There is a lot of confusion as to what is the difference between PPF and PPFD.

The difference between PPF and PPFD by definition

PPF and PPFD are both acronyms that deal with the amount of light for a light source or location:

• PPF - photosynthetic photon flux

• PPFD - photosynthetic photon flux density

What exactly is a photosynthetic photon? A photon is a single particle of light, and can take on a variety of wavelengths. 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.

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.

In our graphic below, each dot represents a single photosynthetic photon. PPFD is the number of photons that land on a particular surface, while PPF is the total number of photons that are released from the light source.

PPF should be the photosynthetic photon flux coming out of a fitting after all losses have occurred and not what a LED is capable of. For example. 
A 100W COB grow light LED may be capable of a PPF of 230 μmol/s at 100W of power to the LED. When you take into account :

1.   Loss of LED efficiency at real junction temperature. Typically 5%

2.   Losses in optics. Typically 3% - 10%

3.   Losses in power supply efficiency. Typically   10%

Taking all this into account would result in the fitting producing  a PPF of about 185 μmol/s.

PPFD on the other hand is the amount of photosynthetic flux that falls onto a surface and is measured as μMol/m²s. This varies based on height and angle from the light source.

An analogy in traditional lightning is:

·         Lumens from a fitting is an analogy for PPF.

·         Lux (brightness measure) is an analogy for PPFD.

·         Lumens per Watt (efficacy) is an analogy for PPF per watt.