Direct positive silver halide photographic emulsions - with internal latent image silver halide grains and nucleating agent comprising bis:condensn. prod. of di:hydro:spiran with salicyl aldehyde

摘要

Direct positive silver halide emulsion comprises grains capable of forming an internal latent image and adsorbed on the surface of the grains, a cpd. (I) comprising the biscondensation prod. of a dihydrospiropyran with salicylic aldehyde or a substd. deriv. of this in an amt. sufficient to allow nucleation of the silver halide grains. Nucleating agent (I) pref. has the formula (I) in which X and Y (same or different) are S, Se or C(R1)(R2); R7 and R8 (same or different) are substd. alkyl gps. of the type normally found in cyanine sensitiser dyes; R3, R4, R5 and R6 are each H, lower hydroxyalkyl or lower alkoxy gps. with 1-5C; R1 and R2 are each 1-5C alkyl or together form a 4-5C alkylene gp.; Z1 and Z2 are the atoms necessary to complete a heterocyclic gp. of the type found in cyanines and opt. contg. substituents or condensed aromatic rings. The (I) have high stability and can be adsorbed onto the surface of the silver halide grains to give direct positive colour or black and white emulsions.