OPTICAL PANEL

Abstract: The panel (10) has an array of cells each actuable to change from an opaque to a light transmissive state, with arrangements actuating the cells to effect a raster-scan of the panel. In this way, a "flying hole" sweeps the face of the panel. As described, the cells are defined by row and column electrodes (22a - 22m, 20a - 20n) on opposite sides of a liquid crystal layer. The panel may be used in a display device, a light source modulated by a video signal being viewed through the panel, or as a pickup device in which light from an image scanned by the panel falls on a photosensor to generate a video signal.

Description: OPTICAL PANEL The present invention relates to an optical panel, which can be used as the basis of devices for displaying an image and for encoding an image into an electrical signal. It is well known to use a video signal to display an image 5 on the screen of a cathode ray tube, but the use of a cathode ray tube is not convenient in many situations. Cathode ray tubes require high power and voltage supplies, and are fragile and bulky. A display device which uses a cathode ray tube and is conveniently portable is therefore hard to achieve. 3.0 Multiplexed liquid crystal displays are well known. For example, in U.K Patent No. 1559074 there is described a liquid crystal waveform display to replace a cathode ray oscillograph in which a matrix of liquid crystal cells is operable by first and second orthogonal series of strip electrodes, the state of a cell 5 being changed by energising the relevant electrode pair, one from each set. Use waveform to be displayed is encoded to generate suitable drive waveforms for the electrodes. Even with the relatively simple requirements of such a display (i.e. only one cell in each vertical column requires energising for any given 0 waveform) the encoding is relatively complex. Although this principle could be extended to display a wider range of patterns, the complexity of the encoding would become substantial. According to one aspect of the present invention there is provided a panel comprising an array of cells individually 5 actuable to change from a first, opaque state to a second, light transmissive state and means for raster-scanning the array by sequentially actuating individual cells. The cells may be liquid crystal cells. Such a panel may be used in a display device, with a 0 light source energisable to direct light through the panel and means .for modulating the output of the light source; or as an input device, with a photo-electric transducer responsive to light passing through the panel . Indeed, the two functions may be combined in a single panel. Colour display/input may easily be 5 provided by using two-or three-colour light sources or sensors and colour selecting filters. Some embodiments of the invention will now be described, by way of example, with reference to the drawings in which: Fig 1 shows a panel suitable for use in the present invention- Fig 2 is a section along the line 2-2 of Fig 1; Fig 3 is a schematic diagram indicating how the panel shown in Figs 1 and 2 is used in accordance with the invention, and Figes 4 and 5 are schematic representations of a display and a video signal generator respectively, each comprising the panel of Fig 1, and Fig 6 shows the panel of Fig 2 adapted to display a colour image. Referring to Figs 1 and 2, a panel 10 embodying the invention comprises first and second, substantially planar plates 12, 14. The plates are rectangular and transparent. Four spacer members 16 extend along the edges of the plates 12, 14 and hold the plates 12, 14 spaced and susbtantially parallel and in register. The plates 12, 14 and the spacer members 16 cooperate to define a cavity 18. A first set 20 of strip electrodes 20a...20n are carried on the interior face of the plate 12. The electrodes 20a...20n of the first set 20 of electrodes are mutually spaced and parallel. The electrodes 20a...20n are also parallel to two edges of the interior face of the plate 12 and extend entirely across the plate 12. At one end, the electrodes 2oa—20n extend through a spacer member 16 for connection to external circuitry. The plate 14 carries a second set 22 of electrodes 22a 22m. The electrodes 22a...22m of the second set22 are arranged in relation to the plate 14 and a second spacer in exactly the same way as are the electrodes 20a...20n of the first set in relation to the plate 12 and the first spacer member. Further, the electrodes of the second set are perpendicular to those of the first set. For reasons which will become apparent, the electrodes are transparent. For instance, the electrodes may be strips of indium tin oxide deposited on the plates, which may be glass. The outer face of each plate 12, 14 carries a polarising filter 13, 15. The polarising axes of the two filters 13, 15 are parallel. The cavity 18 contains a liquid crystal 24, of the twisted nematic type. As is known in the art, in the absence of an applied voltage, a suitable, twisted nematic liquid crystal rotates the plane of polarisation of incident, polarised light. Therefore, in the absence of a voltage applied to the electrodes 20, 22 the panel is opaque. Application of a voltage above a threshold value to a pair of electrodes so alters the structure of the liquid crystal in the cell 23 defined by the intersection of the electrodes, that it ceases to rotate the plane of polarisation.. The cell 23 defined by the electrodes to which the voltage is applied then becomes clear. A voltage of magnitude equal to half the threshold voltage is applied to each electrode of the pair, in opposite senses. Thereby, only the cell 23 defined by the pair of electrodes is rendered clear, for the duration of the application of the voltage to the relevant electrodes. The invention provides that the cells be raster-scanned to sequentially clear the cells. This may be achieved using the apparatus shown schematically in Fig 3. A shift register 26 receives clock pulses from a clock 30. At each clock pulse, the shift register advances to apply the cell-activating voltage to a different electrode. Each time the shift register has received a number of clock pulses equal to the number of electrodes 20a...20n, the shift register 28 advances. The two shift registers therefore cooperate to raster-scan the array of cells in the panel 10. The cells sequentially become clear, to provide the illusion of a hole, repeatedly flying across the entire panel. The device described above may be used as the basis of a display, as shown schematically in Fig 4. One face of the panel 10, which is constantly raster-scanned in the manner described above, is illuminated by a light source 32. The Danel is viewed from the side remote from the light source. Means (not shown inthe drawings) are provided formodulating the output ofthe light source in accordancewith a video signal. Conveniently a modulated voltage is applied tothe light source as the panel is scanned and the output of the light source ismodulated, soa visible image is generated. It will be seen that no special treatment of the video signal is required over that normally provided for a signal displayed on a cathode ray tube. Further, three modulated light sources of appropriate colours may be provided to generate a colour image. The light sources used may be, for instance, neon or other gas discharge tubes. For compactness, the illumination may be provided by abutting a glass plate to one face of the panel and edge- lighting the glass plate, to distribute the light across the panel. Fig 5 indicates schematically how the panel may be used in apparatus for generating a video signal. In the situation illus¬ trated, the signal is generated from a document 34. The document 34 is held in contact with one of the plates 12, 14. On the other side of the panel 10 is a photo-electric transducer 36. As the cells of the panel 10 are raster-scanned, the photo-electric transducer provides a signal in accordance with the intensity of the light traversing the panel 10, which depends on the image on the document 34. The video signal may alternatively be generated from an image focussed on the panel 10 by an optical system. Another embodiment of the optical panel is shown in Fig 6. This panel is an adaptation of the panel shown in Fig 2, to allow a colour image to be displayed. Behind the panel 10 are three gas discharge light sources formed by three chambers 40a, b, c which are separated by clear glass walls 42 and extend over the whole area of the display. The chambers 40a, b, c are filled with gases, for instance mixtures of noble gases. Electrodes are provided within the chambers to provide the current required by the light source. The gas mixtures in the chambers are selected so that one source generates red light, a second blue light and a third green light, so that the full visible spectrum can be produced by varying the combination of sources energised, and their intensities. The transparent walls allow the light from a chamber to pass to the panel through other intervening chamber . A full colour image can be generated by appropriately modulating the light output of the three chambers 40a, b, c while the panel cells are being raster-scanned. It will be seen that a combined device may be provided which is capable of being used as an input device and a display. The combined device could be used as a display or an input device or as both simultaneously. For instance, alternate scans of the panel 10 could be used for display and input respectively. Thus, a keyboard could be displayed. Touching, with a finger, regions of the keyboard displayed prevents external ambient light passing through the cell of the display obscured by the finger. The video signal generated will therefore indicate which region of the displayed keyboard is being touched, and can be used to provide an input to, for instance, a microprocessor. In applications where the ambient light intensity is too low for the device to operate as described, a light pen may be used. The position of the pen can be detected by the increased light intens.ity, at that point. When used with a microprocessor, it is convenient to use a first region of the panel to display a keyboard, which can be a standard typewriter keyboard or a keyboard designed for a particular application. A second region of the keyboard can be used as a display. The microprocessor and the panel can be combined into a truly port-able unit. When used with a micro-processor, the signals from the transducer may be digitally encoded. Other possible uses of the panel include an aircraft position indicator. A transparent sheet on which a map is drawn can be laid on the panel. A point of light displayed by the panel indicates the position of the aircraft. The edge of the map can carry optically coded information about the sheet of the map which is overlaid, which information can be read by the device. The side-lit glass plate described above for illuminating the panel can be used, without being side-lit, in the input device. A sensor or sensors can be used to detect light leaving the edge of the plate. A plurality of sensors responsive to different ranges of wavelength of light, by means of optical filters or otherwise enable colour images to be encoded. Alternatively, a light sensitive coating on the back of the panel may be used. The panel has been described as using liquid crystal cells. Other means such as electro-chromic or magneto-optic techniques to provide an array of cells or "optical gates" having opaque and light trans issive states may, however, be used. With known liquid crystal technology a panel which may be scanned quickly enough to be used as a fast-scan television display nay notbe realisable. However, known techniques enable a smaller, more slowly scanned panel to be used as a display for a microprocessor, for instance, .it is, of.course, possible that future technological developments will enable a television display to be realised. To increase the persistence of the display for relatively low scanning rates, a phosphor coating may be applied to the viewing surface. A shadow mask similar to that used in a television receiver can be incorporated into the device, in order that the cells of the display may be more clearly defined. The display device could be used in a projection television system to project an image onto a larger display.

Claims: CLAIMS 1. A panel comprising an array of cells individually actuable to change between a first, opaque state to a second, light transmissive state and means for raster-scanning the array by sequentially actuating individual cells. 2. A panel according to claim 1, in which the raster- scanning means comprises electrodes arranged to apply an electric field to a cell to be actuated, thereby causing that cell to change state. 3. A panel according to claim 1 or 2, in which each cell comprises a liquid crystal and at least one polarising filter. 4. A device for displaying an image, comprising a panel according to claim 1, 2 or 3, a light source energisable to direct light through the panel and means for modulating the output of the light source. 5. A device according to claim 4, wherein the light source comprises a chamber extending over substantially the whole of one face of the panel, and a transparent partition generally parallel to the panel and partitioning the chamber into regions, the regions being filled with respective gases or mixtures of gases and there being electrodes within each region for creating a gas discharge to generate light of a respective frequency. 6. A device according to claim 4, wherein the light source comprises a transparent sheet extending over substantially the whole of one face of the panel, and means for directing light into the sheet through the edge of the sheet. 7- A device for generating a video signal comprising a panel according to any of claims 1 to 6, and a photo-electric transducer responsive to light passing through the cells from an image to be encoded. . _OMPI 8. A device comprising a panel according to claim 1, 2 or 3, a light source energisable to direct light through actuated cells in at least a first region of the panel in a first direction, means for modulating the output of the light source, and a photo-electric transducer responsive to light passing through an actuated cell in at least a second region of the panel in a second direction, from an image to be encoded, the second direction being opposite to the first direction. g. A device according to claim 7 or 8, for use with a micro-processor in which the output of the photo-electric transducer is digitally encoded. 10. A panel according to claim 7 or 8, for use with a micro-processor in which the output of the phote-electric transducer is digitally encoded. - 11. A panel substantially as described herein with reference to Figs 1 to 3. 11. A display device substantially as described herein with reference to Figure 4. 12. A device for generating a video signal susbtantially as described herein with reference to Fig 5. 13. A display device substantially as described herein with reference to Fig 6.

Inventors: KING LEIGHTON HANNA, KING, LEIGHTON, HANNA
Applicants: KING LEIGHTON HANNA, KING, LEIGHTON, HANNA
Application year: 1984
Main classification: H04N3
IPC codes: H04N3