Reciprocity Law Failure in X-Ray Films

Abstract

The reciprocity law constitutes one of the fundamental rules of photography and of radiography. It states that the quality of a series of photographic or radiographic films will be uniformly constant if the exposure times with which the films are made vary reciprocally with the intensities of the exposing radiation. Thus, the law implies that when other things are equal a roentgenogram exposed for 1.0 second and with a tube-current of 100 milliamperes will be identical to one exposed for 10 seconds and with a tube-current of 10 milliamperes.2 The reciprocity law is based on the assumption that the density or blackening of a photographic film is dependent merely on the exposure or quantity of radiant energy which the film absorbs and is independent of the rate at which the energy is applied (i.e., is independent of the intensity of the exposing radiation).