Radiant power specularly reflected from the surface of a system, \(P_{\mathrm{refl}}\), divided by the incident power, \(P_{0}\): \[\rho =\frac{P_{\mathrm{refl}}}{P_{0}}\]
Source: PAC, 1985, 57, 105. (Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)) on page 115 [Terms] [Paper]