What does the central ray need to be in relation to the film according to geometric principles of shadow formation?

The central ray needs to be as nearly perpendicular as possible to the film to ensure optimal image clarity and accuracy in radiographic testing. When the central ray is perpendicular to the film, it minimizes distortion and foreshortening of the image produced, allowing for a true representation of the object being examined. This setup ensures that the shadow cast by the object is projected directly onto the film without angular distortions, which can compromise the integrity of the radiographic results.

When the central ray is positioned incorrectly, such as being displaced or at an angle (other than perpendicular), the resulting image can suffer from geometric distortions. Images taken at any angle other than perpendicular can lead to incorrect interpretations of the object's dimensions, shape, or position, which is critical in applications like weld inspections or structural evaluations where dimensional accuracy is paramount. Thus, maintaining a nearly perpendicular alignment of the central ray to the film is vital for accurate radiographic testing outcomes.