Emissivity values typically range from

• A. 0 to 1
• B. 0 to 100
• C. -1 to 1
• D. 0 to 2

Answer: (A)

Emissivity is a unitless ratio that measures how efficiently a surface emits infrared radiation compared with a perfect blackbody at the same temperature. Because a blackbody represents the maximum possible emission, emissivity cannot exceed 1. A value of 0 means the surface emits no infrared energy, while 1 means it is a perfect emitter. In practice, real materials fall between these extremes, with values depending on material and finish. In infrared thermography, emissivity is typically used as a number between 0 and 1 (or expressed as a percentage 0% to 100%). The other ranges aren’t correct because you can’t have negative emission or exceed the blackbody limit.