In this review, physical properties of the silver halide crystals that would be closely related to high efficiency of the latent image formation were discussed from the general point of view and then discussions were concentrated on the ionic process that was indispensable to the latent image formation.
Ionic conductivity of the silver halide microcrystals dispersed in gelatin was measured by the dielectric loss method based on the Maxwell-Wagner-Sillars theory. It was shown that the ionic conductivity of the microcrystals was larger by several orders of magnitude than that of the large pure crystals, being proportional to the surf ace-to-volume ratio of the microcrystals and that the incorporation of divalent cadmium ions into the emulsions grains markedly decreased ionic conductivity.These results indicated that interstitial silver ions were the predominant charge carriers in ionic conduction, and that surface sites were responsible for the formation of the positive space-charge layer resulting in the existence of interstitial silver ions with high concentration in the microcrystals. Since the ionic carries were controlled by the surface process, the ionic conductivity was influenced by many emulsion variables, such as the grain size, surface structure of the grains and adsorption of the photographic addenta (for example, antifoggant, stabilizer and sensitizing dye). Effects of sulf ur and reduction sensitization and emulsion pAg on the ionic conductivity were also examined in relation to the photographic effects. Distribution of Frenkel defects in a grain was studied theoretically and was examined experimentally by using iridium-doped AgBr grains. Further discussions were made on the effects of the space-charge layer upon the electronic process of the latent image formation.
An attempt was made to obtain the relation between photographic sensitivity and the ionic conductivity of the emulsion grains. The emulsions containing the grains with low ionic conductivity showed high-intensity reciprocity-law failure probably due to limited rate of the ionic step. The emulsions containing the grains with high ionic conductivity showed no HIRF when they were chemically unsensitized, but they showed strong HIRF due to the dispersion of latent image when they were sulfursensitized. These facts were interpreted by the idea that small image centers were apt to be formed dispersedly when the time for the ionic step was comparable to the electron residence time in the temporary traps.

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An ion chromatography with electric conductivity detector was applied to the analysis of various photographic processing solutions. An anion exchange resin of very low capacity, prepared in our laboratory, was used for the separation of anion components in fixing solutions, and commercial cation exchange resin was used as stripper of the cations in eluent. It was found to be able to obtain the satisfactory analytical results for anion components in fixing solutions, such as Cl^-, Br^-, SO₃^2-, SO₄^2-, CH₃COO^-, and S₂O₃^2-.

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An organic memory coating with a double-layer structure has been investigated. The sensor consists of PVK-TNF and PVK-TNF-Leuco-dye coated successively on a conductive substrate. Light exposure of about 0.3 mJ/cm² resulted in a decrease in its charge acceptance and an electrostatic image could be obtained by uniform charging with positive corona. Furthermore, existence of a high electric field at image exposure proved to reduce the exposure amount required to form a latent image by a factor of about ten (0.02 mJ/cm²).

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Roles of leuco-dye played in photo-induced memory effect were investigated for an organic coating of poly-N-vinylcarbazole (PVK), trinitrofluorenone (TNF) and a leuco-dye. Leuco-dyes form a charge transfer complex (CT) with TNF, and its optical absorption was observed in the visible region (-500nm). The origin of the photo-induced memory effect is relevant to the photo-induced chemical reaction of the CT by illumination with light corresponding to the CT band. The stable radicals are observed as an E.S.R. signal when the CT is exposed to the visible light. The charge acceptance of the organic coating under corona discharge is controlled by injection of counter charges from the conductive substrate. It is considered that the stable radicals caused by light illumination enhance efficiency of the injection of counter charges and thus decrease the charge acceptance in the exposed area.

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A new type of a positive image formation process was investigated by using the photosensitive material which contained a Co (III) complex compound, 2-isopropoxy-1, 4-naphthoquinone, 1-(2-pyridylazo)-2-naphthol (PAN) and a oxidizing agent (N-Bromosuccinimide). By exposuring the photosensitive material imagewisely and by heating it in contact with the image receptive material which contained Ni²⁺ ion absorbed on SiO₂ at the surface of a paper support, the positive image was formed in the image receptive paper. This heat development process is analogous to the diffusion transfer process of silver halide and the recording process was named “Thermal Diffusion Transfer Development” (TDTR).
An oxiding agent, e.g., N-bromosuccinimide is indispensable to obtain the positive image of high density and the fog of low density. The fixation of the photosensitive layer was attained simultaneously by extracting PAN from the photosensitive layer to the image receptive material by TDTR. The mechanism of the positive image formation was ascribed to the thermal diffusion of PAN from the unexposed area of the photosensitive layer to the image receptive material by TDTR process and the formation of Ni (PAN) ₂ at the surface of SiO₂.

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