Why does a hot aluminum surface appear cooler on an infrared camera in a room-temperature environment?

• A. Because it has high emissivity and low reflectivity.
• B. Because it has low emissivity and high reflectivity.
• C. Because infrared cameras magnify the scene leading to error.
• D. Because aluminum does not emit infrared radiation.

Answer: (B)

The main idea is how infrared cameras read temperature: they respond to radiated infrared energy, which comes from two sources on a surface—what the surface itself emits and what it reflects from the surrounding environment. Aluminum is a metal with very low emissivity and high reflectivity. That means it doesn’t emit much infrared on its own when hot; instead, most of what the camera detects from the surface is the infrared radiation reflected off it from the warmer or cooler surroundings.

In a room-temperature environment, the reflected radiation corresponds to the ambient temperature, which is cooler than the aluminum’s actual surface temperature. So the camera’s sensor receives a smaller emitted signal and a larger reflected signal at a lower temperature, making the hot aluminum surface look cooler than it really is.

If you coat the aluminum with a material that has high emissivity, the surface would emit more of its own infrared energy, and the apparent temperature would rise accordingly. It’s also worth noting that metals do emit infrared, but their low emissivity means the emitted portion is small compared with the reflected ambient radiation, leading to the cooler appearance.

The other statements don’t fit because high emissivity would not produce a cooler reading, and the effect isn’t due to magnification or a lack of emission altogether.