What is emissivity?

Usual objects are not blackbodies. They do not absorb 100% of the incident energy and usually select the absorbed wavelengths.

FAQ-emissivity

Consequently, they cannot re-emit all the incident energy. The ratio between the re-emitted energy of a usual object and the re-emitted energy of a blackbody at the same temperature of the object is called emissivity and noted ε. This ratio depends on wavelength and is comprised between 0 and 1. Of course, the emissivity of a true blackbody equals 1.

However, such bodies do not exist and manufacturing “blackbodies” consists in creating optical sources with emissivity value as high and as constant as possible over the widest spectral range. These sources are called grey bodies but practically sources with emissivity higher than 0.9 are also called blackbodies.