Photometrie measurements of grain density in autoradiographs

A technique is described for measuring the grain densities in areas of emulsion from 10 to 200 μm2, using vertical incident illumination which converts the silver grains into bright specks on a black background. The light reflected by the silver grains in the measuring area is integrated with a photometer to give a reading that is directly proportional to the number of grains present over a wide range of densities. Speeds of 200 counts/h can be maintained on suitable material throughout a working day without loss of accuracy. The characteristics of the method, the selection of equipment, and some sources of artefacts are discussed.