Photographic latent image

It has been found that only a few photons, maybe as little as six, are needed to form the latent image. Photographic film is a very sensitive light detector. The final step in the photographic process, fixing, removes the unreacted silver bromide crystals from the emulsion, thus stabilizing the image (Fig. 2.4). 

I was involved in the theory of the latent image, photographic properties of crystals as related to light and to the surface structures of crystals. I was involved in the properties of dyes because they were very important in photographic sensitizing and many other things, some polymers and some not polymers. It kept me busy. 

Chemical Sensitization. After the photographic microcrystals are precipitated but before they are coated on a support, the crystals are treated to enhance their sensitivity to light. Chemical sensitization is a process which improves that abiUty of the emulsion grains to use the absorbed photons, independent of the wavelength. Various methods of post-precipitation chemical sensitization have been developed to reduce the number of photons required to produce a developable latent-image center. 

Despite the fact that not all details of the photographic process are completely understood, the overall mechanism for the production of the latent image is well known. Silver chloride, AgBr, crystallizes with the sodium chloride structure. While Schottky defects are the major structural point defect type present in most crystals with this structure, it is found that the silver halides, including AgBr, favor Frenkel defects (Fig. 2.5). 

Lattice defects and latent image formation in silver halides. Fundamental mechanisms of photographic sensitivity, p. 242. London Butterworth s Sci. Publ. 1951. 

Talbot found that an invisible latent image produced by the action of light on paper impregnated with silver chloride could be developed by brushing the paper with a mixture of gallic acid and silver nitrate. The process worked out by Fox Talbot is the forerunner of the present-day photographic process. 

The developable density of many photographic materials passes through a maximum at a sufficiently high exposure and subsequently decreases with further increase in exposure. This loss of developability with increasing exposure is termed solarization, and probably is caused by a superficial rehalogenation of the surface latent image centers (cf. J. H. Webb, J. Optical Soc. Am. 30, 445, 1940). 

The results just described, while useful in the interpretation of the photographic results obtained by physical development of a latent image, do not yield much information on the ultimate mechanism of physical development. Arens conditions correspond closely to those obtaining during physical development of a photographic material, but the rate of this process is dependent on the rate of agitation of the developing solution (Vanselow and Quirk, 30) and hence is at least partially diffusion controlled. 

Replacement of the latent image silver by gold in the exposed photographic sensitive layer increases the developability (James et at., 31). Since the mass of developed silver depends only upon the number of developed nuclei, the gold treatment has obviously increased the number of active nuclei. Hence, smaller gold nuclei (in terms of numbers of atoms) than silver can initiate development, unless the treatment used to effect a replacement of silver by gold has resulted in something more than a simple replacement. 

Figure 15 Top Photographic latent image formation in undoped (left) and formate-doped and gold-sulfide sensitized AgBr crystals with the hole-scavenging step (center). Secondary reduction step by formyl radical (right). Bottom Sensitometry curves for gold-sulfide sensitized emulsions, undoped or formate-doped, and developed after 5 or 20 min (texp = 10 sec, development with aminophenol and ascorbic acid). The same absorbance is observed for a number of photons absorbed 5 or 10 times less, respectively, than in the undoped emulsion. (From Ref. 200.)...

Note that ZnO or 2 photographic plates in an electrolyte solution exhibit a memory effect the latent image arises on illumination in the absence of metal ions in the solution, and it can be developed subsequently in darkness by placing the exposed sample into the metal salt solution. The mechanism of this effect has not so far been understood completely it may be expected that in... 

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