Lumens are for humans and PAR is for plants. And the two don't have much to do with one another.
Lumens is a measurement of the total amount of photons being emitted by a light source. Lux is the amount of photons being measured on a surface or at a specific point away from the light. This is what LUX meters are for.
By contrast, PAR (photosynthetic active radiation) are the photon light radiation which plants "see" to photosynthesize. PAR is a quantum measurement and it cannot be easily converted to LUX. They are each their own interpretation of the photons being measured.
So Lumens are the spectrum that our eyes perceive. The higher the Lumen 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.
PAR is measured with a specialized quantum sensor which accurately measures PAR. Companies like Everfine, Licor, Apogee Instruments and Innoquest all produce quantum PAR meters and sensors.
These sensors display PAR in PPFD or photosynthetic photon flux density. When you hear that someone grows 1000 umols/ms this means their PAR level is 1000 PPFD.
PAR levels change over distance from the light being measured. If you increase the distance from the light PPFD will drop dramatically. This is because an artificial light source produce a limited amount of photons as compared to the sun. These photons don't always shoot straight out of a light. They shoot in many directions. PAR typically drops by 50% for every 12" of distance away from an artificial light source. These are wasted or lost photons that the plant will benefit far less from.
Cannabis in the flowering stage of growth does best between 800-1000 PPFD. If supplemental C02 is added to the grow area plants can tolerate up to 1500 PPFD. 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 consider 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 lamps in the market today. Some are better than others at distributing energy evenly.
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