Patent GB-557132-A (GB)
Title: "drop-out" half-tone negatives and positives and processes of producing the same
IPC: G03F5/24
Filed: 1942-02-10
Abstract: 557,132. Photo-mechanical printing-surfaces. LIEBOWITZ, B. Feb. 10, 1942, No. 1761. [A Specification was laid open to inspection under Sect. 91 of the Acts, Aug. 11, 1942.] [Class 98 (ii)] In order to eliminate all half-tone screen pattern from the highlight areas of a half-tone positive or negative used in preparing a printing surface, use is made of (1) a half-tone mask negative, prepared from the original drawing or picture to be reproduced, and having substantially opaque areas corresponding to the highlight areas and substantially transparent areas corresponding to the tone areas of the original, and (2) a halftone mask positive made preferably by printing from the half-tone mask negative. If the original includes lines which are to be reproduced solid, a half-tone and line mask negative and a halftone and line mask positive may similarly be used. The half-tone mask negative 4 may be produced by first making a continuous-tone negative from the original 1, after this has been retouched if desired to darken the lightest tones and whiten the highlights, treating the negative with a reducer until the tone areas become transparent, and thereafter, if the highlight areas are too thin, treating the negative in an intensifier. The intensifying step may precede the reducing step. A half-tone and line mask negative 11 may be similarly produced from an original 8. The reducer, preferably of the subtractive type which reduces all densities by the same density stop, may be Farmer's Reducer, a ferricyanide-bromide bleacher followed by weak hypo, acid quinone, or ammonium persulphate free from halogen ions; the intensifier may be a ferricyanide bleacher followed by a sulphide bath, or a uranium intensifier, or a bath of silver nitrate containing a slowly-working reducing-agent. If the original includes lines, broadening of these by reducer action may be prevented by coating the lines of the negative with a reducer-resist such as shellac. It is possible also to convert a half-tone and line mask negative 11 into a half-tone mask negative 4 by opaquing out the lines. If the original 1 contains no lines, a halftone positive for use in preparing a printing- surface, and having no dots in the highlights, may be prepared by making (a) a half-tone negative 2 by photographing the original through a half tone screen and thereafter flashing the image surface through the screen with a sheet of white paper, (b) a half-tone mask negative 4 as described above, and then printing the desired positive from (a) and (b) in registered superimposition ; or by printing through a half-tone screen from a continuous tone negative of the original and a half-tone mask negative 4 in registered superimposition. A half-tone negative having no screen pattern in the highlights may be prepared by photographing the original 1 in turn, on the same sensitive layer, through a half-tone mask negative 4 and a half-tone mask positive printed from it, a half-tone screen being interposed, with the usual spacing from the sensitive layer, when the mask negative is being used. If the original 8 contains lines, a half-tone and line positive having no dots in the highlights may be prepared by making (c) a continuous tone negative from the original with the tone areas opaqued out, so that only the lines are transparent, (d) a halftone and line negative 9 of the original by photographing it through a line screen and flashing, and opaquing out the lines, and (e) a half-tone mask negative 4, and then printing the desired positive in two stages, one from the negative (d) and the mask negative (e) in superimposition, and the other from the negative (c) ; this procedure may be varied by using instead of the negative (d) a continuous-tone negative of the original with the lines opaqued out, and performing the first-mentioned printing-stage through a half-tone screen slightly spaced as usual ; a preferred procedure is, however, to make (f) a half-tone and line negative similar to 9 but with clear lines, by photographing the original 8 through a half-tone screen and subsequently flashing it through the screen with a celluloid mask interposed having opaque lines corresponding to those of the original, and (g) a half-tone and line mask negative 11 made as described above, and then printing the desired positive from (f) and (g) in superimposition. A half-tone and line negative having no screen pattern in the highlights may be prepared by printing from the half-tone and line positive produced by any of the methods just described: alternatively, it may be prepared by making a half-tone mask negative 4, and printing from it a half-tone mask positive, and photographing the original 8 in two stages, one through a half-tone screen with the half-tone mask negative covering the original, and the other with the half-tone mask positive covering it; this procedure may be varied by using instead of the half-tone mask positive a half-tone and line mask positive. It is stated that the halftone mask negatives and positives, and their methods of production are of general photographic and kinematographic application.