How much light do you really need?

[SlownLow86]

I've been doing a lot of research lately on lighting and came across a study that I would like to share and discuss with everyone. I am in no ways an expert in this stuff, but I am learning everyday. First, though, we need to define some terms so we are all speaking the same language. These have been defined in several other threads, but to make it easy I will add them here as well.

PAR- Photosynthetic Active Radiation. This is light in the 400-700nm ("nano meter") range, which basically means light available for photosynthesis
PPF- Photosynthetic Photon Flux. This is a measurement of how much PAR a light puts out. This is measured in umol/s ( "mico moles per second" )
PPFD- Photosynthetic Photon Flux Density. This is the measurement that we really need to be concerned with. What it tells us is how much USABLE LIGHT is hitting our canopy over a given area. This is measured in umol/M^2/s ("micro moles per meter squared per second)

The study was done by the University of Mississippi in 2009, so there may be more recent studies with different findings. In this study they measured the photosynthetic response of Cannabis to different light, temperature and CO2 levels. Here's a link to the study:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3550641/pdf/12298_2008_Article_27.pdf
Here's the nugget from the study: "At 25C, rate of photosynthesis increased with increasing PPFD, but this trend peaked with 1500 μmol m-2s-1 PPFD at 30 oC, and decreased at higher light intensities."

What have you guys been seeing? Is anyone running their setup at 1500 PPFD?
@Rahz @Moflow @CobKits @Chip Green @whytewidow I know you guys are light nerds, too. What do you think?

 

[whytewidow]

I think the minimum for flower is 30w/sqft. You can get by with less in veg.

 

[CobKits]

I think the minimum for flower is 30w/sqft. You can get by with less in veg.

thats more of a rule of thumb because 30W of a high efficiency light source like LEDs run soft can have literally twice the PPF of a fluorescent tube of the same wattage

 

[Big Perm]

I am in no ways an expert in this stuff

You are about to hear from the 'expert(s)'. Like moths to a flame.
Interesting read.

 

[whytewidow]

thats more of a rule of thumb because 30W of a high efficiency light source like LEDs run soft can have literally twice the PPF of a fluorescent tube of the same wattage

That's what we are talking about. Samsung strips. 30w/sqft of Samsung lm561c diodes will flower just fine.

 

[Rahz]

I like 1000 PPFD. Some people prefer to keep things around 700-750. If I wanted to push things I would aim for 1250 average PPFD.

A few years back I converted the data in that graph to a bar chart (discarding the +1500 data). The study was based on leaf material. Some data suggests the yield potential is more linear than the chart suggests, but I still don't see any reason to try and achieve 1500 PPFD. Hot spots will be +1500 and it's not the most efficient plan to begin with. Much better for instance to use 750 PPFD in twice the area if possible.

Easy to figure out the PPFD if the emitters/lamps come with a umol/j rating. Absent the spectral data, if output efficiency is available you can get a good estimate. Basically, 15 PAR watts per sq/ft = 750 PPFD, 20 PAR watts per sq/ft = 1000 PPFD and so on (based on 4.67 umols per radiant watt)