One of the most important factors to take into account when looking at what LED grow light is the best one for your uses is to look at the quality and quantity of light that it produces. However, to do so accurately requires some understanding of a few LED-specific terms. You will see LED grow light manufacturers toss around terms like PAR, PPF and PPFD or units of measurement such as watts or lumens, but what do they actually mean? Let’s dive into the definitions of these abbreviations to help you better understand what is actually important to look at when buying LED grow lights.
Light & Radiation
To understand the units that LED grow lights are measured in, it is first important to understand a few basic facts about light. All light is a form of radiation, which is characterized by the size of its wavelength. Visible light, for example, sits in the range of 400nm to 700nm. As you can see in the chart above, some forms of radiation/light such as UV (ultra-violet) or IR (infrared), possesses a wavelength that is not visible to the human eye. Although we can’t see these types of light, they are still very important for other living organisms – specifically plants. Plants rely heavily on a greater spectrum of light than humans make use of in order to perform photosynthesis, which actually encompasses the visible spectrum that humans use, plus some.
Specifically, the light between the wavelengths of 400nm and 700nm. In fact, the reason most plants are green is that the chlorophyll (the photosynthetic portion of the plant) reflects green light which produces the green color of plant leaves.
What is Lumens used for?
Because humans can only view a small portion of most light in the universe, we use a unit that accurately measures the amount of visible light that is emitted from traditional lights that the human eye can detect. This unit is the Lumen, which is defined as how bright a light is to the human eye. Traditional lightbulbs are often measured in lumens, since that’s a useful metric for everyday life, like when asking the question, “how much light does this lightbulb emit?” However, when we are talking about grow lights for plants – specifically LED grow lights – we need to take into account the light that plants make use of that the human eye cannot detect. The range of light that plants make use of for photosynthesis is known as Photosynthetically Active Radiation (PAR)
What is Photosynthetically Active Radiation (PAR)
In the figure above, we can see that the measurement of lumens encompasses only a certain amount of the PAR range. If you ever see a company advertising their lights using their lights using lumens, this is not very useful information at all. As lumens are simply describing how bright a light is – not how much photosynthetically active light is actually reaching your plants. To accurately measure how much PAR an LED grow light is actually providing your plants, we need to make sure we use a different unit than lumens.
Once again, photosynthetically Active Radiation (PAR) is the range of light/radiation that plants use for photosynthesis. PAR-light falls between the range of 400nm and 700nm. It is important to understand that PAR by itself is not always an accurate measurement to determine how an LED grow light will interact with your plants. PAR of light can be measured using a device called a PAR meter which can determine how much light within the PAR range (400-700nm) your LED grow light is emitting. The amount of PAR is then expressed as the number of moles of photons being emitted within the 400-700nm range (usually expressed in umol).
There is definitely some discord in the growing community about the usefulness of using PAR in determining the strength/quality of an LED grow light. The main reason is that although we can measure the amount of PAR being emitted using the PAR meter, we don’t actually know how much of that light is actually reaching the plant below. Therefore, the position of the PAR meter and the distance from the sensor to the light has a large effect on the PAR value. However, if you know the distance the sensor was placed from the light, PAR can be an accurate value of the strength of an LED grow light.
There are three other important metrics that a PAR meter provide: PPF and PPFD, and spectrum analysis. We’ll cover these next and give our two cents on whether or not you need to take them into account when choosing an LED grow light.
Photosynthetic Photon Flux (PPF)
When you use a PAR meter to measure what amount of PAR is being emitted from a given light, we are given a one-time measurement. You will notice however if you ever take a measurement from under a light, the value changes over time. If we were to track the amount of PAR emitted over a period of time, we would then be provided with the Photosynthetic Photon Flux (PPF) of that light: the number of moles of photons expressed within the 400-700nm over time, in umol/s. Some people claim that PPF is a much more accurate metric to compare LED grow lights, due to the fact that it is tracking PAR over time.
However, we don’t find that this distinction a major issue when comparing grow lights. The fluctuation of PAR values over time for a light is basically negligible. If you were actually to take a one-time measurement of PAR and then compare it with the PAR values that you measure over say, 60 seconds, giving you the PPF – the value wouldn’t be much different. Sure the PPF is more accurate, but it’s just the average of 60 different PAR values over 60 seconds. LEDs produce consistent enough amounts of PAR over time that there is not much difference between PAR and PPF. Of course, we will use PPF whenever available – as it is a more accurate measurement – but if not, PAR is still a great tool for determining the amount of a photosynthetically active light a light is emitting (barring one caveat which we will get to soon) (link to lower down the page where we talk about spectrum analysis).
Photosynthetic Photon Flux Density (PPFD)
Photosynthetic Photon Flux Density (PPFD) is another important factor to consider when choosing LED grow light. While you can define the PPF is the amount of PAR being produced per second, PPFD is the amount of PAR per second that is actually affecting a certain area measured as umol/m2/s. Still with me? In other words, PPFD is the amount of photosynthetically active radiation that is actually reaching your plants! If we know the PPF of a light, and we know the area of our grow space, we can actually roughly calculate the PPFD of our light. For this sample calculation, we will assume a few things: a PPF of 1200 umol/s, a 4×4 ft growing space, and a loss of light of 20% from the canopy of our vegetation (I did say roughly, didn’t I?)
So, from a given distance between light and plant:
PPF = 1200 umol/s
Growing Space = 4×4 ft = 1.2 x 1.2 m = 1.4m2
Loss from canopy = 20% of the PPF -> 80% light reaching -> 0.8 x 1200umol/s = 960 umol/s
PPFD = 960 umol/s 1.4m2 = 685 umol/m2/s
And there you have it! A nice value of PPFD. Note that this value is heavily dependent on two factors that can vary widely: the distance between the LEDs and the plant, and the loss of PPF from vegetation canopy. The closer the LEDs are to the plant, the more PAR will actually reach your plants, resulting in a lower percent loss of PPF. So a value of PPFD is only viable to draw conclusions from if we know the height difference between plant and grow light. As well, the amount and nature of your vegetation canopy will also affect the loss of PPF and therefore alter the PPFD. The actual loss based on the canopy is also a very difficult thing to measure. Because of these variations, PPFD is an extremely difficult metric to use to consistently compare LED grow lights. Whenever possible, we will use these values to describe the lights we review, but keep in mind these values can only be accurate and useful under specific conditions.
Remember when we said there was a caveat to PAR being a good tool for measuring and comparing LED grow lights? Well, we’re getting to that now. What I said earlier about PAR is true – it is a great tool – but you need to know one other crucial piece of information as well: the actual spectral composition of your grow light. Because yellow and green light is contained within the PAR range, even an old-school HPS light could provide a very high PAR value, and if that’s the only metric you’re using to select a grow light, it could lead you wrong. Old HPS and HID grow lights emit a large fraction of yellow and green light producing a lot of excess heat, wasted light, thus harmful to your plants.
So even though HPS/HID pump out a massive PAR value, it’s not necessarily a good thing. To determine whether your PAR value is actually going to benefit your plants, you need to know what spectrum of light your grow light is actually emitting. Ideally, you want a LED grow light with a large PAR value and a spectrum that closely resembles that of the light that the plants chlorophyll maximally absorb.