What is the difference between PPFD and PPF?

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.

 

PPF and PPFD units of measure

• PPF - μmol/s


• PPFD - μmol/s/m²

Both PPF and PPFD measure the total number of photons. This is obviously a very large number, so the unit micromoles (μmol) per second is used. A micromole is equivalent to approximately 6 x 10¹⁷. Further, since we are measuring the rate of these photons, this is counted per second.

PPF is simply micromoles per second, but PPFD is micromoles per second per meter squared. This is because we want to know how many photosynthetic photons land on a square meter per second.

When to use PPFD vs when to use PPF

PPFD should always be accompanied by a distance and location. Most manufacturers will publish PPFD data, but be sure that you take into account:

• Distance from the light source


• Location and whether this is averaged over a certain area

Without knowing this information, you cannot meaningfully compare PPFD.

PPFD can be the result of multiple lamps lighting a single area.

PPF, on the other hand, measures the amount of PAR from a single grow light. You can make meaningful comparisons between lamps by comparing its PPF measurement. That being said, be aware that depending on the beam angle, this can affect eventual PPFD readings depending on the products.

 

Measurement method differences between PPFD vs PPF

Since PPFD is a measure of how much light falls on a surface, even a small, handheld light meter or spectrometer can measure the amount of PAR that falls on a surface. These are typically lower cost and even be connected to smartphones and used in the field.

PPF, on the other hand measures the amount of PAR that is emitted by a single light source, and in all directions. Therefore, slightly more sophisticated instruments are required. Typically a goniosphere or integrating sphere is needed. These devices capture the light emitted in all angles, and then measures the collective light emitted.