Patent GB-1290398-A (GB)
IPC: G06F3/042
Filed: 1970-07-02
Abstract: 1290398 Holography; light guides NIPPON ELECTRIC CO Ltd 2 July 1970 [4 July 1969(5) 16 Oct 1969 11 Dec 1969] 32250/70 Heading G2J [Also in Division G4] Apparatus for providing coded signals representing the position of a movable member comprises a tablet device which in the form of Fig. 1 comprises a record plate 101, means for directing a collimated monochromatic beam to a selected point on the plate so as to illuminate one of a number of holograms 111, each giving a different pattern of diffracted beams, and a plurality of sensors 141-146 arranged to detect the diffracted beams and generate corresponding electric signals identifying the illuminated position. The light, form a source 102, is directed by a flexible self-focusing light guide 103 by a unit 106 which may be connected with the movable member of a control system. There are eight rows of eight holograms 111 so that any hologram may be identified by six bits, three defining the row and three the column. The signals from the six detectors 141-146 are applied to a discriminator 105. The record plate 101 is formed photographically as shown in Fig. 2. Six separate point sources 221-226 are formed from a single coherent light beam by splitting and focusing. Each source can be switched on or off by a signal applied to a terminal 211-216. The beam from each source invadiates the whole surface of plate 101. Another coherent light beam 228 is directed towards the plate 101, a shutter plate 227 allowing it to pass to only one of the areas 111 at a time. A six-bit binary word is applied to terminals 211-216 and simultaneously the corresponding shutter is opened. By this means the desired six-bit word in hologram form will be recorded on plate 101. In the form of Fig. 3 two record plates 301, 302 with elongated holograms are used, one storing positional signals in a horizontal position, the other signals in a vertical direction. Coherent light is passed from source 303 via flexible light fibre 304 to a point on the first plate, the beam passing through both plates. An upper set of detectors 351-355 responds to diffracted light from the first plate 301 and a lower set 361- 365 to diffracted light from the second plate 302, this being derived from the zero-order beam 309 which is not affected by the first plate. The two sets of detectors give signals which after discrimination at 307 represent the horizontal and vertical co-ordinates. The hologram plates 301, 302 are made as shown in Fig. 4, a binary pattern mask 410 having transparent and opaque slits corresponding to the number of elongated holograms to be recorded. The mask is illuminated by a collimated coherent light beam 430 and the transmitted light is projected by lens 420 on to the record plate 400. The plate is also illuminated by a reference beam 450 and the interference fringes formed on the plate 400 between this and the image of 351 mask 410 are recorded. This represents the bits to be derived from the first detector 351. The mask 410 is then changed and the angle of the reference beam altered to make another recording corresponding to the bits for the second detector 352 and so on until all the bit positions have been recorded. In another form Fig. 5 (not shown) the plate is adapted to provided a visible trace of locus of the light guide end to facilitate writing. For this purpose a photochromic glass screen is placed behind the record irradiated by near ultraviolet light to cause it to blacken. The movable beam is of near infrared light which bleaches the photochromic glass. This allows part of a beam of coherent light to reach the record plate through the photochromic glass, this light being diffracted to fall on a detector as before. As the diffracted light beams strike the detector a train of digital signals is given defining the locus of the light beams at the writing tip. The light pen may be formed with arotatable sphere at the tip having a flat which moves in contact with the record plate. A lens inside the sphere collimates the light received along the flexible light guide and allows it to pass in the correct direction towards the record. There may be a switch switching on the light only when the tip is in contact with the record. The moving light beam may be provided on a separate member caused to follow the movements of a writing pen by magnetic attraction. The record plate may be reflective rather than transmissive. Instead of supplying light through a light guide the source may be mounted at the writing tip.