When LEDs arrived on the market, their tremendous efficiency and money saving potential changed the playing field. Lumens, lux, and footcandles finally became obsolete metrics for determining light requirements for plants.
Recently, people began referring to PAR, PPF, and PPFD as ways to measure light in photosynthetics lighting applications. The purpose of this article is to explain what these terms mean, correct some common misunderstandings and help growers understand how the science behind these terms can be used to determine proper light levels to grow happy, thriving plants.
What Is PAR?
PAR (Photosynthetically Active Radiation) is an often used (and often misused) term. It is NOT a measurement or “metric” like feet, inches, or kilos. Rather, it defines the type of light (scientists call light “electromagnetic radiation”) needed to support photosynthesis in plant life. Through photosynthesis, plants convert light energy into chemical energy, which is the food they use to grow and thrive.
Interestingly, plants use roughly the same part of the spectrum that’s visible to the human eye, but the wavelengths we perceive to be the brightest (i.e. green light) are not the most efficient wavelengths for photosynthesis.As we all know, some light (like the light from a candle) is visible to the human eye and some (such as infrared) is not. Scientists define different types of light by their “wavelengths”. These different wavelengths make up the electromagnetic radiation “spectrum”. This spectrum includes X-rays, radio waves and infrared light (none of which are visible to the human eye) and light that we can see such as sunlight, and light from a red or blue LED.
So, the first thing to understand about PAR is that it is the part of the electromagnetic radiation spectrum (light) that is useful to plants and algae to activate photosynthesis -- it is NOT a measurement. What really counts is PPF, PPFD and DLI.
What is PPF, PPFD?
Photosynthetic Photon Flux (PPF)
The first measurement is Photosynthetic Photon Flux or “PPF” which measures the total amount of light that is produced by a light source each second. Put another way, PPF tells us how much PAR is emitted by a light source per second. More technically, PPF measures the “photosynthetically active photons emitted by a lighting system per second”. This measurement is expressed in “micromoles per second” and we’ll get to micromoles in a minute. Note though, that PPF does not tell us how much of the measured light actually lands on the plants or any other surface. It is probably the second most important way of measuring a lighting system, but, for whatever reason, 99.9% of lighting companies don’t list it. PPF is expressed by scientists and light engineers as: μmol/second.
Photosynthetic Photon Flux Density (PPFD)
The second measurement is PPFD which measures the light that actually arrives at the plant or algae. Photosynthetic Photon Flux Density or “PPFD” is a measurement of the amount of light that actually reaches your plants and algae or, as a scientist might say: “the number of photosynthetically active photons that fall on a given surface each second”. PPFD is a ‘spot’ measurement of a specific location on your plant canopy, and it is measured in micromoles per square meter per second. This measurement is expressed by scientists and light engineers as: μmol/m2/s.
Daily Light Integral (DLI)
The third measurement is DLI (Day Light Integral) which measures the total amount of light that is delivered to a plant every day. The grower can think of DLI as the plant’s daily “dose” of light, though a scientist would probably say DLI is a cumulative measurement of the total number of photons that reach the plants and algae during the daily photoperiod. DLI measures the number of “moles” of photons per square meter per day and is expressed as: mol/m2/d.
Obviously, if you leave your lights on longer, your plants absorb more light. Or, put in more technical language, a lighting system with a lower PPF can deliver the same DLI to an aquarium compared to a lighting system with a higher PPF if the photoperiod is extended. DLI is analogous to the total amount of rain that falls during a storm, as opposed to how fast the rain fell (which would be PPFD). DLI is the most important metric for determining the overall growth rate of plants and algae. Once you know the preferred DLI of your plants and/or algae, you can easily set up a lighting system to deliver the required amount of light. Properly designed photosynthetic lighting systems start with defining the required DLI.
Article borrowed from Fluence Bioengineering and revised for content！
In Summary..
PAR represents the fraction of sunlight with a spectral range from 400 to 700 nm which is plants perceive and sensitive light .Like human lumens &lux to identify commercial and residential lighting , PAR is the most important metric to grow lights .
PPF tells us how many photons of light are emitted by a light source each second.
PPFD tells us how densely the fixture distributes the light photons on a one meter square target (i.e. your plants and algae) in one second.
Post time: Feb-23-2021