If you're looking to understand how grow lights compare, you likely have come across the metrics PPF and PPFD.
However, you might be confused about what these metrics mean, and how you can make sense of them to make an informed purchasing decision. In this article we go over the basic differences 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.
The graphic should reveal two additional properties about PPFD.
First, the closer to the light source, the higher the PPFD reading. This is due to the dispersion of light as one moves away from the light source.
Second, the center of the beam typically has the highest PPFD reading. As you move farther away from the center, PPFD will decrease.