reflectance, ρ

https://dx.doi.org/10.1351/goldbook.R05235
Fraction of incident radiation reflected by a surface or discontinuity, ρ λ = P λ refl P λ 0, where P λ 0 and P λ refl are, respectively, the incident and reflected spectral radiant power.
Note:
The reflectance for a beam of light normally incident on a surface separating two materials of refractive indices n 1 and n 2 is given by
ρ λ = ( n 1 − n 2 ) 2 ( n 1 + n 2 ) 2
Reflectance increases as the angle of incidence decreases from 90 degrees.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 413