Reflectivity

Reflectivity is a term in radiometry. It is the ratio between the exitance from the front of a surface and the incident irradiance on the front.

$r_{\lambda }={\frac {M_{front,\lambda }}{E_{front,\lambda }}}$

Note that reflectivity is a unitless quantity in the range [0,1]. It normally varies as a function of wavelength. Reflectivity does not capture the angular dependence of the reflected energy; that is characterized by a BRDF.