Patent GB-681206-A (GB)
Title: Improvements in or relating to photomechanical processes
Inventor: GRESHAM DONALD CHARLES
IPC: G03C7/30
Filed: 1950-02-08
Abstract: 681,206. Photo - mechanical processes. McCORQUODALE & CO., Ltd. Jan. 4, 1951 [Feb. 8, 1950], No. 3267/50. Class 98 (ii). A method of producing positives directly from originals by a reversal technique which method comprises the steps of exposing a photographic plate or film in a camera to light from an original subject, so that the record on the plate or film is made up of dots or lines or both, developing to silver the latent image obtained thereby producing a negative image in silver, producing a coloured image in situ therewith and bleaching the silver image, reexposing the photographic plate or film to light incident on the same side thereof as in the original exposure, the said light being of a colour to which the photographic emulsion is sensitive but which is absorbed by the coloured image, redeveloping the photographic plate or film, fixing the photographic plate or film to remove any residual silver halide, and removing the coloured image. The plate or film may be first exposed through a half-tone screen and the coloured image may be formed by dye-toning, mordanting or, preferably, by effecting the first development by means of an aromatic primary amino developing agent in the presence of a colour former. With plates or films which are sensitive to blue light only, the colour former is preferably one which yields a yellow or yelloworange image and may be included in the emulsion or developer. The second developing agent used is preferably one giving high contrast, e.g. caustic sodahydroquinone developer, and may include an anti-fogging agent such as benzotriazole. The dye image may be removed by bathing the film in an organic solvent for the dyestuff, such as a mixture of methyl alcohol, acetone and glacial acetic acid. To effect tone-correction, the second exposure is effected in the camera at least partially to the original, the remainder of the re-exposure being to uniform light. In the production of colour separation screen positives, the colour rendering is corrected to allow for imperfections in the printing inks used by re-exposing the blue separation screen negative partially to the original via a green filter, reexposing the green separation screen negative partially to the original via a red filter and re-exposing the red separation screen negative partially to the original via a red filter; and a colour former, e.g. thymol and α-naphthol, yielding a blue image, is used in the first development of each screen negative. To improve the highlight contrast on each screen positive, a highlight mask may be registered in front of each negative when re-exposing. If the colour-sensitivity of a screen negative is too low, the negatives may be bathed before re-exposure in a solution of a colour-sensitizing dye, e.g. a solution including pinacyanol bromide, ethyl alcohol, pyridine and triethanolamine. The plate or film may have therein an anti-halation layer comprising a quinone-imine or azomethine dye which is removed at the same time as the coloured image; if an anti-halation layer is used which is removed by the first development, a new anti-halation layer may be coated on the plate or film or the plate or film may be backed by an adhesive black sheet. A suitable yellowforming developer includes N,N-diethyl-pphenylene diamine, acetone, and 2: 5-dichloracetoacetanilide. A suitable blue-forming colour developer includes N,N - diethyl - p - phenylene diamine, acetone and thymol.