Patent GB-623644-A (GB)
Title: Improvements in or relating to colour photography
IPC: G03C7/20
Filed: 1946-09-03
Abstract: 623,644. Colour photography. DU PONT DE NEMOURS & CO., E. I. Sept. 3, 1946, No. 26478. Convention date, Sept. 12, 1945. Drawings to Specification. [Class 98 (ii)] A bipack film comprises a first element (A) consisting of a transparent film base having superimposed thereon in order (a) a waterpermeable colloid layer containing silver halide and sensitive to blue light and containing lightinsensitive colloidal silver, the layer being of a colour absorbing blue light, due to the colloidal silver itself or to the presence of an added dyestuff, (b) a water-permeable colloid layer which is light insensitive and contains no silver or silver halide, and (c) a water-permeable colloid layer containing silver halide and sensitive to a second primary colour of the visible spectrum and a second element (B) consisting of a transparent film base carrying (d) a waterpermeable colloid layer containing silver halide which is sensitive to a third primary colour. Yellow colloidal silver such as that described in Specification 610,941 may be used. Individual colour separation records are made from the bipack film by exposing it in a camera so that the incident light falls on the transparent base of the first element (A), developing, fixing, printing a master positive from the first element, treating it with a hardening agent to harden the colloid in layer (a) where there is colloidal silver, treating it with an aqueous bath to soften the unhardened colloid of layer (b); separating layers (b) and (c) leaving an individual blue separation negative record in layer (a); printing a duplicate negative from the superimposed master positive and blue separation records, this duplicate negative being an individual separation record of the colour to which layer (c) is sensitive; and developing and fixing the second element (B) of the bipack, to provide the third separation record. The colloid in the various layers of the front element (A) is preferably gelatine, hardening may be effected by use of a hardening developer such as one containing pyro, or by ammonium dichromate solution; and softening of the layer (b) for the purpose of removing (c) by immersion of the element (A) in hot water or in aqueous alkaline thiocyanate, ethylene chlorhydrin, acetamide or hydroxyacetamide. In the preferred procedure an aqueous solution containing ammonium dichromate and potassium thiocyanate is used simultaneously to harden layer (a) and soften layer (b), after which a water rinse dissolves (b) and permits easy removal of the outer layer (c). In example (1), a cellulose nitrate film base having a thin gelatine coating was provided with layers (a) of gelatine-silver iodobromide emulsion containing yellow colloidal silver, (b) of plain gelatine, (c) of gelatino-silver iodobromide emulsion containing 1,11-diethyl-2,21-cya.nine iodide as ortho-sensitizing dye, and then a gelatine anti-abrasion layer to form the front element (A); and a similar gelatine coated film base was provided with a layer (d) of gelatino-silver iodobromide containing 1,1<SP>1</SP>- diethyl-2,4-carbocyanine iodide to form the rear element (B) of the bipack; the two films with outer gelatine layers in contact were exposed to a colour scene, developed in a metolhydroquinone developer and fixed; a master copy of the combined images of the element (A) was made and film (A) was then bleached in acidified dichromate, which bleaching operation hardens layer (a); by hot water layer (b) was dissolved and layer (c) removed, after which the support carrying layer (a) was redeveloped in a pyro developer. Ammonium thiocyanate solution can be used instead of hot water to eliminate layer (c). In example (2), the front element (A) of a similar exposed bipack film was developed directly in a pyro developer, fixed and washed in hot water to remove layer (c). Examples (3) and (4) describe hardening of layer (a) by aqueous ammonium dichromate and removal of layer (c) by aqueous potassium thiocyanate, these operations being either simultaneous or successive; and these being the preferred form of the invention because they reduce the distortion characteristics of high temperature processing. The Specification as open to inspection under Sect. 91 describes also an element in which the colloidal silver is present in the gelatine layer (b) instead of in the front silver halide layer (a). This subject-matter does not appear in the Specification as accepted.