Good points. Perhaps in the future we'll be able to tune the spectrum without associated efficiency concerns. It's currently expensive to do testing on the specific things you mention. It would be possible to use the data I've charted to adjust the wattage of each lamp so the PAR values, or the adjusted PAR values were the same. It's not really what I'm after because I do have overall efficacy in mind using the products that are available, and because within the limitations of these cobs the results wouldn't be definitive anyway. At least with the test setup I have in mind we should have a pretty good idea of which sample produces the best yield for the energy despite the fact that we won't know exactly why one is better... unless the yield results closely mimic the Mcree corrected PAR values at which point a correlation can be drawn. I would like for Mcree corrected par values to end up meaning something useful (the point of the test), but I'll be good with whatever takes place.
One thing I want to mention since you keep bringing 700nm up, and I'm sure you're aware, is that with the poor photosynthetic response to raw 700nm light, it probably wouldn't be a good idea for the majority of emission to be 700nm. 700 exhibits high absorption, but since the photosynthetic response is low I suspect the response peak at 665-670 would be the place to invest the majority of photons. Thoughts?
As far as I understand what the current research has found it is that the actual photosynthetic process requires that the photon be 700nm. The further away from 700nm the photon is, the more energy required to condition it. The reference I am using is from 'Photosynthesis' by Hal, Rao and published by Cambridge University Press. You can preview the relevant parts using google preview. Chapter 1-3, figure 1.2 summarizes the energy losses in photosynthesis from sunlight. 24% of energy is lost 'due to degradation of absorbed photons to excitation energy at 700nm photon level' which is conditioning the photon for use in photosynthesis.
That is a pretty big number. The chart also shows that only 5% of the PAR spectrum in sunlight is eventually used for photosynthesis. Reducing that 24% energy loss appears to be the most direct way of being able to significantly influence photosynthesis. These could lead to two possible beneficial outcomes:
1. The increased availability of usable light will result in a corresponding increase in photosynthetic output.
2. Less radiant power is required for the same photosynthetic output since less energy is required for photon conditioning.
Both of those would improve overall system efficiency. How much, who knows, needs to be studied.
As for the other wavelengths, I agree that they have to be present but in what quantities or ratios I do not know. My suggestion of 10% white/90% red as a baseline should provide an acceptable starting point. A good study might be 5/95, 10/90, 15/85, 20/80. 25/75 and from the other way 95/5 (aka Spectrum King) , 90/10, 85/15.
McCree and a lot of the research information available provide good guidance and reference points that often lead to more questions than answers. There is far more study to be done and research has vastly improved since many of the earlier findings. Light is used by the photosynthesis process so it makes sense to provide the most optimal light that produces the best results.
We are all just racing to get there.