Which artifact is most commonly linked to underexposure of the image?

• A. Distortion
• B. Mottle
• C. Saturation
• D. Penumbra

Answer: (B)

Underexposure reduces the number of photons reaching the detector, which increases quantum noise. That extra noise appears as a mottled, patchy pattern across the image—the artifact known as mottle. When there aren’t enough photons, the signal becomes unstable and random variations become visually evident, producing the characteristic grainy look. Other artifacts come from different issues: distortion stems from geometric misalignment or beam geometry, saturation from excessive exposure causing clipped bright areas, and penumbra from unsharpness at edges due to focal spot size and geometry. So the mottled appearance is the hallmark sign of underexposure because it directly reflects photon-starved imaging and elevated noise. To reduce it, increase exposure to boost the photon flux reaching the detector.