About a year ago, when I started reading on the matter, 3500K@80CRI was the "online consensus"; what ever that is. Recently it seems the tides have changed and its 3000K@90CRI. A commercial grower told me recently that a comparative test he did (with 3500/80) got effectively similar results with both, but with 3000/90 finishing to flower 8 days earlier.
The presumption is that while 80CRI is more efficient in terms of lumens per watt the 90CRI has an overall better efficacy in terms of grams per watt and flower time.
That said, it seems all experience -- including my own with 3000vs.4000@80 -- is little more than anecdotal, abiding to Kahneman and Tversky's (sarcastic) "law of small numbers." There are plenty of large scale growers out there, I wish somebody would pick up the glove and do a proper, large scale comparative test to put the question to rest.
Color temperature
Well put.
I've messed around a little myself but my experiments were not well-controlled and ridiculously small in sample size.
I'm positive that good data exists - I'm certain big manufacturers like Cree and Philips who have horticultural lighting departments would have done their homework, but getting this data into our grubby little hands is another story.
I've messed around a little myself but my experiments were not well-controlled and ridiculously small in sample size.
I'm positive that good data exists - I'm certain big manufacturers like Cree and Philips who have horticultural lighting departments would have done their homework, but getting this data into our grubby little hands is another story.
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Not sure if anyone has seen the following Nasa LED.
I think the problem is "There is no single red:blue ratio of light ideal for all species and for every stage of plant growth." So 3000k might be best for one plant/strain and a 5000k better for another...
Some other interesting points from the article however:
Red light. Broad-band red (600-700 nm) light has, by far, the highest quantum efficiency for driving photosynthesis, with a broad peak from about 620 to 660 nm (11). In general, red light promotes stem elongation, leaf expansion, biomass accumulation, and can determine flowering, dormancy, and other important responses of plants, including seed germination.
Blue light. There do not seem to be any simple answers regarding how little or how much blue light is required or any given plant species, or even when to apply it during a given plant life cycle. Even though approximately one-third of sunlight PAR emissions consist of broad-band blue (400-500 nm), plants grown outdoors seem to be not particularly sensitive to blue light, at least at outdoor light intensities (12). Under conditions which tend to involve much lower PPFs than outdoors, the intensity of blue light seems to be a critical factor. Sometimes only a few percent of blue are needed for a particular plant response, above which blue is inhibitory, but that may change during the course of a plant’s life cycle. Plant-growth functions that seem to be particularly sensitive to blue light include stem elongation and leaf expansion, with “too much” blue inhibiting growth in both cases (13). Other plant responses having an absolute requirement for blue light include phototropism, stomatal aperture, leaf thickness, and chlorophyll content.
Green light. Green light (500-600 nm). Green often is disregarded as an unimportant waveband in photosynthesis because absorption spectra of extracted leaf chlorophyll pigments indicate very weak absorption in the green region of the PAR. Because chlorophyll has major absorption peaks only in the red and blue regions, researchers initially selected first red, later blue, LEDs for first-generation LED arrays to support plant growth. However, intact leaves do absorb considerable green light, and in a relative quantum-efficiency curve for photosynthesis vs. PAR wavelengths, some wavelengths of broad-band green actually are more efficient than certain wavelengths of the blue band. Overall, however, broadband green is slightly less efficient than broadband blue. However, when leaf canopies close, red and blue light are absorbed strongly by upper or outer leaf layers, whereas green light penetrates to interior leaf layers, where it subsequently is absorbed and drives photosynthesis of the inner canopy (14). Thus, light sources containing some green can be more effective in stimulating crop growth than are red + blue sources alone, such as when foliar canopies are closed. When applied together with blue light, green has effects opposite to blue on stomatal aperture (15).
Far-red light. The recent availability of far-red (FR, 700-800 nm) LEDs presents opportunities to control plant functions related to photoperiodism and photomorphogenesis. Plant species with a long-day requirement for flowering are hastened to flower when FR is present simultaneously with R light (17).
The 90 CRI should have more far red than a 80 CRI which could be why they perform better. 93+ CRI are also available at least in the vero 18 and might be even better.
I think the problem is "There is no single red:blue ratio of light ideal for all species and for every stage of plant growth." So 3000k might be best for one plant/strain and a 5000k better for another...
Some other interesting points from the article however:
Red light. Broad-band red (600-700 nm) light has, by far, the highest quantum efficiency for driving photosynthesis, with a broad peak from about 620 to 660 nm (11). In general, red light promotes stem elongation, leaf expansion, biomass accumulation, and can determine flowering, dormancy, and other important responses of plants, including seed germination.
Blue light. There do not seem to be any simple answers regarding how little or how much blue light is required or any given plant species, or even when to apply it during a given plant life cycle. Even though approximately one-third of sunlight PAR emissions consist of broad-band blue (400-500 nm), plants grown outdoors seem to be not particularly sensitive to blue light, at least at outdoor light intensities (12). Under conditions which tend to involve much lower PPFs than outdoors, the intensity of blue light seems to be a critical factor. Sometimes only a few percent of blue are needed for a particular plant response, above which blue is inhibitory, but that may change during the course of a plant’s life cycle. Plant-growth functions that seem to be particularly sensitive to blue light include stem elongation and leaf expansion, with “too much” blue inhibiting growth in both cases (13). Other plant responses having an absolute requirement for blue light include phototropism, stomatal aperture, leaf thickness, and chlorophyll content.
Green light. Green light (500-600 nm). Green often is disregarded as an unimportant waveband in photosynthesis because absorption spectra of extracted leaf chlorophyll pigments indicate very weak absorption in the green region of the PAR. Because chlorophyll has major absorption peaks only in the red and blue regions, researchers initially selected first red, later blue, LEDs for first-generation LED arrays to support plant growth. However, intact leaves do absorb considerable green light, and in a relative quantum-efficiency curve for photosynthesis vs. PAR wavelengths, some wavelengths of broad-band green actually are more efficient than certain wavelengths of the blue band. Overall, however, broadband green is slightly less efficient than broadband blue. However, when leaf canopies close, red and blue light are absorbed strongly by upper or outer leaf layers, whereas green light penetrates to interior leaf layers, where it subsequently is absorbed and drives photosynthesis of the inner canopy (14). Thus, light sources containing some green can be more effective in stimulating crop growth than are red + blue sources alone, such as when foliar canopies are closed. When applied together with blue light, green has effects opposite to blue on stomatal aperture (15).
Far-red light. The recent availability of far-red (FR, 700-800 nm) LEDs presents opportunities to control plant functions related to photoperiodism and photomorphogenesis. Plant species with a long-day requirement for flowering are hastened to flower when FR is present simultaneously with R light (17).
The 90 CRI should have more far red than a 80 CRI which could be why they perform better. 93+ CRI are also available at least in the vero 18 and might be even better.
wow thank you for the reading material.. going to have a look at that article tonight.
Excellent little write up, kmb; thanks for sharing. I didn't know that green light penetrated to inner canopy like that.
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They’ll work great. You’ll end up with a color temp somewhere between the 2 when you combine them.
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