Silver specks

This process continues until a small speck of silver is created. It is these clusters of silver that absorb the light falling on the glass. The absorption characteristics of the silver specks depend quite critically upon their size and shape. Photochromic glass production is carefully controlled so as to produce a wide variety of shapes and sizes of the silver specks, ensuring that the glass darkens uniformly. 

Both reduction centers and latent image centers are composed of silver, and to Moisar "it seems safe to assume that the silver specks formed by reduction are identical to those which somehow appear as subspecks and alleged intermediate entities during photolytic silver formation" (93). He proposed that Ag2 centers formed by exposure by light can act as hole traps and Agj centers act as subdevelopable precursors of latent image centers but, as Hamilton and Baetzold comment (96),... 

The speck thus becomes negatively charged silver ions are attracted to it, and a silver speck grows... 

Hada et al. [89] compared experimental characteristic curves with theoretical curves based on a model of exposure and development. A marked difference in the ability of specks to initiate development was found between silver specks smaller than Ag4 and those larger than Ag4. 

Mass transport of developer to, and oxidized developer and halide ions from, the silver speck is calculated by the method of spherical diffusion. The surface concentrations of the active species are then used in the Nemst equation to calculate the surface potential of the developing nucleus. The rate equation (79) obtained is ... 

Figures 6 and 7 show the dependence of the intensity of the light-induced ESR signals of Dye 3 in emulsions upon reduction sensitization by use of stannous shloride and upon addition of Rh ( III ) to the emulsions. In accord with the view that small silver specks formed during reduction sensitization capture positive holes ( K), 11 ), the above-stated reduction sensitization decreased the intensity of the ESR signal, which were related to dye positive holes. In accord with the view that Rh ( III ) captures photoelectrons in AgBr ( 12 ), thus accelerating the separation between photoelectrons and positive holes, the addition of Rh ( III ) to emulsions increased the intensity of the ESR signal.

Figure 72. Silver chloride crystal covered by silver specks viewed along the cube zone...

When silver halide crystals with a cube habit are exposed to electrons, photolytic metallic silver specks are formed in situ in an epitaxial relation to the silver halide substrate. This gives rise to moire fringes parallel to the cube edges due to the difference in lattice spacing between AgCl and Ag (Fig. 72). 

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