Principle of univariance ($Q=\int N(\lambda )a(\lambda )d\lambda$)

A receptor's absorption-dependent response does not independently label the wavelength of an absorbed photon. Different combinations of spectral composition and intensity can yield the same receptor quantum catch. Comparing spectrally distinct receptor classes can supply information unavailable from one alone. In the displayed model, $N$ is a photon distribution and $a$ effective absorptance; an energy spectrum would require the corresponding photon-energy conversion.