Dixana
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Posts: 727
Joined: Thu Apr 01, 2010 3:58 am
Location: zone 4

Gosh you guys are beyond awesome :D
So since your so good at explaining everything ;) can someone give a run through of the differences of the light output blue, red ,etc and which is best for which plants? I hope everyone gets what I'm asking....

TZ -OH6
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Posts: 2097
Joined: Fri Jul 25, 2008 11:27 pm
Location: Mid Ohio

The chlorophyll molecules of all plants absorb both red and blue light, but reflect green and yellow. Blue light has more energy so is more important for growth. Red light is needed to induce flowering, and plants grown under red shifted light such as high pressure sodium plant lights, tend to be undesirably lanky. Plants grown under blue shifted Metal Halide plant lights grow well, but don't like to flower. Seedlings don't need to flower so more blue light is better for their growth.

The human eye has three color receptors, red, green and blue (look closely at the pixels on your TV set, every color we see is a result of mixing red green and blue). Human ancestors evolved in a plant-filled green world so our eyes are sensitive to green light. Fluorescent lights put out white light as a function of three main phosphors making up the white powder coating the glass. The phosphors fluoresce red, green or blue light, but are heaviest on green to look brighter to people.

Cheap cool white tubes have alot of green and look white, or "cooler" than warm whites, cheap "warm white" tubes have alot of red and look "warmer". Color temperature is different, it represents the spectrum put out by an incandescent filament at a given temperature in degrees Kelvin. Hotter temperatures produce more energetic colors, red light is low temp low energy, blue is high temp high energy (red hot vs white hot). This output is a continuous spectrum like a rainbow (ROYGBIV). Fluorescent lights cannot put out a continuous spectrum because the light comes from the inidvidual colors that each phosphor produces, so imagine a rainbow that is black except for the very middle color of red green and blue (no orange, yellow, green-blue etc).

There is alot of word play when fluorescents advertize "full spectrum", and "Daylight". "Full spectrum" means that they have some additional phosphors besides red green and blue, such as yellow and orange (but the spectrum still looks like a black rainbow with some color stripes).

"Daylight" means that they fool the eye into seeing colors similar to daylight hitting the object instead of the icky greenish light of industrial cool white tubes.

Back to color temperatures... 4000K "daylight" tubes look like "warm white" tubes. The low temperature indicates a lot of red light compared to blue light. 6,500K "daylight" have alot of blue, and look whiter (I once bought one that said "arctic" daylight --not good for a homey feeling kitchen).


The new ultra energy efficient LED plant lights only have red and blue lights (and make plants look purple). Fluorescent plant lights have reduced green so they look pinkish, some used to have much less green but people didn't like them.



This site has some images of spectra for sun, plant absorption and fluourescent lights.

https://generalhorticulture.tamu.edu/lectsupl/Physiol/physiol.html

Dixana
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Posts: 727
Joined: Thu Apr 01, 2010 3:58 am
Location: zone 4

:shock: Wow thats a lot to take in! But I finally got what I needed tio understand! 6500k is what I need for seedlings cuz they have more blue light!! Sorry I have to be so pesty and understand the "why" of everything. Thanks again guys!

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