Which statement about penumbra is true?

• A. Penumbra is independent of focal spot size
• B. Penumbra increases with SID
• C. Penumbra decreases as focal spot size decreases
• D. Penumbra is unaffected by OID

Answer: (C)

Penumbra is the geometric unsharpness at the edges of a radiographic image, caused by the finite focal spot size and the geometry between the source, object, and receptor. The width of this blur is roughly proportional to the focal spot size and to how magnified the image is (object-to-image distance relative to source-to-object distance). So, making the focal spot smaller reduces the geometric blur, narrowing the penumbra and producing a crisper edge. Conversely, a larger focal spot increases penumbra.

The other statements aren’t correct because penumbra does depend on focal spot size, and it also grows with greater magnification (larger object-to-image distance or smaller source-to-object distance). Penumbra isn’t independent of focal spot size, and it isn’t unaffected by OID.