radiant (energy) flux, P , Φ

This obsolete term is not recommended. In photochemistry, radiant power, $P$ , is adopted. SI unit is $W$ .

Note:

$P$ is preferred, because in photochemistry, $Φ$ is reserved for quantum yield.

Source:

PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 409