UEi Test Instruments INF155 - Manual (Page 5)

The other common question is regarding emissivity. In simple terms
e m i s s i v i ty is a percentage of energy emitted from a surface compared to
the energy emitted from a black body source. If a surface emits one half
( 1/2 or 0.5) the amount of energy at a given temperature and wavelengt h
as a black body, it is said to have an emissivity of 0.5.
Surfaces closest to the balck body level would be flat black, and those
farthest from it would be mirror or chrome surfaces. With a set emissivity
level some surfaces may measure lower than actual, because their surface
is emitting less thermal radiation at a given temperature. Although you
may have a fixed emissivity of 0.95, most items measured will provide a
reasonably accurate result. Having a fully adjustable emissivity will give
you the ability to fine tune your instrument to a specific applica t i o n .
INF155-MAN P. 4