specular reflectance (reflection factor), ρ in optical spectroscopy

https://dx.doi.org/10.1351/goldbook.S05851
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