Patent GB-581773-A (GB)
Title: Improvements in photographic reversal processes
IPC: G03C1/485
Filed: 1942-09-02
Abstract: 581,773. Photographic reversal processes ; colour photography. KODAK, Ltd., STEVENS, G. W. W., and KNOTT, E. B. Sept. 2, 1942, Nos. 12423/42 and 16280/43. [Class 98 (ii)] A reversed image is formed in a layer of sensitive silver salt emulsion of the kind which forms the latent image mostly inside the grains by (a) exposing the layer to an object or image, (b) without developing the internal latent image so formed, treating the emulsion so as to render it capable of forming a latent image on the surface of the grains, (c) exposing the layer to light to form a reversed surface latent image, and (d) developing it in a bath which will develop surface but not internal latent images. Examples of emulsions of the above-stated kind are Burton's emulsion (Wall : " Photographic Emulsions," 1929, pp. 52, 53), and an ordinary silver iodobromide emulsion which has been bathed in a solution containing potassium bichromate and sulphuric acid ; and an emulsion may be tested as to its suitability as described in Specification 581,772. Examples of developers which will develop surface images only are a developer containing p-hydroxyphenylglycine and sodium carbonate, and a metol - hydroquinone developer containing sodium sulphite, potassium bromide and sodium carbonate. A similar metol - hydroquinone developer containing sodium carbonate or sodium hydroxide will develop internal images if a proportion of sodium thiosulphate is included therein. High contrast in the reversed image is obtained if over-exposure is given in step (a). Treatment (b) above may consist in soaking the emulsion in an ordinary developer containing an organic developing agent, but not a solvent for silver halide, such as one of the surface developers instanced above, and this developer may serve also for step (d) ; or, in the case of certain of the emulsions which are colour sensitive, e.g. one sensitized with the dye 5-[(2-ethyl-1(2)benzoxazolylidene)-ethylidene] - 3 - n - heptyl -1 - phenyl - 2 - thiohydantoin described in Specification 532,098, treatment (b) may consist in soaking in plain water. The exposure to reversal light referred to under (c) above is preferably of low intensity, and the developer used in (d) should be dilute or of low pH value to avoid re-reversal or a reduction of the higher densities. Any tendency to the formation of veiled whites owing to unwanted development in the surface developer of the initial internal image can be prevented by treating the layer after the first exposure, but before immersion in the developer, in a mild silver-oxidizing bath, e.g. containing ferricyanide and bromide of potassium. The application of the process to the production of images in a sensitive material consisting of several emulsions each of the kind referred to on a single support is described in Specification 581,789, with particular reference to colour photography. The method may be applied also to the reversal development of an emulsion layer of the kind referred to which has been exposed through a superimposed multi-colour screen. Specification 524,087 also is referred to.