In medical radiology

Radiology

Radiology is a medical specialty that employs the use of imaging to both diagnose and treat disease visualized within the human body. Radiologists use an array of imaging technologies to diagnose or treat diseases...

, the source to image-receptor distance (SID) is a measurement of the distance between the radiation source and the radiation detector.

The SID is roughly related to the source to object distance (SOD) and the object to image-receptor distance (OID) by the equation SOD + OID = SID. The relative and absolute values of these measurements will affect various qualities of the radiographic image

Radiography

Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....

 as well as the amount of radiation to which the patient is exposed. For example, increasing the OID alone can improve image contrast

Contrast (vision)

Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...

 by decreasing the amount of scattered

Scattering

Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 radiation that reaches the receptor; however it will also result in image magnification

Magnification

Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...

if the SID is not also proportionally increased.