LED Grow Light Terms: PAR, PPFD, PPF (introduction)

This is an introductory article for three commonly used and important LED grow light terms. We circle back to our video on PAR, PPFD, and PPF with this article and offer a little bit more context in writing. Depending on how you prefer to absorb information, text or video, choose the media of your preference.

This is a short and basic explanation of each term but we will do a more in-depth piece of each of them in the future.



Before we start talking about LED grow light terms, let's first go over basic light physics. What we'll need to understand going forward is that all light consists of tiny, tiny particles called "photons". Photons are elementary particles with wavelengths. The intensity of a light source is determined by how many photons that light source emits. As in the example in our video, a car's powerful headlights emit a much more intense light than a small desk lamp. I.e. the car's headlights emit more photons than the desk lamp.


LED street light
Photo by Paul Volkmer on Unsplash


Although photons are incredibly tiny and not visible to the naked eye, there are tools and instruments that can measure:


  1. The total quantity of photons that are emitted from a light source

or

  1. The number of photons that land on a specific area

Note the difference between these two scenarios. The first describes the total light output of a light source whereas the second describes a spot measurement.

A little bit like saying "how fast can this car go flat out?" and "how fast are we going this very moment?"



Spread of photons from LED grow light
Here we see a lamp emitting a certain quantity of photons but as the photons spread, only a portion of the emitted photons actually land on the intended surface; the plant's canopy.

Ok, moving on.



PAR, Photosynthetically active radiation

This term is often used when brands display and market their grow lights. Often this term is misused and confused with PPFD, more on this later.

PAR is, simply put, the range of light that drives photosynthesis in plants. Photosynthesis is the process that drives plant growth. The better we stimulate photosynthesis in a plant, the more the plant will grow. Giving the plant too little light would poorly stimulate photosynthesis. Same thing applies if a plant receives too much light, which could also lead to light bleaching and leaf burn.


The so-called electromagnetic spectrum consists of x-rays, UV, visible, IR, microwaves, etc. Among all of these segments, a portion of the visible light is absorbed by plants and this is what drives photosynthesis. This is the Photosynthetically active radiation, or PAR.


Electromagnetic spectrum with PAR range
By Philip Ronan, Gringer - File:EM spectrum.svg and File:Linear visible spectrum.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24746679

PPFD, Photosynthetic photon flux density.

This is the most important term when it comes to grow lights. PPFD describes how many photons from the PAR-range (the range of photons with the correct wavelength to drive photosynthesis) land on a specific area. This is a spot measurement.

Various lamps emit photons in various directions. A round bulb-type lamp or an HPS bulb usually has a 360-degree spread of photons -- it emits light in all directions; up, down, left, right.

LED bulb 360
Photo by Eilis Garvey on Unsplash



A LED bulb with an E27 socket or a LED grow light on the other hand only emits light in one direction: down.

LED E27 bulb 180

Although the light can spread as much as 180-degrees, the spread is still only half of a bulb, as shown above. The light is more focused. With today’s tools, it’s possible to measure even more specifically how much light ends up on a certain spot, on a plant or on a leaf, for instance. A quantum meter is the right tool for this job. An Apogee MQ-500, for instance. Its sensor takes in how many photons land on it every second and then it translates that value to, let's say, 500 µmol/m2/s.

That would mean that the photosynthetic photon flux density, the PPFD, is 500 µmol/m2/s.


We know what PPFD levels plants prefer during their different stages. If we know how much our plants want, and we're able to measure how much is actually emitted, we'll then be able to set up our grow area and lamp so that we give the ideal amount of light to our plants.

More on plants’ PPFD preference, how to measure PPFD, set up a lamp, and read light footprint maps in a coming article.


Measuring PPFD from LED grow light

MQ 500 PPFD measurement



PPF, Photosynthetic photon flux.

This term tells us how much light, or rather, how many photons a light source emits in total per second. PPF does not take into account where the light lands or how it flows. It just describes the total output of photons from a lamp or any light source. PPF is measured in µmol/s.




Takeaways:

PAR: a specific range of light that drives photosynthesis within the electromagnetic spectrum.


PPFD: the number of photons (quantity of light) on a specific spot.


PPF: the total amount of light emitted from a light source.

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