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the color of light

Posted by sproutly 10 (My Page) on
Fri, May 18, 07 at 10:32

I read with great interest the discussion on red and blue LED lights in another thread. Much of it was way over my head. So I wanted to simplify the answers for all of the unscientific folks like me.

There was a lot of discussion on red being the better spectrum with blue mixed in, but I thought blue is better for growing. Don't you always read that you should use cool fluorescents? Those are in the blue spectrum aren't they?

I've been growing peppers and herbs, now lettuce, under fluoreescents for a couple of years. Everyone over in the pepper forum says you can't get peppers to flower and fruit under lights, but I never had a problem. But I would like to always do it better and cheaper.

So should I keep using cool fluors, or should I look into LED's? And what color?

Follow-Up Postings:

RE: the color of light

Fluorescents are white ;)

"Blue for growth" is a myth. It has arisen mainly because of the responses of plants to two very specific light spectrums, metal halide and HPS. HPS has a distinctly orange spectrum and happens to produce rather etiolated growth in many plants, but does promote strong flowering and fruiting. Metal halide lamps produce more compact growth and so are often preferred for the pure growth stages. These effects are produced as much by the relative strength of red and far red light as by the amount of blue wavelengths. Most plants grow just fine with 10%-25% blue light and the rest red. Blue wavelengths do influence the shape and orientation of plant stems and leaves, and extra blue light can prevent etiolation that would be caused by an excess of far red (and near infra-red) light.

The variations between fluorescents are very slight, you would hardly notice a difference except side by side. Warm white (soft white) fluorescents intended for domestic use just seem to have too little blue (almost none) to grow many plants well, cool white and daylight colours have slightly more and it is enough for virtually any plant. One reason that incandescents are poor plant lights is that they have a huge amount of far red and near infra-red and essentially no blue, plants stretch even if you can provide sufficient intensity and avoid overheating problems.

LED lights, of appropriate power and efficiency, are likely to be cheaper to run than other light sources, but they are likely to be more expensive to buy. You will have to do your own assessment of how many LEDs you need to replace your existing lights, how much you can save on your utility bill, and whether that will offset the upfront cost. Experimental LED systems have concentrated on using light at around 670nm and 420nm, the two wavelengths used most efficiently by plants, and mostly at 670nm since that's where you can get the most photosynthesis for the least power input. Strangely enough, when this approach was tried with fluorescent lighting (ie. first generation GroLux), many growers were unhappy with the results. 670nm LEDs are not always easy to source since they appear very dim to our eyes, but the more common 630nm LEDs miss the photosynthetic peak and essentially just mimic the output of a fluorescent tube at lower efficiency.

RE: the color of light

LEDs' efficiency can be significant from being able to be precisely focused because it is more or less a point source. There will be very little wasted spillage.


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