1 is broadly similar to those in WO 95/27920. Near ultra-violet (UVA) light 20 is directed in collimated fashion from the rear towards a liquid-crystal cell which acts as a modulator and can be of conventional construction. The liquid-crystal material 1 is sandwiched between glass layers 5 and is divided into pixels by orthogonal electrode arrays 3. For a typical LC, for instance of the twisted-nematic type, the cell would also contain polarisers, but for simplicity these are not shown. Directly or indirectly on the upper glass plate 5 is a screen in the form of a photochromic layer 11, shown as continuous though it could also be divided into dots or pixels. This photochromic material takes the place of the emissive phosphors in UVLCDs. Above the photochromic material though not shown is a UV filter to prevent leakage of UV light and unwanted activation by ambient UV light. The UV light is modulated by the LC material, and in those pixels where it is allowed to pass it strikes the photochromic material which then changes colour, as shown at 11a. The viewed display can then be read when front-lit by ambient light 30. The background colour is provided by reflection from the body colour (probably black or white) of the photochromic material; to this end the photochromic material is made up of a carrier, a pigment material making up the background colour, here white, and the photochromic substance or dye itself. The carrier can be of plastics, such as polystyrene, or fibrous, in the manner of paper. When the photochromic dye at a given pixel is inactivated it is transparent and the pixel has the background colour, i.e. white; and when the dye is activated it absorbs certain wavelengths and the pixel appears coloured. An alternative arrangement shown in Fig. 3 illustrates an alternative way of arranging the photochromic material and the background layer. As in all the figures like reference numerals are used for like parts. Here there is a background layer 17 below, i.e. on the UV side of, the photochromic layer 11. The background layer 17 can be a layer applied to the glass cover 5 of the LC cell, or alternatively and particularly simply it can be formed by a frosted surface of this glass. The background layer 17 is required to scatter the ambient light 30 so that the viewer sees a plain background and not the structure of the LC cell below. The layer 17 will therefore unavoidably also scatter some of the incoming UV light 20, though a certain degree of "tuning" is possible so the UV is scattered less than visible radiation. This UV scattering is shown at 20a. The result of this arrangement is that some of the activating UV radiation is lost, leading to a reduced efficiency of the display. However, since in many instances the photochromic dye needs only intermittent activation in any case, this efficiency loss is not a serious disadvantage. For any given application a suitable compromise is struck between the requirement for a uniform background (brought about by increasing the amount of scattering from the layer 17, for instance by a higher degree of frosting) and penetration by UV light (brought about by decreasing the amount of scattering) . Figure 4 illustrates a further variant of the invention which dispenses with the need to produce directed backlighting 20 by a construction analogous to the known "transflective" scheme in which a liquid crystal display is visible by both reflected light and back-lighting. A totally internally reflecting (TIR) backplane 30 is arranged in contact with and under the liquid-crystal cell 1 which is of the scattering type. UV light 20 is input from lamps 32 from the side of the plane 30 and remains within it by TIR while the liquid crystal is not addressed. A white or other coloured background 34 is placed behind the TIR plane 30. The photochromic dots 11 here are transparent and are mounted on a transparent substrate 12 held parallel to the LC cell by spacers 14 as shown, with a small air gap. The air gap is necessary to ensure that light stays within the backplane/LC assembly when the liquid crystal is not switched. When the liquid crystal is switched to its scattering state, as shown shaded, UV is scattered out of the TIR plane and strikes the photochromic dot 11, changing its colour. If the background 34 is white the viewer thus sees a coloured dot on a white background. This embodiment is subject to parallax and shadowing problems when viewed from an angle other than normal to the plane, but for large-pixel displays this defect is not serious and is compensated for by the efficiency of use of the UV light as compared to Fig. 3. In any of the embodiments described use of different photochromic materials with different 'on' colours would allow a full colour display to be built. The power saving comes from the fact that the UV illumination need only be intense enough to cause the chemical transition in the photochromic material (independently of the ambient light level) and the viewed contrast is provided by selective absorption and/or reflection of the ambient light. In addition, if the photochromic material has an appropriate persistence then the UV light could be pulsed to refresh the screen at whatever the display information rate is. On a flight or train arrivals annunciator board, for instance, this might be only one flash per minute giving a much reduced power compared with the continuous illumination required for TFTs or for phosphor UVLCDs, while overcoming problems of mechanical reliability in mechanical displays. If the photochromic material exhibits persistence, various mechanisms can be used to switch the photochromic material off; either: 1. it naturally relaxes after a given period at room temperature; 2. it can be driven back by heating; or 3. it can be switched back by illumination with a different wavelength of light (which may or may not be UV) . The simplest form would be a material of type 1 with a very short relaxation period so that it can be treated like a fluorescent material, requiring continuous illumination by the UV source. The most efficient low data rate display would be of type 3 whereby two light sources are used, the first erasing the whole display and the second writing a new display page which remains fully visible indefinitely. Figure 2 shows a further embodiment of the invention as a printer. Here the photochromic material is made into a sheet form (or incorporated into a sheet of paper) 13 which can be detached from the front of the display. The sheet thus functions both as a display screen and as a hard copy. This could be imagined with a pocket organiser where a monochrome or colour hard copy of a diary/address book could be 'printed' from the display and handled as a sheet of paper, following which a new sheet is adduced as shown by the travelling arrows. Alternatively the sheet could be passed through an enclosed printer so as to ensure that no UV light escapes, the screen here having no display function. Clearly in both cases continuous material rather than sheets could be used. In this case the photochromic material could either be type 1/2 with a very long persistence (degrading like thermal fax paper) or type 3 with an erase wavelength different from the writing wavelength and not present at sufficient intensity in ambient light. The sheets could be made re-usable if a suitable erasing machine were provided, making use of the erase wavelength(s) . Key features of embodiments of the invention are that: a. the system is capable of writing subtractive colours (i.e. not the RGB additive colours of emissive light) to a white screen - this could provide a form of electronic paper; b. the unit is a screen printer, not a line printer - the whole image is addressed directly/ simultaneously as opposed to printer carriage systems when one dimension of output image is provided by the passage of paper past the printhead; and c. the image is viewed directly and can be edited and modified until the hardcopy function is used which instantly stores the exact output on a sheet of paper or similar material. The unit is portable and combined, unlike dedicated free-standing printers like the old azo-dye blueprint machines which used UV in the imaging process. The hard copy can be reversible and rewriteable. As mentioned above, UV light is suitable for many embodiments as the activating radiation. However, using appropriate materials visible light could be used, particularly if only a monochrome display is required. In this case the cover for the photochromic material 11 would have to filter out the relevant wavelength since otherwise the ambient light would continually activate it. This is not such a problem for the UV version since most glasses and plastics will block UV. In another version of the device, operable only as a display, the photochromic material layer 11 can be within the liquid-crystal cell. This reduces any collimation requirements.

Claims: CLAIMS : 1. An image-forming device including a source of radiation (20) , a means (1) of modulating this radiation according to the image to be formed, and an output means (11) towards whicn the modulated radiation is directed and whose reflective or absorptive properties are altered when it receives the radiation. 2. A device according to claim 1, m which the output means incorporates photochromic material. 3. A device according to claim 2, in which the photochromic material switches between a transparent, colourless state and a coloured state (11a) . 4. A device according to claim 2 or 3, m which the output means (13) is detachable so as to preserve the image. 5. A device according to any preceding claim, m which the modulating means is a liquid crystal cell (1) . 6. A device according to any preceding claim and further including a background layer (17, 34) visible through the output means at least in one of its states. 7. A device according to claims 5 and 6, in which the background layer is constituted by a frosting of a cover plate of the liquid-crystal cell. 8. A method of displaying data, in which the data are represented by a material whose reflective or absorptive characteristics are altered by the selective application of incident radiation.

Inventors: BAYLEY PAUL ANDREW, CROSSLAND WILLIAM ALDEN, DAVEY ANTHONY BERNARD, GEAKE VINCENT GLENN, SPRINGLE IAN DAVID, BAYLEY, PAUL, ANDREW, CROSSLAND, WILLIAM, ALDEN, DAVEY, ANTHONY, BERNARD, GEAKE, VINCENT, GLENN, SPRINGLE, IAN, DAVID
Applicants: BAYLEY PAUL ANDREW, CROSSLAND WILLIAM ALDEN, DAVEY ANTHONY BERNARD, GEAKE VINCENT GLENN, SCREEN TECH LTD, SPRINGLE IAN DAVID, BAYLEY, PAUL, ANDREW, CROSSLAND, WILLIAM, ALDEN, DAVEY, ANTHONY, BERNARD, GEAKE, VINCENT, GLENN, SCREEN TECHNOLOGY LIMITED, SPRINGLE, IAN, DAVID
Application year: 1997
Main classification: G02F1
IPC codes: B41J2, G02F1