Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and is defined as the total amount of quanta across a the spectral range of 400 to 700 nm.

The summed amount of light across this range is called the photosynthetic photon flux density (PPFD) and is measured in units of umoles of photons per square meter per second (umoles m-2 s-1).

The term PPFD is often shortened to PPF (photosynthetic photon flux) by plant physiologists. The two terms (PPFD and PPF) are interchangeable.