Patent GB-544035-A (GB)
Title: Improvements in or relating to half-tone screens for photomechanical printing processes
IPC: G03F5/16
Filed: 1940-06-17
Abstract: 544,035. Half-tone screens for photomechanical printing processes. KODAK, Ltd. (Eastman Kodak Co.). June 17, 1940, No. 10468. Drawings to Specification. [Class 98 (ii)] A contact half-tone screen comprises elements of a material which is substantially non lightdiffusing, that is, which if formed as a layer of uniform density of 0.7 has a diffusion, measured as described below, of less than 0.0002 and preferably less than 0.00005. Screens formed by such elements give clearer reproduction of detail and less unevenness of reproduction when the screen is not everywhere in contact with the sensitive surface, as may occur owing to dust particles. The fact that absolute contact is not essential allows the surface of the screen to be very slightly roughened to a degree insufficient to give a matt appearance but sufficient to avoid the formation of interference patterns between it and the surfaces with which it is in contact. The diffusion is measured by arranging a uniformly bright disc-shaped light source parallel to, and subtending an angle of 10 degrees at, the test sample, which is coated with the screen material to a uniform density of 0.7. The brightness of the source is then measured by a brightness meter placed on its axis, the test sample being removed. The sample is then replaced, and the meter is moved through an angle of 45 degrees about the point in the sample at which the axis of the light source meets it, whilst remaining at the same distance as before from this point. The meter now indicates the brightness of the light which is diffused at an angle of 45 degrees by the test sample, and when the ratio of this reading to the first is below 0.0002, and preferably below 0.00005, the material is regarded as substantially non-diffusing. The screen material may be coloured, in which case the light used for the test is of a complementary colour. The density gradation across the screen elements is preferably as described in Specification 544,033. The screen may be made in known manner by exposure of a photographic emulsion to the shadow image of a half-tone screen formed by a uniformly illuminated aperture, with or without an additional exposure of the centres of the elements, as described in Specification 544,033, and cross-lined spaced screen whose interval S is made up of lines and openings, preferably of equal width, may be used, placed at a distance K in front of the sensitive surface where K is between SL/D and SL/4D, L being the distance of the illuminated aperture from the sensitive layer and D its diameter. The contact screen may be developed to a coloured image by the use of colour-forming developers. The production of an orange-red and a magenta screen is described each being made on film known under the Registered Trade Mark "Micro-File," which has a slightly-roughened non-interference coating on each side. The orange-red screen is made on non-colour-sensitized film, preferably by ultraviolet light as described in Specification 544,033 and is developed in a solution containing stated quantities of p-aminodiethylaniline hydrochloride, sodium hexamethaphosphate, sodium sulphite, potassium bromide, sodium hydrate, the substance known under the Registered Trade Mark "Kodalk," and the dye-forming substances 1-p-nitrophenyl-3-methyl 5-pyrazolone and 4-p-toluenesulphonamino-#-benzoylacetanilide. After treatment in an acetic acid stop bath the screen is fixed in a solution of sodium thiosulphate, sodium sulphite, acetic acid, boric acid, and potassium alum. It is then treated in a solution of sodium sulphate, sodium carbonate, potassium ferricyanide, sodium theosulphate and formaline. This screen has a brightness ratio of 0.00002. The magenta screen is made on a panchromaticallysensitized film and is developed in a developer having the same constituents as that for the orange-red screen except for the omission of the 4-p-toluene sulphonamino- #-benzoylacetanilide and the addition of metol, n-butanol, and benzotriazole. After treatment in the stop and fixing baths it is treated in a reducing bath containing ammonium bichromate, sulphuric acid, and sodium chloride, which reduces the overall density and removes the lowest densities. This screen has a brightness ratio of 0.000023. Absorption curves for these screens are given, the density of the orange-red and magenta screens varying respectively about 32 per cent and 45 per cent of their maximum values over the range 400 to 520 mÁ. Other examples of "non- diffusing" screens referred to are a silver-image screen made on a Kodak High Resolution plate by development with the developer given above for the orange-red screen, but omitting the dyeforming substance, and having a brightness ratio of 0.00017 ; a magenta screen produced by colour development on Kodak Commercial Film, with a brightness ratio of 0.000036; and a neutral-grey screen formed by colour development, with a brightness ratio of 0.00016. A red screen may be obtained by the use of a p-aminodiethylaniline hydrochloride developer with the colour couplers parabenzoylacetaminobenzene sulphonamide and 1-paranitrophenyl-3-methyl-5-pyrazolone. The dye-former may be contained in the emulsion and the coloured image be produced by any known process, for example the silver dye bleach process described in Specification 521,746. Specifications 474,911 and 544,034 also are referred to.