Previous thread on it:
http://www.candlepowerforums.com/vb/showthread.php?t=204092
IF you hit the peak photosynthetic responses in the blue and red region, you could in theory be much more efficient. LEDs are not that far behind HID in terms of lumens/W, but the theory is that if you create specifically the wavelengths the plants NEED (the whole green region is reflected, the energy's of no use to the plant), you could get the same photosynthesis rate with far less total lumens on it.
Meeting the blue peak in no problem. The red is a MAJOR problem, it's at 680nm and almost all red LEDs are like 630-635nm or red-orange 615-620nm, which has little response.
Very few makers of "deep red" 680nm LEDs. LEDEngin (Mouser) makes a big one at a sort of reasonable price. The lumens/W is deceptive. The lumen scale is compensated for human eye response, which is poor in the deep red 680nm range and highest in the red-orange range. So it scores low on the VISIBLE lumen scale, but scores much higher in terms of
Photosynthetically Active Radiation, PAR, which is all you care about. In fact, despite widely varying visible lumen figures, I saw deep red, red, and red-orange have very similar" mW light out"/"mW power in" efficiency ratios once the human eye response of the lumen scale was factored out.
The makers of LED "grow lights" frequently sell useless 630-635nm red. Even if you find a supplier claiming "deep red 680nm", there's no telling if they'll actually provide you the wavelength claimed. How would you even tell, without something to measure wavelengths?
As best I can tell, photosynthesis requires BOTH the red and blue wavelengths. However, I'm not certain that this meets the needs of all plants. There are a number of possible channels for photosynthesis, and different plant may not grow the same with the same nutritional content when odd proportions of specific wavelengths are used. For example, that Wikipedia chart suggests the carotenoids may barely be fed at all by a blue/deep red source. I suspect what grows in reef aquariums could be different then Venus flytraps, strawberries, or corn. After all, the leaves are different colors in part BECAUSE the photosynthetic chemistry is different. In fact we can be sure of that with a reef tank because the plants evolved to grow underwater which filters out a lot of the red wavelengths, so they'll probably be evolved to not use red, at least not in the same way. The subject of what each type of plant needs does not seem to be well-researched. In fact the only halfway serious publicly available research seems to be done by pot growers.
They do seem to use many more reds than blues in grow lights. I don't know if they're doing that because they're using 635nm reds with very poor response, or if the plants actually need that much more more red than blue.
Your plants will look pretty sickly with red/blue on it. The photos I saw of plants growing that way showed black leaves with only a weak purple reflection, with the room lit in that reddish-purple hue. It wasn't pretty. The leaves absorbed all the incoming light and the usually reflected "green" wavelengths were absent.
