reflectance, ρ 

Also contains definitions of: reflectance factor, reflectivity
https://doi.org/10.1351/goldbook.R05235
Fraction of incident radiation reflected by a surface or discontinuity, ρ(λ)=PλreflPλ0, where Pλ0 and Pλrefl 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 n1 and n2 is given by ρ(λ)=(n1−n2)2(n1+n2)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 [Terms] [Paper]
Citation: 'reflectance' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.R05235 RIS BibTex EndNote
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Last revised: February 24, 2014