What is spot size?

• A. The resolution quality of the sensor at the current distance.
• B. The ratio of the distance from the camera to the object compared to the size of the spot being measured.
• C. The distance to the object divided by the spot size, expressed as a ratio.
• D. The emissivity of the surface.

Answer: (C)

Spot size is about the footprint of each temperature measurement on the target. It depends on how far away you are and the camera’s optics. In this context, describing spot size as the distance to the object divided by the spot diameter on the object, expressed as a ratio, captures how large the measurement area is relative to distance. This ratio is a practical way to think about spatial resolution: a larger ratio means the spot on the object is small compared to distance (better detail), while a smaller ratio means the spot is larger (more averaging). Emissivity is a separate property of the surface that affects how much infrared energy is emitted, not the size of the measurement footprint. So the spot size is defined as the distance to the object divided by the spot size on the object, shown as a ratio.