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Geometric magnification

A finite focal spot at the top emits a divergent X-ray beam. Two rays pass through the edges of an object and continue to the detector below, forming similar triangles. Because the object sits above the detector, its projected image is wider than the object. The magnification factor M equals the source-to-image distance divided by the source-to-object distance, which also equals image size divided by object size and 1 plus OID over SOD. When the object touches the detector the object-to-image distance is zero and M is one; a larger object-to-image distance gives more magnification, and M is always at least one so minification cannot occur.

Geometric magnification in projection radiography A finite focal spot at the top emits a divergent X-ray beam. Two rays pass through the edges of an object and continue to the detector below, forming similar triangles. Because the object sits above the detector, its projected image is wider than the object. The magnification factor M equals the source-to-image distance divided by the source-to-object distance, which also equals image size divided by object size and 1 plus OID over SOD. When the object touches the detector the object-to-image distance is zero and M is one; a larger object-to-image distance gives more magnification, and M is always at least one so minification cannot occur. Geometric magnification in projection radiography small focal spot divergent beam object detector / film object size image size = M × object size SOD OID SID M = SID / SOD = image size / object size = 1 + OID / SOD SID source-to-image · SOD source-to-object · OID object-to-image distance OID = 0 (object on detector) → M = 1 · larger OID → more magnification · M ≥ 1, so no minification macro / magnification radiography uses a large OID with a small focal spot to limit penumbra: Ug = F (M − 1)
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