Emulsions unsensitized

Color sensitizers are dyes added to silver halide emulsions to broaden their response to various wavelengths. Unsensitized emulsions are most responsive in the blue region of the spectrum and thus do not correctly represent the light spectrum striking them. Widely used sensitizers include the cyanine dyes, the merocyanines. the benzooxazoles, and the hcn/n11iki/oIl1 s Cryplocyaniiie sensitizes the extreme red and infrared. 

Some chemical sensitization may occur even in emulsions prepared with the treated gelatin because of incomplete removal of chemical sensitizers by the treatment used. For this reason it is more precise to refer to these emulsions as "without deliberate chemical sensitization" rather than as "unsensitized," but the latter term is less cumbersome and is used rather commonly in the literature. I shall use it in this chapter with the understanding that it means simply that a deliberate chemical sensitization step was not included in the preparation of the emulsion. 

FIGURE 1. Reciprocity curves for an unsensitized silver bromide emulsion coated at pAg 9.0. (A) Curve 1, exposure in room air ... 

FIGURE 2. Dependence of sensitivity on the concentration of dry oxygen and 45% RH oxygen unsensitized silver bromide emulsion exposed 100 s. 

FIGURE 4. Spectral distribution of Herschel effect at 77K. Exposure, 2.8 x 106 ergs/cm to radiation specified on abscissa, o, Unsensitized AgBr emulsion, 0.2-pm grains A, same, except S-sensitized emulsion, AD/D0, fractional loss of developable density as result of long wavelength exposure. 

Although S-sensitization decreases low intensity reciprocity failure it usually does not eliminate it. In our experiments with monodisperse fine-grain silver bromide emulsion, vacuum outgassing of the S-sensitized emulsion eliminated the LIRF, just as it did for the unsensitized emulsion. Moreover, the sensitivities of the two emulsions under vacuum were nearly the same. Whatever may be the role of S-sensitization in this emulsion, it became inconsequential for exposures made under vacuum. However, the degree of increase in sensitivity caused by S-sensitization of the fine grain emulsions for exposures in air is much smaller than can be achieved with coarse-grain poly-disperse emulsions. 

The photographic attributes of (S+Au)-sensitization are similar to those of S-sensitization. Both act primarily to increase the sensitivity for surface image formation. Both usually decrease the sensitivity for internal latent image formation in emulsions that show considerable internal sensitivity in their unsensitized form. Bahnmueller (163) and Farnell and Solman (164) observed that, in emulsions of this type, the combined (S+ Au)-sensitization produced a greater increase in surface sensitivity and a greater decrease in internal sensitivity than S-sensitization alone, but sensitization with gold alone increased surface sensitivity without impairing the internal sensitivity (163). 

They arrived at a size of the order of 100 silver atoms for a bare metal nucleus, but only 10 atoms for a nucleus embedded in gelatin. Moisar and associates (205) estimated the minimum size as four silver atoms. Ronde (206) revised their calculations and obtained 10 for an unsensitized silver bromide emulsion and six for a S-sensitized emulsion. The validity of the Gibbs-Thomson equation for such small sizes, however, is questionable. 

The crossover also depends on the chemical sensitization. Gilman (263) found that a dye with ER = -0.5A V could spectrally sensitize a (S+Au)-sensitized emulsion to some extent, although the crossover for the corresponding unsensitized emulsion was between -0.79 and -0.89 V. Iridium-doped emulsions can be sensitized for internal image formation by dyes with even less negative potentials. Emulsions without chemical sensitization also can be sensitized by dyes with ER as high as -0.25 V when the exposure is made under vacuum (235). 

Tani [90] has examined the properties of silver clusters by means of redox buffer solutions, and showed that the oxidation potential of latent images formed by sulfur-plus-gold sensitization was much more positive than for those formed in unsensitized, sulfur-sensitized, reduction-sensitized, and iridium-sensitized emulsions. The oxidation potential of fog centers with excessive sulfur sensitization was much more positive than that of fog centers with excessive reduction sensitization. In general this reflects the relative ease of bleaching of silver centers compared with silver sulfide centers. 

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