Are temperature measurements and temperature differences reliable when emissivity levels are incorrect?

• A. Yes
• B. Sometimes
• C. No
• D. Only for small differences

Answer: (C)

Emissivity sets how efficiently an object emits infrared energy compared to a perfect blackbody, and infrared instruments convert measured radiance to temperature using a specified emissivity value. If that emissivity value is incorrect, the radiance-to-temperature calculation is biased, so the reported temperatures are off. Since a temperature difference comes from subtracting two biased temperatures, the difference is also unreliable. In short, both temperature measurements and their differences depend on accurate emissivity, so they aren’t trustworthy when emissivity is wrong. To improve reliability, determine or estimate the correct emissivity for the material (or use methods less sensitive to emissivity), but with incorrect emissivity, expect biased readings.