Lumens vs PAR

We often get asked about measurements of Lumens vs PAR and what that means. Essentially, lumens are the measurement of light output that is visible to the human eye. When we buy a 60w light bulb it will show as approximately 700 lumens. There is no PAR measurement for a human light bulb. PAR (photosynthetically active radiation) is the light output that plants need to grow. It is a plant's version of lumens. When we find a manufacturer advertising lumen output rather than PAR, we are very skeptical.

One of the best ways to know what the PAR output is on a light fixture, is to test it with a light meter. We use a calibrated LiCor with the Quantum sensor. Measure a light at the manufacturer's recommended mounted height. This is important because some fixtures are created to be veryclose  to the canopy for high PAR values but it is difficult to work under your plants with the lights mounted so close to the plants. Also, a fixture that requires close mounting height is a sign of a fixture with fairly low PAR levels. If you raise the light up, your light levels will drop dramatically.

Different crops will be looking for different PAR output. For example, lettuce will be looking for an average of 10 umol evenly across the canopy where cannabis in the flowering stage of growth will be looking for between 800-1000 umol.

What do we mean by evenly? Many light sources are directional, they don't have optics to spread the light out. A light fixture like this will measure very high light output directly under the light but as you move the meter further from the center point, the levels will drop drastically. You might see a measurement of 1200 umol under the center and only 200 umol just 2' off center. It's important to find out what the PAR is all the way across your canopy, not just directly under the light.

Nanometers vs umol? You will hear phrases that say "plants respond to a spectrum between 400-700nm" and it is often confused with PAR. The nanometer measurement is with regards to COLOR SPECTRUM and is not a measurement for PAR. We'll save that for another blog!

As always, any questions you have about this or any other blog you see here, please contact us at and we are happy to help you!