Radiographic Examination – Exposure

The exposure of the photographic film by radiation passing through the workpiece is determined by the intensity of the radiation multiplied by the time of the exposure. The optical density of the developed negative increases with increasing exposure. ASME Code, Section V, requires a radiograph to have a density between 1.8 and 4.0 for proper visual interpretation. The exposure must be adjusted if the density of the radiograph is not between these limits, by changing either the time of the exposure or the intensity of the incident radiation.

Flaws that reduce the thickness of the workpiece through which the radiation passes will increase the exposure, and, therefore, the density of the radiograph. However, the density of the flaw image compared to that of the surrounding workpiece cannot be relied upon to give an accurate indication of the depth of the flaw. The threedimensional shape and orientation of the flaw can significantly affect the density of the image and are not always revealed by the two-dimensional silhouette in the radiograph.

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