The fraction of light absorbed by a system is independent of the incident radiant power (P λ 0). This law holds only if P λ 0 is small, scattering is negligible, and multiphoton processes, excited state populations, and photochemical reactions are negligible.
See: absorbance, Beer–Lambert law
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2251 (https://doi.org/10.1351/pac199668122223)