Recent work on the ultimate sensitivity of radiation detectors is reviewed and discussed. It is shown that the validity of Clark Jones’ treatment is more restricted than was supposed. By an extension of his argument a general method is obtained giving the ultimate sensitivity throughout the region of the electromagnetic spectrum in which the limit is set by temperature radiation. It is found that Clark Jones’ method, suitably interpreted, is a useful approximation in most practical cases. The method is applied to a number of detectors. The well-known result for the noise level in a radio antenna is confirmed. The performance of certain photo-multipliers and photo-conductive infra-red cells is assessed. It is found that the sensitivities of recent lead telluride cells closely approach the ultimate limit corresponding to room temperature. Consequently, improvements to these cells cannot give rise to substantially increased sensitivity unless the cells are used in cooled enclosures.
© 1949 Optical Society of America
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