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. Film density is used to describe the degree of blackness of the radiograph. The reason for this distinction is that dense body parts such as bone have little film density, while air in lung is not a dense part of the body, yet produces an area of high film density. The contrast of the radiographic image can be defined as the difference in opacity between 2 regions of the radiograph. The highest or ultimate contrast is when an image is either black or white.