Study on Fog Centers of Pre-fogged Direct Positive Photographic Emuisions

The distribution of redox potential of fog centers on the surface of the silver halide grains of the direct positive emulsion pre-fogged by thiourea and stannous chloride respectively was determined. Their characteristics of the distribution of redox potential were compared. The distribution of the critical radius and the atom number of fog centers were quantitatively investigated wiyg Gibbs-Thomson equation. That why the direct positive emulsion fogged by stannous chloride has low reversal speed was explained.