Additive color reproduction may be produced by exposing a photosensitive silver halide emulsion through an additive color screen having filter media screen elements each an individual additive color such as red, green or blue and by viewing the reversed deposit of silver image formed by transfer to a transparent receiving element through the same or similar screen which is suitably registered with the reverse positive image carried by the receiving layer. As examples of suitable film structures for employment in additive color photography mention may be made of U.S. Pat. Nos. 2,861,885; 2,726,154; 2,944,894; 3,536,488; 3,615,427; 3,615,428; 3,615,429; 3,615,426; 3,894,871; and 3,364,482. Co-pending applications Ser. Nos. 697,104, filed June 17, 1976 and 649,201, filed Jan. 14, 1976 and commonly assigned, also disclose photographic diffusion transfer film units for use in additive color photography.
Co-pending application Ser. No. 383,261, filed July 27, 1973 and commonly assigned is directed to an additive color diffusion transfer film unit which includes a transparent support, an additive multi-color screen, a photosensitive silver halide emulsion layer and a layer comprising an antihalation dye adapted to be discharged as a function of the contact of the layer with photographic processing composition.
U.S. Pat. No. 3,704,126, issued Nov. 28, 1972, discloses and claims photographic silver diffusion transfer processes and film units wherein the positive silver image is provided with enhanced stability by employing in the film unit a noble metal compound, which noble metal is less reactive than silver, that is, below silver in the Electromotive Force Series of Elements. In a preferred embodiment the noble metal compound is an organometal compound which is substantially water insoluble, alkaline solution-soluble and may specifically comprise a compound of the formula M-X, wherein M is a noble metal less reactive than silver and X is an organic complexing ligand which preferably provides a substantially water insoluble complex. The disclosure of U.S. Pat. No. 3,704,126 is incorporated herein by reference in its entirety.
In the above-described film units employed in additive color diffusion transfer processes, a noble metal compound may be disposed in a processing composition permeable layer distal to the support. Thus, the layer comprising the noble metal compound is often located next adjacent the photosensitive silver halide layer.
It has been found, however, that as to silver diffusion transfer film units, employing such noble metal compounds may result in some degree of desensitization of the silver halide emulsion. While not intending to be bound by theory, it is believed that the organic ligand portion of the noble metal compound may contribute to a desensitization effect observed in the silver halide emulsion layer by displacing optical sensitizers on the silver halide grains, but the exact mechanism is not known.