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4 Must Know LED Grow Light Metrics: PAR, PPF, PPFD, & DLI Decoded

Posted by Logan Fulcher on

   LED Grow Lights and the improvements in horticulture accompanying them have brought us into a new era of growing – with this comes new and better ways of applying horticultural principles. Measurements such as lumens, footcandles, and lux (measures for human perception of light) are now obsolete in horticulture; taking their place are the more applicable PAR, PPF, PPFD, and DLI... the ways plants 'see' light, represented by acronyms difficult enough to make anyone's brain parts shudder, but let's break 'em down and simplify. Nice & easy. 


PAR- Photosyntetically Active Radiation: Refers to the comparison of light wavelengths that promote plant growth.

PPF- Photosynthetic Photon Flux: The measurement of light photons that leave a light fixture.

PPFD- Photosynthetic Photon Flux Density: The number of photons from a fixture that reach a square meter area per second. Written as: μmol/m2/s

DLI- Daily Light Integral: The number of photons that a given plant requires per day

Glossary- List of words and other abbreviations used in the article.


   First, definitions. PAR is Photosynthetically Active Radiation... Radiation? The stuff thaLumens are for Humans - PAR (Photosynthetically Active Radiation)t makes you bleed out of every orifice and glow green? Well, kind of yes; through the lens of science, light is just one small range of 'radiation' inside of a larger spectrum called electromagnetic radiation, which includes x-rays, microwaves, radio, and more!  

    For our uses, let's know that the R(adiation) in PAR means light (but check out the handy image of the electromagnetic radiation spectrum below to visualize what we're talking about.)

Now then, we should have you on board with 'radiation,' let's talk about the photosynthetically active kind. 

LED Grow Light Visual Radiation


   Photosynthesis is the process through which plants convert light into energy (If you need a refresher check out this crash course for photosynthesis and see if it rings a bell.) Simply put, “Photosynthetically Active” references the type of light [radiation] that turns on [activates] photosynthesis. To really grasp this, let's bring it back to us: most humans have eyes built to see every color from deep violet to red, with an especially high acuity recognizing the color green; of course, we can't see anything outside of the visual light spectrum such as Ultra Violet (UV) light and Infra Red (IR) without special instruments. Do plants react the same way we do to the different wavelengths/colors of light? Interestingly, plants react to all of the same wavelengths that humans see, but they do it in a very different way. They see PAR!  Let's try to visualize things the same way plants do. This telling chart, designed by Lush Lighting, shows us how different proteins in a plant react to the different wavelengths of light.

Photosynthetically active radiation chart

   As you can see, there are two main peaks of photosynthetic activity around Blue-(460nm) and Red-(630nm), yet there are always some proteins reacting to every wavelength/color, even some of the 'colors' humans don't perceive, such as UV and Infrared. Like us, plants react to green, yellow, and amber light, but they are much less sensitive to those wavelengths than humans are and tend to be less affected by them. That being said, there is a lot of debate over how much of each wavelength is necessary for cultivating monster plants, but most agree that red and blue are necessary for healthy plant growth. So, PAR, simply put, is the spectrum of colors that plants need most.  (We've written more about spectrum! Check out the LED Head Grow Lights article: Red, White, or Blue? Choosing the Right LED Color)

  Great, we know what PAR is... so what? Well, it's a huge factor in deciding what the best LED grow light is for you, so unless you want a large light emitting paper weight, you'll check! The Good News: 99% of all manufacturers are acutely aware of PAR and employ it carefully in their LED light designs, so we have that going for us. However, what we really want to know isn't only the quality of light that plants crave, but the quantity of PAR that is reaching our plants- so now we're going to need those other acronyms to describe the amount of PAR.


   Stands for Photosynthetic Photon Flux. We already know what photosynthetic is, but a photon? A photon is the smallest unit of light. Flux means 'continuous change, or flowing out' and refers to the number of photons leaving a lighting unit. I know, it's kind of hard to imagine, but the folks over at Fluence Lighting have imagined a great visual metaphor for this: think of each photon as a single drop of rain during a storm; in this storm, there are millions of drops leaving the cloud from many different points - constantly. Imagine how many drops of water leave that cloud in just one second... a massive number! Hold that thought though...

… that's PPF.

LED Head Grow Lights PPF vs PPFD Demonstration Image

      From our analogy, the storm cloud is our LED Grow Light, and PPF is the amount of light [photons] that leave our light every second; but how many photons is that? They are incredibly Not a Micro-mole for led light measurementssmall and come in mind-boggling quantities. Ready for the mind blow?: We measure the photons in micro-moles... Huh? A micro-mole? No, it's not a teeny tiny subterranean mammal. It's a number used to express awkwardly large numbers. To be exact, one 'micro-mole' equals 602,214,150,000,000,000... AKA ~602 quadrillion. Scientists write this more simply as micromoles per second: “μmol/second.”

   Head overflowing with the digits? Don't waste that precious brain energy storing zeroes! Focus on this: How do we use Photosynthetic Photon Flux? PPF is incredibly useful for gauging potential growing-power; the amount of light emitted from your unit should be high, the higher the better, but it should be said that all of the photons are worthless if the light doesn't get to where you need it.  Just as in our storm analogy, we don't care as much about the water leaving the cloud as we do about where that water ends up. PPF really is just a stepping stone to utilizing our home-run metric: PPFD.


   We've worked our way up - stands for: Photosynthetic Photon Flux Density...  PPFD is the measurement of how many light photons hit a given square meter per second. You'll see this expressed as μmol/m2/s (which just stands for micromoles per square meter per second.) PPFD can be measured with an instrument called a PAR meter; ideally, a manufacturer will measure several points in the light footprint and take their average from multiple heights for inclusion in LED Light specs.

LED Head Grow Lights PPF vs PPFD Demonstration Image


  Typically, quality LED grow lights have a good amount of high readings spread evenly over the growing areas with quickly diminishing numbers towards the edges. Here are some examples of PAR maps for illustrative purposes:


LED Grow Light PAR Map Styles Example Images


   We need to be weary of individual measurements that appear high (~ Over 700,) as there is a bad habit of taking these readings directly under the center of a light - solely to get a good-on-paper high reading; when available, LED Head Grow Lights will post a PAR map that includes PPFD readings from multiple points and/or distances from the light, and we encourage folks who are in the market for a new light to check and or request PAR maps whenever possible.


   At this point, you should either be either so numb or so well informed that this last acronym should fit in pretty easily. While PPFD is a measure of photons in an area per second, DLI refers to how many photons a plant receives in a given day – DLI is Day Light Integral and can be used to calculate how much lights-on time is need for your plant to get enough light for a day; it's your plants daily dose of light. Depending on what you grow, the DLI changes, and we recommend researching your plants' needs before purchasing any light.



PAR - the wavelengths of light that plants use to grow, different from what humans see.

PPF - the quantity of photosynthesis-activating light that a source outputs per second.

PPFD - how much photosynthesis-activating light is reaching a square meter (your plant) per second.

DLI - how much photosynthesis-activating light is reaching your plant in a day.

Micromole - a measurement that groups very large numbers into a more manageable unit – particularly light photons.  Approximately 6.022141×1017

Photon - a single unit of light – simultaneously a particle and a wave: see here to learn more about photons.

Photosynthesis - The process through which plants convert light energy into growth. A very complex and interesting process can be studied easily online here.

Radiation - a massive topic, but in the article describes the wavelengths of visible light, measured in nanometers (nm.) Image is included above!

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