That "Chandra" chart has always been a bit suspect. All in all it's pretty much like any light response curve, but it seems a bit too steep. People who tried different light intensities (including myself) did not see the exponential gains in yield/Watt which you would expect from that chart when lowering the light levels. In reality the relation between light intensity seemed much more linear between 400umol/s/m2 and 1000umol/s/m2.
A couple of years ago on the YOR (Yield-O-Rama) site they were looking into which factors attributed most to increased yields. For this they collected a lot of yield information from many growers. In total something like 175 grow reports were submitted.
I plotted those results in a chart showing the yield in g/m2 on the vertical axis and estimated light intensity in umol/s/m2 on the horizontal axis. The red line is a smoothed average. Blue points are without CO2 and green with CO2.
View attachment 3788419
This is based on grow reports with different genetics, different lights, different temperatures, different medium (hydro/soil), different stock (cuttings vs seeds) etc etc etc. Yet on average it looks pretty much like the chart we have all been using and it shows that adding more light well over 1000umol/s/m2 still increases yield. Only above 1500 umol/s/m2 does it really seem to taper off, but there is not enough data there to say anything definitive.
Still, the biggest difference compared to the "Chandra" chart, is that it indeed shows a much more linear slope between 400 and 1000 umol/s/m2 range.Based on the average line you could expect an average yield of 0.56g/umol/s/m2 @ 400umol/s/m2 and 0.48g/g/umol/s/m2 @ 800umol/s/m2. That would make it around 15% less efficient to run with 800PPFD instead of 400PPFD, but you would also get 70% more yield from the same space. Most likely more compact and harder buds too under the higher light intensity. So less grows needed for a certain amount, more yield from the same area and better (higher priced) product overall. That's why people tend to average around the 600 to 800umol range instead of 400.
It also demonstrates another major mistake in FUD's reasoning that the "optimal" value would be the saturation point. This chart indicates the average saturation point for cannabis is around 1500umol/s/m2. Yet virtually no one is growing at this "optimal" value. Or the other way around, even if the saturation point was around 400umol, it would be much more efficient to grow at 200umol/s/m2.
:edit: Added distinction between CO2 enrichment and no CO2
