X-ray beam intensity is usually defined as mAs. The inverse square law is used to calculate exposure factors when FFD is changed. It is also an important concept to understand because it tells us that harmful scatter radiation intensity decreases by the square of the distance from the source of scatter. We will return to this in our discussion on radiation safety. While radiographic film density refers to overall film blackness, we must now discuss patient opacity as seen on the radiograph.
Opacity refers to the appearance of the radiographic image of the patient.
Bone is not as opaque as metal, as some of the x-ray beam gets through to the film. Soft tissue does not attenuate nearly as much of the x-ray beam as bone. Fat is less opaque than soft tissue, but not by much. Air is the least opaque, and is usually referred to lucent . In summary, the terms opaque and lucent are used to refer to the patient as observed on the radiograph.