What is filtering in industrial radiography?

All materials, for example a metal layer between the radiation source and the film, cause absorption and filtering. The position of the metal layer plays an important role in the effect it has. A metal layer in front of the object will “harden” the radiation because it filters out the soft radiation. The degree of hardening depends on the type and the thickness of the material. This phenomenon is used to reduce excessive contrast (variation in density) when examining objects of which the thickness varies greatly. A metal layer between the object and the film filters the soft scattered radiation that occurs in the object, thereby increasing the contrast and consequently the image quality. This method of filtering is for example applied in the use of Cobalt60 in combination with exposure time reducing intensifying screens, which are sensitive to scattered radiation. Lead, copper and steel are suitable filtering materials.