Which of the following factors mainly influences the geometric unsharpness in radiographic testing?

The factor that mainly influences geometric unsharpness in radiographic testing is the source-to-film distance. Geometric unsharpness is a result of the geometric configuration of the X-ray source, the object being examined, and the film or detector. When the source-to-film distance increases, the penumbral effect, which contributes to unsharpness, decreases. This is because a larger distance minimizes the angular spread of radiation that reaches the film or detector, thus providing a sharper image.

While other factors, like film thickness, film density, and radiation type, can affect image quality and contrast, they do not directly relate to the geometric unsharpness caused by the distance between the source and the film. In radiographic testing, maintaining an optimal source-to-film distance is crucial for achieving a clear and defined image of the tested object, making it the primary factor in controlling geometric unsharpness.