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.
Lumens are for humans and PAR is for plants. And the two don't have much to do with one another.
The way to measure PAR output for grow lighting is with a PAR sensor and meter such as the Apogee MQ-500 which is the gold standard for full spectrum light measurement.
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 50% 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) of photosynthetic light. Veg plants on the other hand prefer light levels between 250-400 umols. And clones and baby seedlings can't tolerate much more than 100 umols.
Distance is just one consideration for PAR (PPFD) 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 sensor further from the center point, the levels will drop drastically. You might see a measurement of 1200 PPFD 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.
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.
As always, any questions you have about this or any other blog you see here, please contact us at firstname.lastname@example.org and we are happy to help you!