Lighting which to use to dial in DLI
- Thread starter gqjeff
- Start date
My statement didn't say anything about DLI.
The issue is the light saturation point which is a PPFD of about 64µmols.
If you look at the intercept of the curves in the chart from the Chandra paper, you will see that they intersect the X axis at about that value.
My metric was "light compensation point" which is the metric that describes the level of light below which a cannabis leaf is no longer a net photosynthetic contributor.
You asked for a cite for "for saturation @64DLI?". That's a different metric than the light compensation point. The light saturation point is the light level above which a plant can no longer process the PAR photons that are impacting it. In a non-CO2-enahanced environment, that is considered to be a PPFD of 800-1000µmols.
The Chandra paper, like every other bit of cannabsi research that I've ever seen, deals with photoperiods so it's safe to assume 12/12 in flower but I don't know about the photoperiod in veg. It could well be 24/0 because, based on cannabis' biology, there's no particular reason to not veg cannabis with a 24 photoperiod.
Until auto flowers can be cloned rather than grown from seeds, the amount of research that can be done on autos is very limited because the variability of seeds.
Unless there's a need for a PAR meter, I'd go with a lux meter and use the conversion factor. The Uni-T is about $35 and the numbers you get from are, in practical terms, as accurate as numbers from an Apogee. I bought my Apogee about two years ago and got the Uni-T last year to test is against the Apogee.
My concern about that model of PAR meter its reliability and because the sensor, it it's the same gallium arsenide sensor as the PhotoBio, cannot read light > 660nm. That tidbit is from a paper on the Apogee site where they tested a handful of PAR meters. The inability to read the full PAR range may or may not be important for your situation but, for me, the bottom line is that the Uni-T will give you accurate readings and it's ¼ the price of the entry level meters on Amazon.
Below is lighting data that I took yesterday. The plant (I only grow one in my 2' x 4') is a Chemdog photoperiod, 38 days above ground. The lights are a Growcraft X2 veg light + a Vipar Spectra XS-1500 that is suspended between the two light bars of the veg light. I've added the XS-1500 to add red photons to the grow and it's toward one end of the plant so it acts as a "fill light".
The readings were taken with the tent flaps closed. The Apogee sensor is on the wand and I insert the wand between the flap doors. The Apogee wand allows me to get accurate readings and the readings are about 100µmols at the front of the plant when the flaps are closed.
The average PPFD is 896, the average DLI is 68. The standard deviation of the PPFD is 45, and the StDev of the DLI is 3. That's a lot of light and the StDev values indicate that it's very uniform light. After taking the readings with the Apogee, I took the readings with the Uni-T by just sticking my arm in between the tent flaps and I tried to get the sensor of the Uni-T as close as I could to where I took the reading using the Apogee. There were differences in sensor locations of at least a couple of inches, I'd guess, and that change is enough to throw off the readings by a couple of dozen µmols either way.
Looking at those five values, the 0.161 is an outlier. I'd toss that out and go with 0.15. The lower conversion factor will result in a higher PPFD value. And that's good because that builds in a little safety margin that can make a difference at higher PPFD levels.
The numbers that I took fell in line with the testing I've done previously — if you're using a light meter, convert the lux reading by 0.155 and you'll get a value that is very close to the reading from a PAR meter.
The readings were taken with the tent flaps closed. The Apogee sensor is on the wand and I insert the wand between the flap doors. The Apogee wand allows me to get accurate readings and the readings are about 100µmols at the front of the plant when the flaps are closed.
The average PPFD is 896, the average DLI is 68. The standard deviation of the PPFD is 45, and the StDev of the DLI is 3. That's a lot of light and the StDev values indicate that it's very uniform light. After taking the readings with the Apogee, I took the readings with the Uni-T by just sticking my arm in between the tent flaps and I tried to get the sensor of the Uni-T as close as I could to where I took the reading using the Apogee. There were differences in sensor locations of at least a couple of inches, I'd guess, and that change is enough to throw off the readings by a couple of dozen µmols either way.
Looking at those five values, the 0.161 is an outlier. I'd toss that out and go with 0.15. The lower conversion factor will result in a higher PPFD value. And that's good because that builds in a little safety margin that can make a difference at higher PPFD levels.
The numbers that I took fell in line with the testing I've done previously — if you're using a light meter, convert the lux reading by 0.155 and you'll get a value that is very close to the reading from a PAR meter.
Last edited:
Similar threads
- Random Tasked User
- New Growers Forum (Q&A)
