radiant power, P

Power emitted, transferred or received as radiation. SI unit is $J s −1 = W$ .

Notes:

Mathematical definition: $P = d Q / d t$ . If the radiant energy $Q$ is constant over the time interval, $P = Q / t$ .
In radiometry, flux ( $Φ$ ) is used with the same units as $P$ . The symbol $Φ$ is reserved for quantum yield in photochemistry. Therefore, the use of flux ( $Φ$ ) is not recommended.

Source:

PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 410