X-ray films for general radiography basically consist of an emulsion-gelatin containing
radiation-sensitive silver halide crystals (such as silver bromide or silver chloride). The
emulsion is usually coated on both sides of a flexible, transparent, blue-tinted base in
layers about 0.012 mm thick. An adhesive undercoat fastens the emulsion to the film
base and a very thin but tough coating covers the emulsion to protect it against minor
abrasion. The typical total thickness of the X-ray film is approximately 0.23 mm.
Though films are made to be sensitive for X-ray or gamma-ray, yet they are also
sensitive to visible light. When X-rays,
gamma-rays, or light strike the film, some
of the halogen atoms are liberated from
the silver halide crystal and thus leaving
the silver atoms alone. This change is of
such a small nature that it cannot be
detected by ordinary physical methods
and is called a “latent (hidden) image”.
When the film is exposed to a chemical
solution (developer) the reaction results
in the formation of black, metallic silver.