Photographic imaging employing silver halide grains comprises a stage of forming a latent image.  At the time of exposure, conduction band electrons are produced and these electrons migrate in the grain and are trapped at specific sites where the latent image is formed.  In ordinary, non-sensitized, silver halide grains, dispersion of the sites promotes a faster subsequent development but enables only low sensitivity levels to be achieved.  The purpose of the sensitization is to concentrate these sites in order to increase sensitivity.  However, in doing this, the development speed is reduced. 
Means have therefore been sought for improving the concentration and localization of the latent image sites in silver halide grains.  These means in the prior art include using compounds which are adsorbed selectively on certain sites in the grains, introducing distortions in the grain or in the crystalline morphology of the grains, as described in European patent 96 726 or U.S. Pat.  No.  5,045,443, modifying the grains by epitaxy, as described in European patent 462,581, or again producing grains with a complex crystalline form, as described in U.S. Pat.  No.  4,710,455.  None of these different means is entirely satisfactory.