specular reflectance (reflection factor), ρ in optical spectroscopy

Radiant power specularly reflected from the surface of a system, $P refl$ , divided by the incident power, $P 0$ :

$ρ = P refl P 0$

Source:

PAC, 1985, 57, 105 (Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)) on page 115