Two infrared images of a very small distant hotspot were captured: one in perfect focus and one slightly out of focus. Which statement is correct about accuracy and temperature readings?

• A. More accurate: Out of focus; Hottest temperature: Focus; Correct temperature: Neither
• B. More accurate: Focus; Hottest temperature: Focus; Correct temperature: Neither
• C. More accurate: Out of focus; Hottest temperature: Out of focus; Correct temperature: Focus
• D. More accurate: Focus; Hottest temperature: Focus; Correct temperature: Neither

Answer: (C)

When imaging a very small hotspot, how the light from that spot is sampled by the detector depends on focus. A perfectly sharp image concentrates the hotspot’s radiance into just a few pixels, which can make the local temperature reading highly sensitive to pixel-to-pixel variations and calibration quirks. Slightly defocusing spreads that energy over more pixels, which tends to smooth out those short‑scale inconsistencies and noise, giving a more stable temperature estimate for the region as a whole. That broader, steadier readout is why the out-of-focus image can be more accurate for describing the region’s temperature in general. However, because blur dilutes the peak brightness, the exact hottest point (the true peak temperature) is best recovered when the image is in focus, where the radiance from the hotspot isn’t smeared. So you get more accurate overall temperature readings in the out-of-focus frame, the hottest temperature appears more reliably in the out-of-focus frame, but the true hotspot temperature is most accurately determined in the focused frame.