4 STRUCTURE OF PRINTING-OUT PAPERS

In printing-out papers, the sensitive material is silver chloride associated with an excess of soluble silver salts capable of absorbing halogen liberated by the action of light. The fast darkening of POPs is a consequence of this excess of soluble silver salts contained in the support itself, in the binder, or in the emulsion. The image becomes visible upon exposure to light, without the action of a developer. It consists of metallic silver in a very fine state of division (photolytic silver). The tone of the image depends on the size, distribution, and morphology of the silver particles (smaller particles absorb the smallest wavelengths and display a warm, reddish tone; bigger particles tend to give prints of cooler tone). Unlike albumen papers, where the silver particles are small and concentrated near the surface of the binder, emulsion papers have large silver particles that are spread uniformly in the binder (Lav�drine 1990).