radiant power, P

Power emitted, transferred or received as radiation. SI unit is J s −1 = W.
  1. Mathematical definition: P = d Q / d t. If the radiant energyQ is constant over the time interval, P = Q / t.
  2. 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.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 410