What is indicated by black lines or thin radiolucent lines on corners of a radiographic film?

The presence of black lines or thin radiolucent lines on the corners of radiographic film typically indicates that the film has been creased. When a film is creased during handling or processing, it can result in areas that are not properly exposed or developed, leading to these visible lines. This creasing creates an area where the emulsion is disturbed, allowing for greater penetration of developer chemicals or a change in how the image appears on the film.

In contrast, double exposure would usually result in overlapping images rather than distinct lines. Film bending could cause some distortion in the image but would not specifically produce the described lines. Over development tends to darken the entire image rather than create localized lines in specific areas, making creased film the correct interpretation for the described phenomenon.