Latent image mechanisms

Liquid toners are suspensions of toner particles in a fluid carrier. The carrier is typically a hydrocarbon. Dielectric, chemical, and mechanical properties of the Hquid must be compatible with the photoreceptor, the suspended toner particles, and the materials of the development equipment. Liquid toners are capable of producing higher resolution than dry toners because of the smaller (3—5 -lm) particle size achievable. Development of the latent image occurs as it passes through a bath of toner and the charged particles are attracted to the oppositely charged surface. 

In the previous section, five basic mechanisms of photopolymer reactions were identified. In order to utilize these photopolymer mechanisms in practical systems, sensitive visualization methods must be used to develop the latent images. Six different classes of visualization techniques are used in most commercial applications of photopolymer technology. 

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). 

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. 

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. 

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... 

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. 

Physical development. The developer used in this process contains a soluble silver salt, and the developed image is obtained by reduction of this salt. The process is chemical, but the term is retained for historical reasons. Development by the same mechanism can occur when silver ions from silver halide grains pass into solution and are subsequently reduced at latent image centers or developed silver. This process is termed solution-physical development. 

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 Gurney-Mott mechanism has been successful in bringing order to a variety of latent image phenomena and in predicting experimental results. However, uncertainties remain about important details of the mechanism, and in recent years several rival mechanisms have been proposed. The first serious challenge to the Gurney-Mott mechanism was made by J. W. Mitchell. 

Mitchell s views on the mechanism of latent image formation are based on theoretical calculations and extensive experimental work on silver bromide sheet crystals (55,56). He accepts the participation of separate electronic and ionic processes in the formation of latent image centers, which he termed the Gurney-Mott principle, but changes the order of the two processes. 

No consensus has been reached on the details of latent image formation, and it remains possible that latent image centers can be formed by more than one mechanism. The preceding discussion raises several questions of detail which either have not been answered or are in dispute. 

There are several theories as to the constitution of the silver subhalides in the latent image. The molecular theory regards the subhalides as definite chemical compounds. The adsorption theory regards them as adsorption-compounds of colloidal silver and subhalides. The molecular theory is advocated by Trivelli,1 who considers the colour-changes to indicate the existence of several silver subhalides, which yield solid solutions with each other and with the silver halides. He also regards the mechanism of reduction with ammonium persulphate as favouring the molecular theory. 

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