A high light plant such as Cannabis needs a minimum of 22 mols/m2/day of photons to as high as 65 mols/m2/day which is the range of optimal growth. Yes if you don't have that much light, cannabis will still grow, but you will typically see lower yields, less dense buds, lower THC percentages because the plant isn't operating at its best.
Converting DLI to PPFD
Now how do we get from DLI Mol/m2/day to PPFD or micromoles/m2/second.
The formula to convert this would be:
You would use this formula...
PPFD x 0.0864 = DLI
OR
DLI / 0.0864 = PPFD
So lets look at cannabis, knowing that cannabis needs at least (Minimum for optimal growth) 22mol/m2/day DLI at minimum for optimal photosynthesis lets break it down.
22 / 0.0864 = 254.6 micromoles/m2/s-1 (PPFD)
This is the amount of photons you need every second, averaged over 1 square meter, for the whole 24 hours if usuing a 24/0 daylight schedule (no dark period).
So what happens if you are using a 18/6 veg period or a 12/12 flowering light schedule... simple...
If using a 12/12 flowering schedule, you are only using 12 hours of the 24 hours of the day, which means you would take the 254.6 Micromoles/m2/s-1 and multiply it by 2, since you are reducing the amount of daylight by 1/2 (12 hours), then each hour of the daylight period you need double the amount of photons... so in this example you would need 509.25 micromoles/m2/second.
If using a 18/6 schedule, you are using 3/4 of the amount of daylight with 18 hours on, 6 hours off, therefore you would multiply the 24hr PPFD by 1.5 because you are using only 3/4 of the day, or 1 1/2 times the 12 hour requirement. In this example a DLI of 22, under a 18/6 schedule would amount to 381.5 micromoles/m2/second.
So for cannabis, bottom threshold for optimal growth and photosynthesis is a DLI of DLI of 22 would be:
****Going lower than these numbers will still grow cannabis, however yields, bud density, potency and THC percentage may suffer.
24/0 schedule: 254.6 micromoles/m2/s-1
18/6 schedule: 381.5 micromoles/m2/s-1
12/12 schedule: 509.25 micromoles/m2/s-1
For Cannabis, the Top threshold for optimal growth and photosynthesis is a DLI of 65 moles per day.
***extremely important notice, only go up to these amounts if you are using supplemental CO2, do not go this high if you are not using supplemental CO2 as you will actually slow down photosynthesis and waste energy.
24/0 schedule: 752.31 micromoles/m2/s-1
18/6 schedule: 1128.465 micromoles/m2/s-1
12/12 schedule: 1504.6 micromoles/m2/s-1
The generally accepted guidelines for artificial light PPFD in flowering are this:
in a 12/12
PPFD of at least 510 micromoles/m2/s-1 for the low end of optimal intensity
PPFD of at least 800-1100 micromoles/m2/s-1 for perfect optimal lighting without additional CO2.
PPFD of at least 800-1500 micromoles/m2/s-1 for perfect optimal lighting WITH additional CO2.
So now that we know what all these numbers mean, how to we interpret these PAR charts or 4'x4' grids with PAR readings?
Typically if a Lighting company presents a PAR chart "4'x4' grid" with measurements, you will see a bunch of readings all over the place.
Even though PAR measurements are in micromoles/METER/second this is with a constant and even light source what the average would be... grow lights, especially LED's do not have an even footprint and intensity can change rapidly over just 6" or 1" away from the center of the light. This is why these 4'x4' PAR grids are important because the light source is not even, it shows us the Actual area that will produce the OPTIMAL amount of PHOTONS for driving photosynthesis at each point of the grid.
So now knowing for flowering you need to see a grid with a bunch of 510's all over it or higher for flowering, you can see exactly how far a light will spread to OPTIMALLY drive your plant. Also, usually when a company makes these grids, they do it at heights of 12", 18", 24" and 36". By comparing the charts, you can tell at what height a certain light will spread providing 510's or higher and at what height would be optimal to hang the light. The closer the light is to the canopy/or meter, the higher the amount of photons that will be hitting it, but the narrower the coverage area will be (less light is scattering). The further away you move the light, the more photons that scatter leaving less to hit the canopy resulting in a larger coverage area but less Photon Density (PPFD).
Now if you don't see 510's or higher across your chart for your light, does that mean it wont work...NO.. it will just produce less photosynthesis which typically results in more airy buds, less potency and THC, and lower yields.
You can also look at these charts to see how you would hang multiple lights, by combining 2 charts together... lets say 2' out from the center of your light, the chart is only reading 250 micromoles/m2/s-1, well you know if you added a 2nd light and combined their outputs, with them spaced apart 2 feet from each other, then that center point between them (the 250 micromole/m2/s-1 mark) would double, now making that area optimal for flowering in a 12/12 schedule.
This is why it is important that companies provide this information, so we can all make accurate decisions on plant lighting for our plant species, photoperiod usage, and making sure to have a even light footprint over our canopies.
So all and all the things you will want to remember from this are:
*Light plants use is in tiny packets called photons.
*Plants count photons, this count directly drives photosynthesis.
*Only photons with a wavelength of 400nm-700nm matter for photosynthesis (some say 380nm-780nm)
*All colors of Light/wavelengths in the PAR range will absorb, at 80% or higher rates
*PAR measurements are taken with a quantum meter or spectroradiometer and are measured in micromoles per meter squared per second (umol/m2/s-1)
*When choosing a lights by PAR measurements we are looking for a PPFD of 510 umol/m2/s-1 or more across the entire canopy for optimal flowering results, less will still work but not at optimal levels for best results
*Having a 4'x4' grid with PAR measurements at different heights is the best way to judge a plants actual optimal footprint/coverage area based on plant type and photoperiod. Also aids with hanging height and blending of multiple panels/lights.
Guess I will give it a shot and post in the infirmary section. blech.
