When it comes to growing your own weed indoors, there are a few technical terms that you to understand to get the most out of your plant. PAR is definitely one of them.
“What Is PAR Light Measurement?” is a question we get asked a lot by our readers.
In this article, we’ll explain exactly what PAR is and why it’s important for your plants.
What is PAR Light Measurement?
PAR ( Photosynthetically Active Radiation ) is what causes photosynthesis ( your plants growth ) and happens when your plant consumes photons of light wavelengths between 400 and 700 nanometers. See the graph above!
Why is PAR Important?
Simply put, grow lights beam out lots of different light wavelengths at your plants. However, your plants will only absorb some of these wavelengths to eat and grow bigger. The remaining wavelengths will reflect away from the plants, making them pretty much useless to your plant’s growth.
In general, more PAR = more growth and healthier plants.
Therefore, in general, you want to choose a grow light with a high PAR output so that your plants are getting as much usable light as possible.
How is PAR Measured?
PAR is measured in two ways:
PPF = Photosynthetic Photon Flux – This measures the light’s intensity by determining how many photons per second are emitted from the light. This is measured as micromoles per second (μmol/s).
PPFD = Photosynthetic Photon Flux Density – This measurement is even more important because it considers both the intensity of the light and the amount of that light that actually reaches the plant. It is measured as micromoles per square foot per second (μmol/m²/s)
If you want to measure the exact PAR output of your grow light, then you need to purchase a PAR meter.
What is a good PAR output?
Your plants require a lower PAR output during the vegetative stage and a higher PAR output during the flowering stage; this is because flowering plants use more energy and therefore need more food.
As a general rule of thumb, here are the PAR levels you should consider for each growth stage:
200-400 PPFD – for new seeds or clones and parent plants.
400-600 PPFD – for plants in the vegetative stage.
600-900 PPFD – for plants in the flowering stage.
Generally speaking, the higher wattage your grow light is, the higher the PAR output will be. However, that’s not always the case.
Check PAR Output before buying a grow light
It’s important to check the PAR output of a grow light before purchasing it. Some manufacturers will only provide PPF measurements, or they will provide a single PPFD measurement from the center point of the light. These are tactics used by lower-quality manufacturers to manipulate their lights to look better than they really are.
To get a full understanding of your grow light’s PAR, look for detailed diagrams that show the PPFD at various heights or distances. Most good manufacturers will show this.
Here is an example of a PAR output diagram that is provided by a good quality led grow light manufacturer:
This is the PAR output for the VIPARSPECTRA 600w LED grow light notice how it shows the full PPFD at various distances from the center of the light. It also shows the PPFD of the light when it is hung 24” above the plant and how the light dispurses over 45 inches of growing space.
Tips to increase your PAR
Your grow space setup can have a big effect on your PAR output.
1. Investing in a good quality grow tent with reflective mylar lining can increase your overall PAR output by up to 35%!
2. Another way to increase your tent’s total PPFD is to daisy-chaining multiple lights together in the same tent.
Video explanation of PAR
This article provides a basic overview of PAR – as much as you need to know for growing cannabis. However, if you’re looking for a more technical overview, check out the video below which goes into a bit more detail: