Reciprocity failures

Reciprocity failure of the film has two practical implications. The first is seen with the use of color-reversal film. Each of the three color-sensitive layers of the film has a different reciprocity failure factor resulting in incorrect color reproduction during long exposures. To correct for this problem, color-compensating filters can be placed in the light path to the camera. 

Reciprocity failure. High-irradiance/short-time exposure and low-irradiance/long-time exposure may be less efficient than exposures of the same energy but intermediate irradiances and durations. These effects are designated high-intensity reciprocity failure (HIRF) and low-intensity reciprocity failure (LIRF). 

Emulsions prepared with gelatin that had been treated to minimize sensitizing impurities have relatively low sensitivities for exposures made in air, particularly at low irradiances. They show pronounced low intensity reciprocity failure (LIRF). The pure silver bromide emulsions do not show high intensity reciprocity failure (HIRF) for direct development (65,66), but may do so for physical development (66). Faelens obtained HIRF in silver chloride emulsions for both physical development and the same developer he used for direct development of his silver bromide emulsion (67). Silver chloride emulsions, however, are more prone to unintentional chemical sensitization than silver bromide, and it is uncertain to what extent some chemical sen-... 

Reduction sensitization produces a general increase in sensitivity, but the effect is greatest in the region of low irradiance for exposures made in air. It greatly decreases and in some cases eliminates low intensity reciprocity failure. 

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 sensitivity of our (S+Au)-sensitized emulsions was increased by vacuum outgassing, and the low intensity reciprocity failure was nearly eliminated (108). When the vacuum out-gassed emulsion was equilibrated with water vapor before exposure, sensitivity decreased to a minimum at 40-60% RH, then increased at higher RH, unlike the behavior of emulsions with other types of sensitization where sensitivity continued to decrease. The cause of the increase at high humidities is uncertain. One possibility is that incorporation of gold into the latent image takes place more readily at the high humidities. 

The last three involve the capture of a charged carrier at an oppositely charged center. In all of these events except the free-hole trapped-electron recombination, the free carrier is the electron and the trapping center has a charge of +e/2. The key assumption is that the cross section for electron capture is determined by the coulombic attraction. On this basis, Hamilton derived an equation that includes one term to cover low-intensity reciprocity failure and another which is a first-order approximation of high-intensity reciprocity failure. Its predictions were in good accord with experimental data on the effects of sulfur sensitization. 

Because the critical size of a silver aggregate for latent image aggregates becomes smaller with increasing intensity of the exposing radiation, Moisar holds that not only is low-intensity reciprocity failure explained, but it is required. 

This, however, is in conflict with results obtained when fine grain monodisperse emulsions were exposed under vacuum (Section IV.A). We found that several emulsions of this type showed no low-intensity reciprocity failure under the best vacuum conditions, and in some the highest sensitivity occurred in the region of lowest irradiance, in direct contradiction with the requirements of the supersaturation mechanism. 

Reciprocity failure in aggregate photoreceptors has been described by Mey et al. (1979), as shown in Fig. 3. The exposure wavelength was at the absorption maximum, 680 nm. A loss in sensitivity of 0.14 log z was observed for positive surface potentials and 0.12 log z for negative surface potentials. The reciprocity failure was explained by Langevin (1903) recombination in conjunction with a field-dependent photogeneration process. 

Figure 3 Reciprocity failure in an aggregate photoreceptor with low-intensity emission-limited exposures (solid circles) and 6 ns flash exposures (open circles). The measurements were made with positive surface potentials (upper figure) and negative potentials (lower figure).

A remarkable reciprocity failure was observed in the inorganic resist described in Section 3.4.2 (210). The effect of dose rate on the total dose required for response is shown in Figure 3.71, where the number of pulses required for exposure is plotted as a function of pulse energy density. As a pulse energy is increased, a drastic reduction in the required dose is observed. The reciprocity failure was attributed to a locally induced temperature rise. Further investigations are needed to clarify the reciprocity behavior of resist systems under excimer laser radiation. Multiphoton mechanistic studies are clearly warranted. 

Key words Film, Photobleaching, Photographic emulsion, Photomicrography, Reciprocity failure... 

A second important decay mechanism for the iridium-associated trapped electron center involves the thermally assisted excitation of the trapped carrier to the conduction band. It is retrapped many times at other Ir3 + sites, so that the time before the electron is permanently annihilated by silver formation at an intrinsic sensitivity center or recombination is increased. When this decay mechanism predominates, the addition of Ir3 + reduces the emulsion s high intensity reciprocity failure (see Section I.B). 

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