The spectral response of two typical x-ray films was determined as a function of the truly absorbed energy and also as a function of the number of absorbed quanta needed to produce a given optical density. With decreasing wave-length a better utilization of the kinetic energy of the photo and recoil electrons takes place in the process of photographic image formation. The number of absorbed quanta needed to produce the same photographic effect decreases rapidly with decreasing wave-length. Correspondingly the average number of grains made developable per absorbed quantum increases. These data, together with those of the range of electrons in silver-bromide and of their energy loss per unit path-length permitted the determination of the inherent sensitivity of the average grain. Comparison between the sensitivities to visible radiation, and to x-rays, reveal great differences in efficiencies of latent image formation between films. The average grain of Non-Screen film requires the formation of 210 latent image silver atoms by photo or recoil electrons, and 105 silver atoms by exposure to blue light. The corresponding figures for High Speed x-ray film designed for medical work with calcium tungstate intensifying screens are 270 silver atoms for electrons and only 8 silver atoms for blue light. This relative inefficiency of direct x-ray exposures is predominantly due to the formation of large amounts of internal latent image.
© 1949 Optical Society of AmericaFull Article | PDF Article
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