Horticulture Lighting Terms

What are PAR, PPF, PPFD, PPE?

PAR

There are some parameters you need to know about grow lights: PAR, PPF, PPFD, PPE. Learn these terms so you can make an informed purchasing decisions and achieve high-quality plant production.

PAR

The spectral range of solar radiation from 400 to 700 nanometers that plants are able to use in the process of photosynthesis is called photosynthetically active radiation, often abbreviated as PAR.

When measuring the irradiance of PAR, values are expressed using units of energy (W/m2), which is relevant in energy-balance considerations for photosynthetic organisms. However, photosynthesis is a quantum process and the chemical reactions of photosynthesis are more dependent on the number of photons than the energy contained in the photons. Therefore, plant biologists often quantify PAR using the number of photons in the 400-700 nm range received by a surface for a specified amount of time, or the Photosynthetic Photon Flux Density (PPFD).

This spectral region corresponds more or less with the range of light visible to the human eye. It is the optimal range, which is not harmful for living organisms. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere. While the longer wavelengths have less energy to carry out photosynthesis.

PAR and visible light

PAR is not a unit of measurement, and if you want to know how much light a grow light produces and how much light a plant can utilize, you should turn your attention to these two parameters: PPF, and PPFD.

PPF

Photosynthetic photon flux(PPF) refers to the number of micromoles of photons radiated by a light source per second in the wavelength range of 400-760nm, in umol/s. It tells us how many photons the light source emits per second.

PPFD

Photosynthetic Photon Flux Density is the number of micromoles radiated by the light source per second per square meter, the unit is umol/m2s, which is the concept of density. PPFD is the PPF expressed in one square meter. PPFD does indicate how many photons fall on a plant, but a single PPFD measurement will not give you the number for the entire growing area. The center of the light source usually has the highest PPFD reading. The PPFD decreases as you move away from the center. PPFD measurements also depend on the distance of the light source. The farther the light source is from the canopy, the smaller the PPFD. Therefore, it is necessary to make useful comparisons between grow lights by measuring PPFD values over the entire illuminated area and at various distances above the tree canopy.

So you can see that grow light suppliers usually provide PPFD charts like this:

The following picture shows what is the difference between PPF and PPFD:

PPE

Photosynthetic photon efficiency is photosynthetic photon flux PPF divided by input power, the unit is μmol/J, PPF (μmol/s)/input power(watts)=PPE(μmol/J)

The higher the PPE, the more efficient the plant is at converting light into growth, and the higher efficiency of the lighting system. High PPE bulbs use less energy overall, which means you'll save electricity while still giving your plants all the light they need to thrive.

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