Emissivity

Emissivity is a term in radiometry. It is the ratio between the observed exitance and ideal blackbody exitance:

$\epsilon _{\lambda }={\frac {M_{observed,\lambda }}{M_{blackbody,\lambda }}}$

Note that emissivity is a unitless quantity in the range [0,1]. It normally varies as a function of wavelength. Objects with a uniform emissivity are often called gray bodies.