The Eleventh Renwick Memorial Lecture: The Nature Of Photographic Sensitivity

Abstract

This paper reviews the work on photographic sensitivity which has been carried out by the author and his colleagues, and presents the conclusions which have been drawn from it. The earlier concept of the sensitivity speck has been replaced by that of the sensitivity centre. The sensitivity speck was assumed to consist of silver or of silver sulphide and to have the function of concentrating silver atoms: the sensitivity centre is a region of enhanced reactivity associated with a crystal imperfection. The conclusion is now that the principal function of the silver sulphide is to trap positive holes and thus to prevent the regression of latent image specks. The theory of dye sensitization has been extended and a number of new features introduced. According to present ideas, the latent image is formed in two stages. In the primary stage, electrons and positive holes are liberated and the positive holes are trapped with the production of interstitial silver ions. In the secondary stage these combine with the conduction electrons to form first latent pre-image specks, then latent sub-image specks, and finally stable latent image specks. A discussion of reciprocity failure is given. The new approach to the theory of photographic sensitivity, in which the role of kink sites Is emphasized throughout, leads to the enunciation of four basic principles which must be satisfied by any theory of the formation of the latent image. Of these, only the Gurney-Mott principle has previously been recognized.