GB1395793A - Photographic silver halide emulsion - Google Patents

Abstract

1395793 Silver halide emulsions EASTMAN KODAK CO 19 June 1972 [17 June 1971] 28605/72 Heading G2C A silver halide emulsion in which the grains are substantially free of any chemically-induced sensitivity, both internally and on the surface, contains an electron-accepting compound having a cathodic half-wave potential more positive than -1.0 and an anodic half-wave potential more positive than +0.4. The compound may be a methine or polymethine dye, particularly 3, 3'-indolocarbocyanines substituted in the 2, 2' positions by aromatic groups, symmetrical imidazo [4, 5-b] guinoxaline trimethine cyanines, trimethine cyanines containing pyrrolo [2, 3-b] pyridine nuclei, and cyanines substituted by the desensitizing nitro group e.g. nitrobenzylidenerhadanines, nitro, nitrophenyl and dinitrothiacarbocyanines, and nitrothia -2'-cyanines. Other electron acceptor compounds include 4- nitro-6- chlorobenzotriazole; 2, 3, 5- triphenyl- 2H- tetrazolium chloride, 2- (4- iodophenyl)- 3- (4- nitrophenyl)-5-phenyltetrazolium chloride; 1-methyl-8-nitro-quinolinium methylsulphate and 3, 6-bis [4- (3-ehtyl-2-benzothiazoliumylidene)-2- butenylidene]-1, 2, 4, 5-cyclohexanetetrone. The silver halide is predominantly bromide the grains are monodisperse e.g. having a grain size of 0.2Á, and the emulsion may also contain a spectral sensitizing dye. The emulsion gives a direct positive image when imagewise exposed to light of relatively low intensity, then exposed overall to light of relatively high intensity and developed with a surface or internal silver halide developer.