GB606120A - Process of and apparatus for making half-tone negatives - Google Patents

Abstract

606,120. Photo mechanical process and apparatus. POWERS PHOTO ENGRAVING CO. July 11, 1945, No. 17726. Convention date, Aug. 23, 1944. [Class 98 (ii)] Half-tone negatives are made by twice exposing a light-sensitive element located in the focal plane of a camera to light reflected from copy through a screen which has spaced intersecting lines wider than the spaces in between the lines, one exposure through a smaller stop aperture and the other through a larger aperture which surrounds an opaque area similar to the smaller aperture, the spacing of the screen from the element and stop apertures, and the sizes of the apertures, being such that the larger aperture will produce exposures surrounding those of the smaller aperture. The apertures may be square or the larger one may be of quadrilateral shape, the outer periphery of each side being convexly curved, and they may be aligned at an angle of 45 degrees to the lines of the screen. The negative may be the usual wet plate, or, as shown in Fig. 1, a film 19 which is exposed to light through lens 11, stop 12, with aperture 13 and half-tone screen assembly indicated generally at 14, the latter having a screen mounted in a frame (Figs. 2, 3 and 4, not shown), and spaced apart from a transparent plate 15 so that the screen is at the correct distance from the film 19, being held there by camera screen bars 16, while the film 19 is pressed against plate 15 by a further plate 17. Fig. 5 shows a stop with smaller aperture 36 and a larger aperture which has opaque centre 38, and Fig. 7 shows how the larger aperture is used to produce exposed areas 42 around the exposure 41 of the smaller aperture.