GB640001A - Improvements relating to the production of multi-colour printing surfaces - Google Patents

Abstract

640,001. Copying-telegraphy. YELLAND, N. W., Feb. 24, 1947, No. 5287. [Class 40 (iii)] [Also in Groups XL (a) and XL (c)] In a system for reproducing a design having areas each coloured in one of a limited number of distinct colours the design is optically scanned, the reflected light being received by at least one photo-electric cell and the whole of the cell output is applied to a device controlling a set of independent recording devices corresponding in number to the distinct colours to be reproduced. A coloured original E, Fig. 1, is mounted on a continuously-rotated drum A and scanned by a light-spot traversed by a leadscrew C. The reflected light is applied to two photo-electric cells 11, 12 of different coloursensitivity (e.g. red and blue) which may be attained by the use of suitable optical filters. The output of the cells is amplified and applied respectively to the X and Y plates of a cathoderay tube L. The beam is thus deflected to a co-ordinate position dependent on the red and blue components of the colour being scanned at any instant. By suitable biassing of the cathode-ray tube a white area on the original E is arranged to deflect the beam to a position W, Fig. 2, and black to a position O. Six colours intermediate red and blue will then deflect the beam to, e.g., positions 1 ... 6. The cathoderay screen is covered by a mask having suitable apertures opposite these positions 1 ... 6 and opposite each aperture is mounted a photoelectric cell detector N, Fig. 1. When a particular colour is being scanned it therefore energizes its associated detector N whose amplified output energizes a discharge lamp to expose a corresponding photographic film F1 ... F6. Thus six separation negatives are obtained and may be used to produce printing blocks for textiles and the like. The scanning head G may be driven by a separate lead-screw from that driving the reproducing heads P1 ... P6 to allow of enlarging or reducing the width of the reproductions. In order that the height of the reproductions may be varied, the original E may be mounted on a detachable and interchageable portion of the cylinder A. The films F1 ... F6 may be covered by a half-tone screen. At the boundary between two colours U, V, Fig. 8, special considerations apply. So long as the scanning spot T is on one colour or the other a single detector N is energized but when the spot overlaps both colours the cathode. ray beam is deflected to a point between apertures in the mask and no detector is energized. There would therefore be a separation between the two reproduced colour areas equal to the diameter of the scanning spot but since the reproducing spot is of finite diameter d the separation is reduced to D - d and the amount of this separation can be arranged by altering the size of the scanning spot so as to produce a good printing fit of the reproduced negatives. A third scanning photo-cell may be employed to register a third colour component but this component cannot be indicated on the twodimensional screen in the same way as the red and blue components. The output from the third cell is used therefore to vary the intensity. or frequency of the cathode-ray beam. The scanning head photo-electric cells 11, 12 may instead be directly connected to the X and Y plates of six small cathode-ray tubes each differently biassed and each covered by a mask having one suitably positioned aperture, so that for a particular colour only the corresponding beam penetrates its aperture to expose the corresponding film. The cathode-ray tube of Fig. 1 may be replaced by one having six collector electrodes which are manually adjustable to the required co-ordinate positions (see Group XL (a)). A particular colour then energizes a corresponding collector electrode whose output energizes a lamp focused on a film.