The science behind the LED Grow Light

The science behind the LED Grow Light (what to believe now?)

We are going to talk about the science behind (LED) grow lighting and why it can work so well. It's a complicated matter but we try to explain it as simply as possible without being incomplete.

This is probably familiar to you

You are planning to purchase an LED lamp for the growth and flowering of your plants, first look up some information.. Forget it, as soon as you go on the internet you will be showered with sense and nonsense from everyone who wants to sell their lamps . The highest wattages, the highest amount of lux, lumen, candela and efficiency of photons etc. They all promise the best results for your plants, but none of the above metrics are correct. What should you believe now? In horticulture and greenhouse construction, it's all about the PAR value. To make this clear, we will explain the difference between a number of quantities such as lumen, lux and PAR in LED grow lights below. We hope to be of service to all growers struggling with these questions.

Lumens (lm)

Lumen says something about the total light intensity of the light source. Lumen (lm) is the unit for luminous flux and is concretely measured in light quantity per time unit (e.g. per second). The range of the lamp and the direction of the light are not important here, it is only about the amount of light that the lamp emits. So it doesn't matter if there is one LED grow light a lens angle of 60, 90 or 120 degrees is used. The amount of lumen of this lamp always remains the same. A lamp with more wattage or less wattage can of course contain more or less lumen.

Candela (CD)

Unlike lumens, the direction of the light is important with Candela. Candela is the amount of light that is emitted by, for example, an LED Grow Light to a certain angle. With a light angle of 60 degrees, it is therefore about the amount of light that is within those 60 degrees. Candela is also not directly important for the growth and flowering of plants and flowers. A grow lamp with a high value measured in Candela therefore says little about its operation.

Lux (lx)

And then there is lux, very important in all kinds of applications such as in sports or in the office, but again not important in an LED grow light. When people say that 200 lux is present, they often refer to the average lux value on a certain surface. For example on a sports field of 90 by 50 meters. The amount of light can always be calculated back to a surface area (for example, a square meter). The further the light source is from the surface, the lower the amount of lux. Of course, this also differs per light source, lux only says something about the light quantity or light intensity. Lux is also the amount of lumen/m2.


PAR stands for Photosynthetically (photosynthesis) Active Radiation (active radiation) and indicates which light plants need to achieve photosynthesis. Lumen, Candela and Lux ​​are used by people as measurements and units for the amount of light. These data are all perceptible to the human eye, that is not with PAR, PAR is not a unit of measurement.

When we talk about LED grow lights , the problem when using the lux value is that you get different values ​​than when you measure the amount of PAR. Blue light between 400nm and 500nm and red light between 600nm and 700nm are less present within the perceptible spectrum for humans. Nm stands for nanometers. Are you already falling asleep? Hold on a little longer, we'll get to the heart of this story.

So Lumen is for humans and PAR is for plants, so only the PAR value should be used when talking about LED grow lights. In the nanometer regions mentioned above, plants are sensitive to light, which activates photosynthesis.

How is the PAR value determined?

A quantum sensor can be used to determine the light intensity of an LED grow lamp. A quantum sensor uses an optical filter that is sensitive to PAR. In combination with a light meter, the total light intensity and the PAR value can be determined. Below you see an image

Measure PAR

What should I pay attention to when buying LED grow lighting?

  • How much PAR is emitted by the grow light (Photosynthetic Photon Flux - PPF)
  • How much of this PAR value is available/usable for the plant (Photosynthetic Photon Flux Density - PPFD)
  • How much energy is used by the grow light to make the PAR available to the plant (Photon Efficiency - PE)

Let's explain that..

Photosynthetic Photon Flux – PPF

This is the amount of PAR per second produced by a grow light, or micromoles per second (µmol/s). This still says little about the amount of light absorbed by the plants, but it does indicate how efficient a (LED) grow lamp is in producing PAR. For the geeks: PPF is measured with a special spherical measuring device that captures all the photons emitted by the grow light. The light is let in through an opening in the dome. Below is an image.

Measure PPFD

Photosynthetic Photon Flux Density - PPFD

PPFD is measured in micromoles per square centimeter per second (µmol/m2/s) and indicates how much light reaches the plant at a certain location. It is a difficult quantity to measure, because an LED grow lamp, for example, naturally contains more μmol in the center of the light beacon than on the sides. It is therefore important to have a good average value. So one measurement is never enough to determine the PPFD of a grow light. The PPFD also depends on the height at which the lamp hangs during the test. To get perfect measurement results, you will therefore have to take both horizontal and vertical measurements at different places under the light source (grow lamp). When Het LED Warenhuis talks about PPFD, this is almost always measured at the recommended suspension height of the lamps.

Photon Efficiency - PE

Wattage is a commonly used factor when purchasing an LED grow light. However, what many people don't know is that the wattage tells something about the lamp and the electronic input, not the output of the lamp. What can be done with wattage is to measure how efficient a grow light is, because you can divide the PPF by the wattage. Wattage is measured in Joules per second (J/s) and PPF as discussed in micromoles per second (µmol/s). The efficiency of the lamp is then measured as micromoles per Joules (µmol/J). You will then have found out how good a grow lamp actually is at converting electricity into PAR photons.


Within the light spectrum, plants and flowers mainly use wavelengths between 400nm and 700nm to achieve photosynthesis. It is therefore important that you purchase a lamp in which this light spectrum is represented. The 12-band spectrum (also called Full Spectrum) also contains the red and far red (towards 700nm) that the plant needs to flower. Then pay attention to the average PAR value in combination with the wattage.

We hope that all this information has been useful to you, but we can imagine that this raises more questions for you, so please feel free to contact us.

Good article? Share it easily with your friends and acquaintances via the buttons on the right side of this page.