Sources:
Orange Book, 2^nd ed., p. 250 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2282 (https://doi.org/10.1351/pac199668122223)