In photometry and heat transfer, reflectivity's the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that's a function of the reflected direction, the incident direction, and the incident wavelength. However it's also commonly averaged over the reflected hemisphere to give the hemispherical spectral reflectivity: ho(lambda) = frac Reflectivity's an important concept in the fields of solar thermal energy, telecommunication and radar.… (More on Reflectance)