GB1405803A - Photographic direct-positive silver salt material - Google Patents

Abstract

1405803 Photographic direct positive silver salt material AGFA-GEVAERT A G 18 May 1973 [20 May 1972] 23776/73 Heading G2C A photographic material comprises at least one fogged direct-positive silver salt emulsion layer having silver salt grains containing internal electron traps and which are heterodisperse and irregular and have a grain size distribution in which at least 10% of the number of the grains have a diameter which deviates by at least 40% from the average grain diameter. The heterodisperse and irregular salt grains may have cores containing electron traps and an outer shell of silver salt on the cores. The silver salt may be silver bromide which may contain up to 20 moles per cent silver iodide. The silver salt grains may be fogged with a reducing agent in the presence of a gold compound or a compound of another metal more electro-positive than silver. The emulsion layer may contain a sensitizing or desensitizing dye (see EX9). Electron traps may be produced by forming ripening nuclei e.g. by chemical sensitization with gold or iridium salts or sulphur compounds such as thiosulphate, or by treating the cores with aqueous solutions of polyvalent metal salts e.g. trivalent bismuth. The directpositive emulsions used in the above materials are easily produced and have a flat gradation as is required for continuous tone images.