Today we're talking about PAR output. PAR is one of the biggest factors in whether your grow lights will provide you with the best yield for your garden. PAR is a part of our benchmark measures when testing grow light systems.
It can be used to test all types of HID grow lights, CMH grow lights, LED grow lights and T5 grow lights. What makes a grow light suitable for your needs is the PAR output (or "PAR rating") of a given grow light.
But to understand what makes a PAR reading so meaningful, we're going to discuss PAR, PPF, and PPFD and what it all means for your garden.
What is PAR?
PAR stands for Photosynthetic Active Radiation. What does that mean? Well, let's break it down: In scientific terms, light has properties of both photons and waves and in the spectrum of visible light. The usable light that plants can absorb is used for photosynthesis. In simple terms: grow lights (that mimic the light of the sun) gives off tons of wavelengths (colors) of light when it shines. However, plants will only absorb some of those wavelengths and use them to eat. The rest of those wavelengths are reflected from the plants. So what does that have to do with PAR? Some people think PAR is a measurement of all the light coming from a light source. This is not the case. PAR is the amount of light that’s usable to plants- those wavelengths that will be used for photosynthesis. Just like the difference between a growing area and a canopy, PAR is not the measure of all of the light a grow light gives off, but the measurement of the wavelengths plants will absorb.
So if PAR is just a term that describes the type of light that plants absorb, how do you measure it? You measure PAR with a PPF meter (aka "PAR meter")PPF stands for Photosynthetic Photon Flux, which means how many photons of light per second make contact with your plants. When we use a PAR meter to test how well light fixtures emit PAR, we are measuring how many photons of light hit our sensor per second. In other words, we are measuring the intensity of light being emitted from your light source that plants use to photosynthesize. The Sun System PAR Meter measures the visible light spectrum, which is between 400-700nm wavelengths.
Okay, so what's the connection between PAR and a grow light source?
To demonstrate this idea, imagine your grow light as a cloud and the light coming out of it is rain. When it comes to grow lighting, you don't want a drizzle, you want a full-on storm! Most plants that are cultivated for consumption require light to grow. Plants will use that light to help break down the water and nutrients they suck up from their roots and turn it into carbohydrates, or energy, to grow big and strong. A PAR reading will tell you how much light energy/intensity a grow light has at different points over a given growing area. To use our rain analogy, a PAR reading will be the measurement of rainfall on the ground it covers.
So that means the more intense PAR a light has the better it is, right?
Yes! Well, sort of... What's most important is making sure the quality of spectrum your light gives off is the best one for your plants, and that your grow light has a nice, even spread across your canopy.
PAR meters measure the intensity of light but it does not tell us the quality of the spectrum that light produces. Visible light is a mix of different reds, blues, greens, oranges, and yellow wavelengths.
While a given light may have high PAR readings, it may not necessarily have the wavelengths of light you need to grow a successful, harvest-ready plant. We should note that while most plants need a blend of all of these wavelengths to grow properly, targeting specific wavelengths isn't necessarily a bad thing, as is the case in supplemental LED's.
Light Coverage and Spread
PAR output for grow lights is best when the output is spread evenly in your grow area. You may have great PAR readings directly under a grow light, but if your PAR output drops dramatically from moving only 2 feet away from the center of your grow area, then you might want to consider upgrading your grow light.
PPFD Measurements and What's Best for Your Plants
As we've mentioned, PAR is the type of energy plants use from light and PPF is a way to measure light efficiency or intensity. PPFD, or Photosynthetic Photon Flux Density, is a measurement of light density in a given area.
So let’s talk benchmarks and the best PPFD/PAR range for your plants!
- Between 200-400 PPFD: This is great for seedlings, clones, and mother plants.
- Between 400-600 PPFD: This is great for early to late stage vegging cycles.
- Between 600-900 PPFD: This is great for the flowering, fruiting, or budding stage of plants.
Between 200 PPFD and 400 PPFD, plants growth rate potential crawls around 30 - 55% which is recommended for mother plants or young, weaker plants that need a light that's not super intense.
If you were to double your PPFD from 400 to 800, the growth rate potential reaches about 85%. You would think that doubling the intensity would push the growth rate potential pass 100% but the truth is, there’s a loss of power in return the more intense your lights become.
Once you surpass 1000 PPFD and go into Double Ended Grow Light territory, your plants stop becoming light-limited and start becoming carb limited. At that level of PPFD, increasing CO2 levels is highly recommended to take advantage of that intense light. But for most growers, CO2 is not necessary.
So a common balance between performance and efficiency is in the 600-900 PPFD range. Keep in mind that these benchmarks we mentioned represents the potential growth rate on the lighting side and doesn’t factor in other environmental conditions or genetics for optimal growth.
Testing the PAR/PPFD Reading of Yield Lab HID Grow Lights
- We measured a 4x4ft area and plotted 33x PPFD points on a grid in that space
- We turned each Yield Lab Cool Hood on and hung them at 18” and 24” above the PPFD meter
- Each grow light has two PAR readings: one at 18" and one at 24"
Take a look at our graph: each square has a number, and that number represents a PPF reading of that one spot. The total of all of the spots in the testing area is the PPFD of the grow light. Now that you know what's being measured and how here are the results of our tests:
Results - Yield Lab Brand HID Grow Light Systems
We completed the PAR rating for all HID Yield Lab Grow Light Systems. The graphs you see here are just a sample of the testing. This is the 1000w, 600w, and 400w HPS grow light system measured over a 4x4 area at 18 and 24” in height. All of them are housed in a standard Yield Lab cool hood reflector.