GB782751A - Improvements in or relating to apparatus for determining areas - Google Patents

Abstract

782,751. Photo-electric area determination. CINEMA-TELEVISION, Ltd. Dec. 14, 1955 [Dec. 16, 1954], No. 36458/54. Class 40 (3). In order to determine a plane area (ABCDE) contained within a given irregular boundary a scanning element is caused to traverse the area in an appropriately large plurality of equiangularly spaced lines radiating from a point P within the area. The sum of the squares of the distances from P to the boundary alone each of the lines is determined and the area is directly proportional to this sum. If the area is such that the whole of the boundary cannot be " seen " from a single point P (as may be the ease with re-entrant boundaries) the area has to be sub-divided. In one embodiment the scanning is effected by a light beam originated by the setting up of a suitable radial scan on a cathode-ray tube 7 by means of scanning current generator 8. The screen 6 of the tube is focused on an area of table 3 within which is placed a transparent or translucent sheet 1 with an outline 2 of the area marked on it in opaque pigment. The sheet 1 is so placed that the origin of the scan occurs at a suitable point within the area. Light passing through the sheet 1 and the table 3 is directed to photo-electric cell 10, the output from which is applied to amplifier 11. When each line commences a control signal is applied from generator 8 to open gate 12 to allow signals from a source of oscillations 13 to pass to counter circuit 14. The gate is closed by the pulse generated when the scanning light beam passes over the area boundary. In order that the signals fed to the counter circuit shall correspond to the square of the length scanned from the origin to the boundary the scan generator 8 may be arranged so that the time taken for the spot to travel a given distance is proportional to the square of that distance and the oscillator 13 of constant frequency. In a modification the scanning may be linear and the oscillator 13 triggered at the commencement of each scan to initiate an oscillation whose repetition rate increases according to a square law. If the frequency of oscillator 13 is constant and the scan linear it is necessary to introduce a digital squaring device between gate 12 and counter 14. Counter 14 sums the trains of signals fed to it, yielding a number proportional to the sum of the squares of the lengths of the scanning lines from the origin to the boundary. This number, displayed at 16, indicates the area to be measured. The apparatus may be arranged so that the derivation of a count proportional to the square of the radial line lengths is not commenced at the origin but at a constant distance therefrom; the area of the centre circle being added to the area measured. A monitor display cathode-ray tube may be provided scanned synchronously with tube 7 and having its beam suppressed except when the scanning beam passes over the boundary of the area to be measured. In another embodiment of the invention the area to be determined is focused on to the sensitive element of a television camera tube. The electron beam of the tube is arranged to perform the above-described scanning and the signals derived are utilized in the same way as those from cell 10.