Blurring at the edges of an object on a radiograph is referred to as what?

• A. Magnification
• B. Penumbra
• C. Quantum mottle
• D. Ghosting

Answer: (B)

Penumbra is the gradual transition at the edges of an object on a radiograph. It occurs because the X-ray beam is divergent and the focal spot has finite size, so rays from different parts of the focal spot illuminate neighboring margins differently. This creates a partial shadow around edges instead of a perfectly sharp boundary. The amount of penumbra depends on the focal spot size and the geometry: a larger focal spot or greater object-to-image distance increases the edge blur, while a longer source-to-image distance reduces it. Magnification changes the overall size of the image, not the edge sharpness, and other terms describe graininess or nonuniformities rather than edge blur.