April 15, 2021
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.
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.
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.
Why do we want to measure PPFD and why is it important for grow lights?
Since it’s possible to measure both the PPFD output from grow lights and also determine the ideal PPFD levels for plants, the grow light’s output should match the plant’s needs.
Let's say you’re growing tomatoes in a 2x2’ (60x60 cm) tent and you know that tomato is a high light intensity crop that likes PPFD 600 µmol/m2/s and above. Naturally, a grow light matching this plant’s needs should be selected.
Or, if you’re growing microgreens, which are considered low light intensity plants, they will do well with PPFD 100-200 µmol/m2/s. These two scenarios would need very different grow lights.
General PPFD recommendations in µmol/m2/s:
|
Plant/maturity Clones, cuttlings, and propagation:
Young high-light plants, early veg: |
PPFD (µmol/m2/s) 75 150-300
200-300 300-400 |
Understand your plants’ or grow area’s PPFD needs then find a grow light with adequate performance.
Although plants don’t need maximum PPFD levels over their entire surface, averaging somewhere between 300-600 µmol/m2/s on most of the plant during flowering should be aimed for.
Here’s a tomato plant receiving 600 µmol/m2/s in the center and around 80 µmol/m2/s on its outer leaves.
To determine the optimal grow light size, it is important to consider the optimal PPFD (density of light) for growing cannabis and how that Optimal PPFD converts to an Optimal PPF (amount of light).
The data from Chandra et al. confirm that the optimal photon density for peak cannabis photosynthesis is between 500 and 700 µmol/m2 (PPFD). It also shows that we should avoid going over 1000 µmol/m2 (PPFD) which could lead to damage. With artificial lighting, the distribution of light is never perfect. Therefore, we want to ensure that all areas of the canopy get at least 500 µmol/m2 (PPFD) and that no spot receives more than 1000 µmol/m2 (PPFD). We recommend an average of 700 µmol/m2 (PPFD). With most grow lights, an average of 700 will ensure that you stay within the optimal range for peak photosynthesis in all regions of the canopy.
PPFD is a density measurement which is expressed as micromoles per square meter. To convert PPFD into a quantity measurement, we multiply it by the area in square meters. Since the optimal average photon density is 700 µmol/m2 (PPFD), the optimal number of photons is 700 µmol Usable PPF per square meter. This converts to 65 µmol Usable PPF per Square Foot. To calculate the total amount of light that you need for your grow space in Usable PPF, simply multiply the square footage by 65 (Sq. ft x 65 = µmol Usable PPF).