Table 2.

Overview of basic quantities in a photon-based system used for life science applications.

simplified definition	name	unit
emitted/incident number of photons	number photons $(N)$	micromol $(\mu \mathrm{mol})$ or microeinstein $(\mu \mathrm{E})$
emitted/incident number of photons per second	photon flux $(PF)$	micromol per second $(\mu \mathrm{mol}\bullet {\mathrm{s}}^{-1})$ or microeinstein per second $(\mu \mathrm{E}\bullet {\mathrm{s}}^{-1})$
	photosynthetic photon flux $(PPF)$
	yield photon flux $(YPF)$
emitted/incident number of photons per area and per second	photon flux density $(PFD)$	micromol per square meter per second $(\mu \mathrm{mol}\bullet {\mathrm{m}}^2\bullet {\mathrm{s}}^{-1})$ or microeinstein per square meter per second $(\mu \mathrm{E}\bullet {\mathrm{m}}^2\bullet {\mathrm{s}}^{-1})$
	photosynthetic photon flux density $(PPFD)$
	yield photon flux density $(YPFD)$

Next to a simple definition of each quantity, the particular name of each quantity in a unweighted and a weighted system (weighted with the BWF PAR) and weighted with the average plant response curve (McCree 1971) including their SI units are summarized