Latent image formation

As in chemical sensitization, spectral sensitization is usually done after precipitation but before coating, and usually is achieved by adsorbing certain organic dyes to the silver haUde surfaces (47,48,212—229). Once the dye molecule is adsorbed to the crystal surface, the effects of electromagnetic radiation absorbed by the dye can be transferred to the crystal. As a result of this transfer, mobile electrons are produced in the conduction band of the silver haUde grain. Once in the conduction band, the electrons are available to initiate latent-image formation. 

Exposure and latent image formation. The sensitized photoreceptor is exposed to a light and dark image pattern in the light areas the surface potential of the photoconductor is reduced due to a photoconductive discharge. Since current can only flow perpendiculai to the surface, this step produces an electrostatic-potential distribution which replicates the pattern of the image. 

Image erasure. The potential differences due to latent image formation are removed by flooding the photoreceptors with a sufficiently intense light source to drive the surface potential to some uniformly low value (typically I00V corresponding to fields of 10 Vcni ) the photoreceptor is then ready for another print cycle. 

Latent-image centers, 19 189 formation of, 19200—201 Latent-image formation... 

Photothermographic materials, 19 211-212. See also Photothermographic/ thermographic imaging materials Photothermographic process binder role in, 19 359—360 latent image formation in, 19 353-355 mechanism of, 19 353—360 thermal development and toner recycling in, 19 358-359... 

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

Photography, Silver Halide, Chemical Sensitization, Spectral Sensitization, Latent Image Formation (James). 

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.)...

Although silver halide photography dates from 1839 when Daguerre and Talbot disclosed their inventions, there is no general agreement on the mechanism of latent image formation. 

Why and how are nuclei of silver formed by the photolysis What becomes of the other photolytic product, the hole or halogen, and how important is it to photographic sensitivity How do crystal defects and impurities influence sensitivity and latent image formation How can radiation absorbed by dyes at the crystal surface induce formation of silver My intention in this chapter is to review recent experimental and theoretical work bearing on these questions and to offer some conclusions as to what we know, what we think we know, and what can be decided only by future investigations. 

The theory of how chemical sensitization acts to increase photographic sensitivity and the theory of photolysis and latent image formation are interconnected. The aim in this section is to provide a summary of current experimental information on chemical sensitization as a basis for discussing the various mechanisms that have been suggested for latent image formation. 

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. 

In summary, there is evidence that the multitalented sulfur sensitization product can trap electrons, can trap holes to reduce recombination, can stabilize photolytic silver atoms, and can accelerate reduction sensitization. Conceivably, each of these properties could be of importance for latent image formation under at least some conditions. The silver sulfide centers are not of uniform size, they probably are not uniformly related energetically to the silver halide matrix, and they may differ in chemical consititution. 

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