Sensitivity of photographic plates in the region 2500-2000 A.

The effect of sensitizing photographic emulsions for the wave-length region 2500 to 2000 A. by bathing them in fluorescent substances before exposure, or by decreasing their gelatin content, is examined experimentally. The characteristics of seventeen different types of plates are compared by a simple method of sensitometry in which the plates are given stepped exposures to the γ bands of nitric oxide under reproducible conditions. Direct density-comparisons made by visual estimation are then sufficient to give the information as to relative sensitivities which is most likely to be required by the practical spectroscopist. Apart from the Ilford Q2 plate, which has special characteristics that are fully discussed, it is found that a measurable record can be obtained with the least possible exposure on an ordinary plate. In this sense, therefore, the methods of sensitization at present available do not have the effect which is generally ascribed to them. Their main use is for increasing contrast and saturation density, not threshold sensitivity. A table gives the relative threshold exposure times needed for the different plates at various wave-lengths, and spectrograms on representative plates are reproduced.