PPF and PPFD are both deal with the amount of light for a light source or location, firstly we need to know what is a photosynthetic photon?

A photon is a single particle of light, those that are capable of contributing to photosynthesis are considered a photosynthetic photon. Specifically, this includes photons with wavelengths between 400 nm and 700 nm.

Then we can understand what are PPF and PPFD:

• PPF - photosynthetic photon flux

PPF represents the total amount of photons in the PAR range emitted from the light source per second, expressed in μmol/s (micromoles per second)

• PPFD - photosynthetic photon flux density

PPFD measures how many photons effectively hit the plants per unit time per unit area (μmol/sec/㎡ ).

PPF and PPFD measure the quantity of such photons. The critical difference is that PPFD measures the density of these photons falling on a particular surface, while PPF is a measure of the total number of photons released from a light source.