Emissivity

The real objects do not radiate as much heat as an ideal black body. They radiate less heat than a black body and are termed as gray bodies. To get into account the fact that real objects are gray bodies, the equation below is modified to be of the form as shown.

Q = εσAT⁴

Here:

ε = emissivity of the gray body (dimensionless)

Emissivity is merely a factor by which we multiply the black body heat transfer to get into account that the black body is the ideal situation. Emissivity is a dimensionless number and contains an utmost value of 1.0.