Which of the following characteristics most directly causes a hot aluminum surface in a room-temperature environment to appear cooler on infrared instruments?

• A. High reflectivity
• B. Low emissivity
• C. High reflectivity and low emissivity
• D. Low reflectivity and high emissivity

Answer: (C)

In infrared imaging, what you see from a surface depends on how much it emits versus how much it reflects from its surroundings. A hot aluminum surface has high reflectivity in the infrared and low emissivity. Because of that, most of the light entering the camera from the surface is actually reflections of the cooler room environment, not energy the surface itself is emitting. Since the surface emits only a small amount of infrared energy due to its low emissivity, the camera registers a lower radiance, making the surface appear cooler than it really is. If a surface emitted more (high emissivity) or reflected less (low reflectivity), it would look warmer because the camera would pick up more of its own emitted energy rather than ambient reflections. The combination of high reflectivity and low emissivity is what drives the cooler appearance in this case.