specular reflectance (reflection factor), ρ in optical spectroscopy

specular reflectance (reflection factor), $Math - symbol$

Radiant power specularly reflected from the surface of a system, $Math - symbol$ , divided by the incident power, $Math - symbol$ :

$Math - e$

Source:

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