GB191409929A - Improved Methods in the Production and use of Screens for Colour Photography and General Photographic Purposes. - Google Patents

Abstract

9929. Rheinberg, J. April 22. Colour photography, screens and screen processes for; photomechanical printing-surfaces.- In the manufacture of screens and screen plates for half-tone processes or colour photography, and in the production of colour pictures made by multicolour screens, use is made of chemical changes caused in a colloid film coated with a resist vehicle, by the action of light which alters the permeability of the resist to solvents whereby dyes or pigments can be extracted from the underlying film, or whereby the underlying film can be dyed through the permeable parts of the resist, without any removal of the superimposed resist. To produce a three-colour screen, a collodion film dyed red is coated with an albumen resist which is sensitized by immersion in water containing green ferric ammonium citrate and uranium nitrate. The plate is exposed under a black-line screen having 200 lines to the inch, the black and white spacing being equal. The exposed lines on the resist are rendered more permeable to alcoholic solutions, and the plate is immersed in acidified alcohol to extract the red dye from the exposed lines. The plate is then immersed in an alcoholic solution of a green dye, by which means the clear lines from which the red dye has been extracted are dyed green, giving a screen of equally spaced red and green lines. The plate is then washed to remove the water soluble chemicals, resensitized with the same mixture, and exposed through a black-line screen with 200 lines to the inch, the opaque lines being twice the width of the clear spaces, and the lines being placed across the red and green lines. The plate is ther treated in acidified alcohol to remove the red and green dye from under the lines exposed through the screen, leaving red and green rectangular areas, ach colour occupying one-third of the plate area. The plate is then immersed in an alcoholic solution of violet dye to obtain violet lines., The whole of the resist may then be removed by warm water, to which a little acid or alkali may be added. In lieu of starting with a coloured film, a transparent film may 'be used and dyed up progressively. Various alternative substances are described for the underlying film, the resist vehicle, and the sensitizers, the governing condition being that both the underlying film and the resist vehicle must be permeable to the solvent used for extracting or conveying the dye to or from the underlying film. The underlying film may be of the same colloid as the resist vehicle, and the two films may be separated bv a third film also permeable to the solvent used. In the application of the process to screen photographs, the colour particles may be selectively removed from the screen in the white or light portions of the picture. This method is described in Specification 22,938/13 for the particular case of pictures made up of multicolour particles which act additively and are converted into a picture in which the particles act subtractively.