The change in spectral sensitivity of unsensitized photographic emulsions with temperature and its connection with light-absorption are investigated.
In the first part, the influence of temperature upon the shape of the spectral-sensitivity curves is considered and graphs are given showing the ratio of the sensitivities at −195°C and room temperature and the ratio of the sensitivities at +50°C and room temperature as a function of wave-length. For pure silver bromide emulsions, these curves show a minimum at about 470 mµ and a maximum at about 500 mµ. For emulsions containing chloride or iodide, the curve shape is somewhat changed but not in its essential features.
In the second part, the absolute change of spectral sensitivity with temperature is discussed. The sensitivity of the basic light-sensitive material, silver bromide, is found to be comparatively little temperature-dependent. The large drop in sensitivity, on lowering the temperature, which is found in commercial emulsions, is due to the chemical sensitization becoming less effective.
In the third part, a tentative explanation is given for the results of the first part. It is assumed that the maximum of the sensitivity-ratio curves at 500 mµ is due to a different temperature-dependence in the light-absorption of the silver bromide and that of the reaction product of the chemical sensitizer with the emulsion (probably silver sulfide) which, in this region, acts as an optical sensitizer. Measurements of the spectral light-absorption of a pure silver bromide emulsion show that the decrease in sensitivity with temperature of this emulsion at 400 mµ is, at least in the main, due to decreased light-absorption and only to a smaller extent to a genuine decrease in light-sensitivity.
It is concluded from the experimental results that there exist two kinds of sensitivity specks: one kind being numerous and consisting of shallow traps, which are mainly responsible for the sensitivity of chemically non-sensitized grains and for that of the interior of chemically sensitized grains; the other kind being few in number and consisting of deep traps, which are responsible for the surface sensitivity of chemically sensitized grains.
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