The SpotOn Quantum PAR Meter Displays PAR in umols/ms.
How Lumens Compare to PAR.
Lumens are for humans and PAR is for plants. The two don't have much to do with one another.
Lumen is a measurement of human eyes ability to perceive the brightness emitted by any light source, and its measured in foot candles or LUX.
By contrast, PAR (photosynthetic active radiation) is light energy measured in quantum photon flux which plants "see" to photosynthesize.
Because PAR is a quantum measurement and it cannot be easily converted to LUX. Different color wavelengths of light travel at very slightly different speeds, so a complex quantum equation is applied to each wavelength to provide us with PAR.
PAR is the total amount of energy in 400-700nm wavelengths of energy emitted by the sun, which is associated with optimal plant photosynthesis.
LUX is the total brightness of a light source, which is associated with what our eyes perceive. They are each their own interpretation of the photons of light. One for plants and one for us. The higher the LUX level the brighter the light source is to our eyes.
PAR is the spectrum that plants absorb or perceive. The higher the PAR the more intense the light source is for the plants.
Can LUX meters be used to measure PAR? The short answer is no. They can be used for an approximate measurement of full-spectrum white light and a simple equation can be applied to give you a general comparison to PAR. However lights that contain more blue energy or more red energy will throw off this calculation significantly.
PAR is measured with a specialized quantum sensor. Companies like Everfine, Licor, Apogee Instruments and Innoquest SpotOn all produce high quality quantum PAR meters and sensors.
These sensors display PAR in PPFD or photosynthetic photon flux density. When you hear that someone grows at 1000 umols/ms this means their PAR level is 1000 PPFD.
PAR levels change over distance from artificial sunlight being measured. If you increase the distance from the light fixture, PPFD will drop dramatically and exponentially. This is because an artificial light source produces a limited amount of photons as compared to the sun. PAR typically drops by 50% for every 12" of distance away from an artificial light source. And there is no standard that grow light makers use to measure PAR.
There is a standard used in lighting laboratory measurements called PPF (photosynthetic photon flux ) which is the total amount of photosynthetic energy a light outputs. But this measurement does not accurately predict PAR levels at the distance where your plants are from the grow light. This is because of the size of the light and how close all the diodes are to one-another. A 600 watt grow light that is 4’ x 4’ in size as compared to a 600 watt grow light that us 5’ long by 6” wide will have very different PAR measurements over different distances from the light, but could have the same rated PPF output.
Plants in the flowering stage of growth do best between 800-1000 PPFD, and can be succesful up to 1500 PPFD depending on the grow environment and available CO2 levels. If supplemental C02 is added to the grow area plants can tolerate higher levels of PAR. Veg plants on the other hand prefer light levels between 250-400 PPFD. And clones and baby seedlings can't tolerate much more than 100 PPFD.
An additional consideration in lighting layout for grow is what is called crossover. This is the area between two light sources where the light from each source crosses over. These are the photons that shoot at an angle and are still beneficial to the plants. Crossover is an important consideration and can significantly improve PPFD levels. Lighting layout that considers the distance between fixtures based on the amount of PPFD each light can produce allows light loss to be recaptured creating less wasted photons and more beneficial light for your plants.
There are many different types of grow lights in the market today. Some are better than others at distributing energy evenly.
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