Measuring an extremely small, distant hot spot against the sky with an infrared camera tends to be what?

• A. More accurate
• B. Accurate
• C. Inaccurate, lower than it should be
• D. Inaccurate, higher than it should be

Answer: (C)

When you image a tiny, distant hot spot against the cold sky, the reading tends to be lower than the true temperature because of spatial resolution and pixel averaging. Each pixel captures the average radiance over its footprint. If the hot spot only partly fills the pixels, those pixels blend hot radiance with cold sky radiance, pulling the measured brightness temperature down toward the background. The result is an inaccurate, lower-than-actual temperature. Atmospheric effects and low contrast can amplify this, but the main idea is that small targets get diluted by the surrounding cooler background.