Abstract: Silver bromide emulsions with monodisperse cubic grains, which were sensitized internally by iridium, were prepared. The emulsions were fogged by thiourea dioxide and gold salt. It was found that if the fogging proceeds with gold, fog centers may form predominantly on the grain surface, and if fogging proceeds without gold salt, fog centers may form predominantly in the interior of the grain. The emulsion grains were etched with Na₂S₂O₃ solution for different times and spatial distribution curves of fog centers were obtained. When emulsions were fogged by (1) a small amount of thiourea dioxide and gold salt, fog centers distributed on the grain surface and sub-surface; (2) a more amount of thiourea dioxide and a small amount of gold salt, fog centers distributed on the grain surface and interior, but from the surface to the center, fog centers became less and less; (3) a more amount of thiourea dioxide, fog centers mainly distributed in the grain interior. It is clear that the function of gold in the direct-positive process is to suppress the internal fogging which in turn to eliminate the competition in hole trapping and to increase the reversal efficiency.

Key words: direct-positive emulsion, fog center photobleaching