Patent GB-1234966-A (GB)
Title: Scanning and color analyzing systems
IPC: H04N9/11
Filed: 1968-12-16
Abstract: 1,234,966. Colour analyser. PRINTING DEVELOPMENTS INTERNATIONAL S.A. 16 Dec., 1968 [22 Dec., 1967], No. 59617/68. Heading H4F. [Also in Division G2] A colour analyser 99, Fig. 6, comprises neutral beam splitters 110, 121, e.g. halfsilvered mirrors, which divide an input light beam 103 into a plurality of secondary light beams 111, 122, 123, and a plurality of dichroic filters 115, 120, 125, 130 which are disposed in the secondary beams and transmit different wavelength ranges, e.g. red, green and blue light, on to photo-responsive devices 117, 127, 137, e.g. photo-multipliers or photo-transistors. The analyser may be used in connection with scanning apparatus for deriving colour separations from a scanned original colour transparency, and as shown in Fig. 6 a cathode-ray tube 90 is arranged to scan a transparency 100. Alternatively, the transparency may be mounted on a rotating glass drum and illuminated from within the drum. The dichroic filters 115, 120, 125 and 130 which transmit, red only, blue and green, green and red, and blue only, are disposed so that the angles of incidence of the respective secondary beams thereon are less than 45 degrees and preferably less than 20 degrees, and since the peak energy wavelength transmitted by a dichroic filter varies with the angle of incidence, fine tuning of the colour bandpass curves for the analyser is provided by arranging for the dichroic filters to be rotatably adjustable, e.g. through 20 degrees, to vary the respective angles of incidence thereon of the secondary beams. The input beam, e.g. from the cathoderay tube phosphor, comprises light having a wavelength distribution curve having a peak energy value in the green, which curve when multiplied with the wavelength-energy transmission curves of the respective dichroic filters defines for the respective light beams incident on the photo-responsive devices a plurality of wavelength bandpass curves which are sequentially overlapping in wavelength and intersect at points which, when such curves are normalized, each have an energy value substantially less than 50% of the peak energy value of the normalized bandpass curves. Due to the dichroic filters being adjustably mounted, one edge of the red and blue bandpass curves is adjustable in wavelength position and both edges of the green bandpass curve are adjustable in wavelength position. A small fraction of the input light beam may be diverted on to a monitoring photo-multiplier 97. Light reflected from the dichroic filters is absorbed by the walls of the analyser housing, Fig. 7 (not shown). To line up the scanning action of the cathoderay tube with the area of the transparency which is desired to be scanned an image of the transparency is projected on the screen of the cathoderay tube by means of a chopped beam of white light (169), Fig. 7 (not shown), which is directed back through the neutral beam splitter 110 and transparency 100 by means of a 10% reflective mirror (181) positioned between splitter 110 and dichroic filter 115. The chopping of the light preferably takes place at a rate greater than that at which the flicker of the light pulsations would be invisible to the human eye. The light projected on to the screen of the cathode-ray tube is reflected back by the screen to pass through the analyser in the same way as the light from the scanned spot of the tube. Since the brightness of the image projected on to the screen of the tube is such that the reflected light masks the light from the scanned spot of the tube when the chopped beam is " on", measurement of the colour component signals are made during the dark or " off" periods of the projected image light by using gate circuits synchronized with the chopping action in combination with integrating circuits. To avoid this gating of the colour signals the chopped beam may be pulsed on between each line scan.