These tests are based on the absorption and dispersion of X-rays or gamma rays passing through the material. By means of a luminescent screen or photographic plate, point of varying radiaion intensity that occurs at faults can be detected.

as radiation sources, X rays produced by betatron devuces ( electron centrifuge) or gamma rays produces by radio – active decay processes are used. In these methods the radiation is passed through the metal being examined and then allowed to impinge upon sensitive film.
Thus,if an object has an internal defect, more radiation will be passed through the deffective area then through the sound region and the defect will show up as a dark area on the film.
Unfortunately, scattering tends to obscure the defect, so that small defects may not be detected. In general, X- rays are preferred for laboratory testing since they offer greater control over intensity and they produce sharpher pictures.
X-ray equipment is generally large and bulky, where as gamma radiography depends upon the use of small, mobile sources of radioactivity, such as radioactive cobalt,iridium,samorium and other radioactive elements.
In addition gamma rays, through of the same nature as X-ray, have a generally shorter wave length and therefore, more effective in the study of thick sections.it shows the basic principle of X-ray inspection.
In radio graphic tests, the inclusions appear darker, while blow- holes, cavities and porosity appear lighter, than the surrounding metal.

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