In infrared thermography, a surface with which characteristics tends to yield the least reflected ambient infrared?

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

Answer: (C)

The key idea is that what you detect in infrared depends on how a surface both emits its own infrared energy and reflects ambient infrared from the surroundings. A surface with high emissivity emits infrared strongly at its temperature, so the signal you see mainly comes from the surface itself rather than from reflections. At the same time, low reflectivity means it does not bounce much ambient infrared into the camera. For opaque materials in the IR, emissivity and reflectivity are linked: higher emissivity generally means lower reflectivity. So the combination that minimizes any ambient infrared being reflected into the detector is a surface that emits well (high emissivity) and reflects little (low reflectivity). If a surface reflected a lot of ambient IR, or emitted very little, the measured signal would be contaminated or weak. Therefore, high emissivity with low reflectivity gives the cleanest reading with the least reflected ambient infrared.