Joe, yes, red light is still most efficient in McCree curve (action spectrum of photosynthesis), but there isn't a lot of dip in between red and blue (green region). That was my main point. In addition to absorption by other pigments, the energy from the absorbed light has to reach to certain molecules in the chloroplast (called reaction centers) to initiate the subsequent reactions. So how energy get transmitted from molecules to molecules (in addition to absorption) influences the efficiency of photosynthesis. McCree curve tries to capture the entire deal.
Description below could be a bit too much, but you are correct that this McCree curve is not the end of the story; there are more complications. YPF uses the McCree curve to adjust the photosynthetic photon flux (PPF), a measurement of photosynthetically active radiation (PAR). PPF weighs each photon equally, but as McCree curve shows, red photons are about 30% more efficient than blue photons. So YPF try to weigh the effectiveness of each photon. So it seems to be a good idea, but as you mentioned, YPF is not always better than PPF.
You might want to take a look at this poster (
link), which briefly talk about this issue. One reason is that the plant can respond differently when multiple wavelengths are hitting the leaf. McCree curve is measured from monochromatic irradiation. One example is what Ray mentioned. It has been recently shown that when there is a lot of light, adding a bit of green light is better than adding additional red light. This is partly due to how light travels through the cells of the leaves.
The other issue is something called photomorphogenesis, different wavelengths influences different aspects of physiology (e.g. via molecules called cryptochromes and phytochromes). So photosynthesis efficiency is not the only thing relevant to plants.
Finally, nothing wrong with red/blue based grow light. They work ok; many people used them a couple years ago. But some people (including me) have to pay lots of electricity cost, so we pay attention to the efficiency and the total cost of ownership (initial fixture + electricity). Under this criteria, white LEDs are quite a bit ahead of mono's at this moment. This will eventually change once there are more demands for plant specific LED. But company is still pouring resources to white LEDs (larger demands at this moment). As you mentioned, white LEDs are much more common than monos.
White LEDs are basically blue emitter + phosphor (to get light of longer wavelengths). So there exist efficient blue (and red) monochromatic LEDs. But if you want to squeeze out the efficiency, you have to pay lots of money to make red/blue grow light. Because of the cheap packaging of white LEDs (driven by demands) like Cree or Citizen COBs or Samsung mid-power linear strips (an amazing model is coming out any time now), you can achieve the same level of efficiency at much lower initial cost.
I understand that efficiency isn't relevant for some people, and that there are market for the inexpensive, low-efficiency LED fixtures. For example, gdupont, who wants to raise the temp by light, would be ok with low efficiency light. In this sense, it is not wasting energy. Even with low efficiency light, if you give enough light, the plant will grow.
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