 Surface-illuminant ambiguity

In a diffuse spectral model $N(\lambda )=E(\lambda )R(\lambda )$, choose any positive $g$ for which $R/g$ remains an admissible reflectance. Then $E^{\prime }=Eg$ and $R^{\prime }=R/g$ preserve $N$ exactly. Thus outgoing spectral information alone does not uniquely identify reflectance and illumination; three-channel receptor sampling introduces further ambiguity.