GB813116A - Process and apparatus for photo-mechanical screening - Google Patents

Abstract

813,116. Photo-mechanical half-tone screen processes. SCHUMACHER, K., and SCHUMACHER, E., [trading as KLIMSCH & CO.]. July 8, 1955, No. 19908/55. Class 98(2) In a half-tone screen process in which a screen wherein the area of each transparent portion or window is small relative to the area of the associated opaque portion is moved continuously in its own plane during exposure in such manner that the image of each window traces out a predetermined path over the surface of the light-sensitive material, the speed of movement of the images and/or the intensity of light reaching the sensitive material is varied as the images travel along their path. The screen may be so moved that each image passes over substantially the whole of the field defined by the associated opaque portion or that bridges are formed between the integrated dots formed by the image movements to permit the resultant negative to be used in a rotogravure process. The images may be caused to move each in a spiral path inwardly from the periphery of the field or outwardly from the centre of the field, or the movements of the images may be a series of rectilinear movements. The fields may be rectangular or hexagonal. Part of the exposure may be effected with the screen stationary. As shown, cams 2 and 3 rotatable on a shaft 4 operate on the ends of levers 5 and 6, the lever 5 and an auxiliary lever 7 being pivoted at 8 and 9 and engaging the lower horizontal edge of the frame of the screen 1 while the lever 6 pivoted at 10 engages the vertical edge of the screen frame, which is urged by springs 11 against the ends of the levers. The cams are designed to move the images in any desired paths and at any desired speed at each point in such paths. The screen may also be moved by other mechanical means or by electrical or hydraulic means, and a variable speed drive for the cams may be operated electrically or mechanically, e.g. by an additional cam operating switches controlling the speed of an electric driving motor. The light intensity may be varied by varying the intensity of the light source or by means of polarising, neutral grey, or complementary coloured filters, or by means of a Kerr cell. Variations in the opacity of the elementary dots formed on the sensitive material may be obtained by replacing the iris diaphragm by an appropriately shaped stop in the case of projection printing or by placing the stop in front of the light source in the case of contact printing. In making half-tone colour separations, different paths of the images for each colour may be used to prevent 'moirÚ' or a set of screens having different angular positions of the screen pattern may be employed. An uncemented glass screen having its etched and brackened surface adjacent the light-sensitive surface is preferred, but ruled, printed. photographic, or lenticular screens or screens with a dyed ground may be used. A holder may be provided on the body of the screen moving device for a corrective mask or the negative. Various ways of using the screen in optical copying from an original and in projection and contact printing using a negative or diapositive are described.