Abstract: The role of iridium salt in AgBr emulsions was investigated in this work. It was prepared that monodisperse cubic AgBr grain with edge length of 0.3μm. which are surface and internal sensitized with (NH₄)₃IrCl₆or (NH₄)₂IrCl₆; It was found that the surface sensitivity of the emulsion with Ir surface sensitization is increased. The surface sensitivity of the emulsion with Ir internal sensitization is decreased, but its internal sensitivity is increased. The Ir internal sensitized and fogged emulsion can form direct positive image after exposure and development. The Photoelectrons may be trapped by the internal Ir-centers forming internal latent image and the surface fog centers on the grain may be destroyed by the holes.

Key words: iridium sensitization, surface development, internal development, chemical fogging, direct position image