Patent GB-506581-A (GB)
Title: Improvements in photographic and photomechanical colour printing
IPC: G03F3/04
Filed: 1937-08-28
Abstract: 506,581. Colour photography ; photomechanical printing processes. HENRY, P. S. H.. Aug. 28. 1937, No. 23585. Drawings to Specification. [Class 98 (ii)] A colour photograph or photo-mechanical print produced from three subtractive colour components is characterized in that in the areas in which grey is present, equal densities of the three colours arc replaced by a density of black substance equal to the major part of the density of the colour present in least quantity and the areas in which grey is not present are free from black substance. Preferably the whole of the colour present in' least quantity is replaced so that only two colours and black are present. In preparing the record for supplying the black, three subtractive colour records are produced each including an image of a step wedge and are processed to the same scale of densities. A positive transparency is made from one of these, e.g. the green record, having the same density range as the negative (a "normal '' transparency) and is registered with a negative of another of the records, e.g. the red record. A " sub-normal " transparency is made from the combination, i.e. one in which an emulsion having a short too to the characteristic curve is used and the exposure is such that only a faint image is produced under the superimposed images of the wedge. This transparency is registered with the red negative and a " normal " transparency is made from the combination and is registered with the blue negative. From this combination a " subnormal " transparency is made which is registered with the blue negative and the combination forms the key negative for producing the black printer. Corrected colour components may be made by preparing " normal " positive transparencies from each of the negatives, registering the green positive with the red negative, the blue positive with the green negative, and the red positive with the blue negative and making " sub-normal" transparencies from the combinations which are registered respectively with the red, green, and blue negatives and normal transparencies made from these. The resulting transparencies are registered respectively with the blue, red, and green negatives and the corrected positives printed from these. The first pair are also used to print a " subnormal " transparency which in combination with the blue negative forms the key negative. in the case of negatives produced on colour-screen plates having a screen of parallel lines, the combination printing is performed with a positive from this superimposed on the negative and shifted laterally by the width of one line, either by contact or projection. The process may be shortened when one of the component colours is always predominant by using the negative of that colour as the key negative, or when one of the colours is never predominant by using the first " sub-normal " transparency of the main process in register with the red negative to print the key negative. When printing by contact register may be obtained by applying petroleum jelly between the transparencies to allow of minute relative adjustment. In using the process for photomechanical printing employing a half-tone screen, a screen having a reduced number of lines to the inch, e.g. ¢ or 1/3 is used when preparing the black printer or the screen for the black printer may be set at an angle to the screens for the component records. The grey component in colour photographs or kinematograph films may be provided by silver or a black dye.