Pressure marks (crimp marks).
Produced by careless film handling – if the film is crimped or buckled either
before or after exposure crescent-shaped images in the processed radiograph
will result. Light marks indicate crimping before exposure, dark marks crimping
after exposure but before film processing.
It is usually possible to identify crimp marks by viewing the film in reflected
light. They should appear as indentations in the surface of the film. Lead
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Interpretation of radiograph (Part 3).