Patent GB-1388026-A (GB)
Title: Appratus for the optical-electrical scanning of a drawing
IPC: G01J3/46
Filed: 1972-03-23
Abstract: 1388026 Measuring colour photo-electrically FRANZ MORAT GmbH 23 March 1972 [17 April 1971] 13742/72 Heading G1A In an apparatus for point-by-point scanning of a drawing consisting of a large number of differently coloured points, in which the scanning head includes a plurality of photo-cells 31 r, b, g, each responding to a definite spectral range, the output from each photo-cell is supplied to a plurality of colour discriminators 411, 412, 413, each of which produces a colour signal characterizing the point being viewed only when all the photo-cell outputs, or signals derived therefrom, be within respective upwardly and downwardly limited tolerance ranges defined by the discriminator. To calibrate the discriminator 411 for a first colour the scanning head including photo-diodes or photo-transistors 31 is brought above a point of that colour. The photo-cells 31 r, b, g have associated therewith filters or dichroic mirrors to render them responsive to red, blue and green light respectively. In this embodiment the points B 1 and B2, G1 and G2, R1 and R2 are connected together, so that the amplifed blue output signal is applied to the pair of differential amplifiers 43 b1, b2. Amplifiers 43 b1 provides an output only when its input is below a certain level, whereas 43 b2 provides an output only when its input is above a pre-set level. This tolerance range is adjustable by means of resistor 49b, which changes both upper and lower limits symmetrically. Initially the resistor 49b is set to provide the narrowest tolerance range, and resistor 37b of pre-amplifier 33b is adjusted so that the blue output lies in this range, at which point lamps 69b light up. This procedure is repeated for the green and red signals. Next the scanning head is placed successively over a number of points of the same colour but of different quality of covering, i.e. the white background may be showing through. Resistors 37 must now be kept fixed, and the tolerance ranges are widened, using the resistors 49, so that eventually all points of the same colour on the drawing will produce an output signal from the AND-gate 451 of the colour discriminator. This process is repeated for all the other colour discriminators 412, 413 ... After this calibration each colour of the drawing will produce a colour signal from only one discriminator. In the above mode of operation, the resistor chains 47-50 are supplied with a stabilized voltage source from a battery and a Zener diode 59, via a switch position 53 (4) and an operational amplifier 51. This may lead to errors if the illumination intensity changes. To overcome this, the output level at any one of the pre-amplifiers 33 may be used as a reference voltage level for the pre-amplifiers 33 by moving the contact of switch 53. The chosen output level may be monitored by depressing push-button 38 to produce a display on the appropriate dial 39. In a further embodiment, the threshold amplifiers 43 do not receive directly the amplified photo-cell outputs but "normalized" versions thereof, i.e. signals which relate the photo-cell outputs to the white background level. To do this there is interposed between points B1, G1, R1 and B2, G2, R2, circuitry (77) which stores white signal values derived from a calibration viewing of uncoloured points and compares these values (at 97) with the photo-cell outputs. Fig. 3 (not shown). The "normalized" photo-cell outputs may be supplied to a circuit arrangement in which each threshold limit is independently adjustable by having its corresponding variable resistor (115- 129) Fig. 4 (not shown). In this case one of the three "normalized" photo-cell outputs may be used as a reference signal to adjust for variations in illumination level. Another embodiment employs the circuitry of Fig. 5 positioned between the points B1, G1, R1 and B2, G2, R2. By forming the quotient values shown errors due to temperature, illumination or voltage fluctuations, which affect both top and bottom lines, are eliminated. The outputs from the circuit of Fig. 5 may be applied to a modified threshold circuit, Fig. 6 (not shown). In this embodiment, each of the two quotient outputs at B9, G9 is subtracted (145, 147) from a fixed base value. These two "differential quotient" signals are then applied to a further dividing circuit (149) which calculates the absolute value of their quotient. This final quotient signal is applied to two threshold amplifiers 151, 153. If the output from the divider (149) is within the variable threshold a signal is supplied to an AND gate (161). This gate also receives signals indicative of (a) the sign of the quotient and (b) whether or not the sum value (B + G + R) - see Fig. 5 - lies within a certain threshold value. The information (a) and (b) locates the colour within the colour triangle, and the AND gate (161) produces an output if the colour being scanned is the one for which the discriminator was originally calibrated. The apparatus may be used in a device according to Specifications 1170947 or 1190600, in which case the "white" stored reference signals Fig. 3 (not shown) are arranged to be up-dated each time a white margin is scanned. Alternatively, the drawing to be scanned may be clamped on  the periphery of a rotating drum.