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Lumens vs PAR

We often get asked about measurements of Lumens vs PAR and what that means for cannabis growing. Essentially, lumens are the measurement of light output that is visible to the human eye. For example a 60w light will output about 700 lumens.

By contrast, PAR (photosynthetic active radiation) is the light radiation which plants "see" to photosynthesize.  When we see a grow lamp manufacturer advertising lumen output and not PAR as well, it's evidence they don't have much experience with plant biology and light physics. 

The way to measure PAR output for grow lighting is with a photospectrometer such as Apogee paired with a quantum sensor. PAR is a quantum measurement but we'll leave the deeper technical comparison between lumen and PAR measurement for another day. 

PAR level changes with the distance from the lamp so always measure a light at the manufacturer's recommended mounted height or the targeted distance to your plant canopy. If you raise the light up, your light levels will drop dramatically. In fact up to 75% of PAR is lost when you double the original measurement distance. 

Cannabis in the flowering stage of growth does best between 800-1000 umol/ms (micromoles). 

Distance is just one consideration for PAR coverage. Many light sources are directional, they don't spread the light out evenly. 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. This is where PAR charts come in and we have them available for all of our lamp designs. Other manufacturers should as well. 

An additional consideration in lighting layout for grow is what we call crossover. This is the area between two lamps where the light from each lamp crosses over. Crossover is an important consideration because it will determine the ideal distance between the lamps to create the most even PAR coverage across the plant canopy. There are many different types of lamps in the market today. Some are better than others at distributing energy evenly. And there is a new school of thinking that some uneven spots over the canopy may simulate the shade created in nature by competing plants. We've observed many grows with simulated shade which causes more competition between flowers to reach the lighter areas. So although we do want consistent energy across the canopy in the grow room, we might also consider designing a checkerboard of light to imitate shade and foster flower competition, depending on strain and grow style.

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