Radiographic Examination – Photographic Film
The radiation which passes through the workpiece is recorded by photographic film. The radiographs are usually interpreted visually with the aid of a high intensity light source (light box), but optical densitometers or image analyzers are occasionally used.
Two primary characteristics of the film can affect the sensitivity of RT for detecting flaws: gradient and grain. Gradient is the difference in optical density of the negative resulting from exposure by different intensities of radiation. A film with high gradient exhibits relatively large differences in density when exposed by radiation of relatively small differences in intensity. In other words, a high gradient results in a relatively high contrast in the negative, which makes small differences in the intensity of radiation passing through the workpiece visible. Therefore, films that have a high gradient will provide the greatest sensitivity for detecting small flaws that cause only a small attenuation of the radiation.
Grain results directly from photo-sensitized crystals in the film. The darkened crystals impart a visually apparent “graininess” to the transparent negative that limits the detail that can be resolved by viewing the negative, regardless of the difference in density of adjacent areas of the film. Therefore, very fine flaws can be obscured by the grain of the film can obscure very fine flaws, so the flaws and may not be visible in a radiograph, although they significantly reduce the effective thickness of the workpiece. Fine grain films usually also have a high gradient, and are preferred for RT to obtain the greatest sensitivity for the detection of flaws.
Another characteristic of film that can affect the sensitivity of RT for detecting flaws is speed, which is a measure of the sensitivity of the film to the radiation passing through the workpiece. A high speed film requires less exposure to radiation to produce the same optical density in the developed negative than a low speed film. Therefore, less exposure time is required for high speed films to produce satisfactory radiographs than with low speed films for the same intensity of incident radiation. The reduction of exposure time can be significant, especially for relatively thick workpieces, when low intensity radiation sources are used for in-service inspection. However, high speed films provide less gradient and have coarser grain than low speed films. It is usually inadvisable to sacrifice the greater sensitivity of fine grain films with a high gradient for detecting flaws in order to obtain the advantage of shorter exposure times.
The film is loaded into flexible cassettes in a darkroom, and the cassettes are usually held against the surface of the workpiece with magnets. The cassettes incorporate a “radiographic screen” that improves the image recorded on the film, by both intensifying the exposure of the film by the radiation passing through the workpiece, and by filtering out scattered radiation to reduce fogging. Lead foil is commonly used for the radiation screen. Radiation penetrating the workpiece interacts with the lead atoms in the foil, which causes them to “fluoresce,” intensifying the exposure of the film. The lead foil will also absorb most of the low energy scattered radiation, while having little effect upon the intensity of the higher energy radiation passing through the workpiece. In this manner, fogging of the film by the scattered radiation is greatly reduced, but exposure of the film by the transmitted radiation is not significantly affected.
Categories: In-Service Inspection | Tags: Radiographic | Leave a comment