Latent sub-image

The best point of attack for a determination of the surface electrochemical properties of latent images is at the level of latent sub-images. There can hardly be more than three sizes (2, 3,4) without gold sensitization, and only one size (N=2) with gold. Indeed, the continued presence of optical latensification effects after the application of optimal gold sensitization established the development threshold at N=3, not 2, in the presence of gold atoms. 

The isolated silver atom formed in this reaction could take part in latent image formation either by diffusion to a latent sub-center or by thermal dissociation with the transfer of an electron to the conduction band. Hence, it is theoretically possible that the absorption of one photon by the silver halide results in two latent image silver atoms. Reduction centers larger than Ag2 could not contribute additional silver to the latent image in this way until their size had been decreased to two atoms by reaction with holes. 

The absorption of energy by the grains produces conduction electrons and either free or trapped holes. The conduction electrons and the holes diffuse initially by a three-dimensional random walk. In chemically sensitized crystals, the holes are trapped by products of chemical sensitization which thus undergo photo-oxidation. Rapid recombination between a trapped hole and an electron is avoided by the delocalization as an interstitial Ag ion of the nonequilibrium excess positive charge created at the trapping site. Latent pre- and sub-image specks are formed by the successive combination of an interstitial Agi ion and a conduction electron at a shallow positive potential well. 

A variation of the typical anodized aluminum is Photosensitive Anodized Aluminum. This method uses the porous nature of unsealed anodized aluminum to create a sub-surface image. For this special case, the anodized aluminum is impregnated with a silver compound that creates an activated latent image when exposed to a source of light. After the developing and fixing processes take place... 

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