X and Gamma rays also have the property, like light, of partially converting silver
halide crystals in a photographic film to metallic silver, in proportion to the intensity of
the radiation reaching the film, and therefore forming a latent image. This can be
developed and fixed in a similar way to normal photographic film.
Material with internal voids is tested by placing the subject between the source of
radiation and the film. The voids show as darkened areas, where more radiation has
reached the film, on a clear background. The principles are the same for both X and
Gamma radiography.