This is a well-established technique that gives a permanent record and is widely used to detect internal flaws in weldments and castings and to check for mis-constructions in assemblies.

Radiography uses X-rays or gamma rays to produce an image of an object on film. Thus, if an X-ray source is placed on one side of a specimen and a photographic film on the other side, an image is obtained on the film of the thickness variations in the specimen, whether these are surface or internal.

When the radiograph is placed on an illuminated screen for visual examination, the parts of the film that have received more radiation during exposure - the regions under cavities, for example, appear darker.

The source of radiation is either an X-ray tube or a pellet of radioactive material emitting gamma radiation. Conventionally, thin to medium section components are tested using an X-ray tube with thicker sections being exposed to gamma radiation.