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Thread: light color temp?

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    Plant Heathen's Avatar
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    light color temp?

    When I learned about lighting for planted tanks everyone said 65k Is the best. Now on this forum I hear otherwise. So now I'm on the hunt for a light and found two LED lights. One is 4000k the other is 5000K. Are either of these any good?
    Earth laughs in flowers. ~Ralph Waldo Emerson,

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    nimbulan's Avatar
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    I don't think there's really any merit to the "6500k is best" argument that many people throw around - it's simply the color temperature of most bulbs that come with T5HO fixtures so people stick with it because it works. I've heard anecdotal evidence from a number of people that they get better results by mixing 3000k and 6500k bulbs (an average of 4750k, though I'm not sure if the scale quite works that way.)

    In any case, color temperature is not nearly as important as light intensity as long as you stay in that range. I've used 6500k, 5400k, 4000k, and 3500k but the brightest light always wins.

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    Hi
    the effect of light spectra on plant growth is a pretty complex issue. I can't answer the question fully but I can help point out some of the important factors. To make things a bit easier lets split the problem into
    smaller, more manageable parts.

    1. How does the spectrum of light (colour) affect the rate of photosynthesis?
    2. How is the colour of light affected by the "emision temperature"?
    3. How does the rate of photosynthesis affect plant growth?

    1.To start with let's trace the light as it leaves our light source. Light is emitted in discrete packets called photons, you can imagine those photons as tiny little spheres filled with energy if you'd like. This energy is carried from place to place by the photon. The amount of energy the photon carries can be different, these different energies are what we perceive as colour. Blue colored photons carry more energy than red coloured photons.

    light color temp?-visiblespectrum-gif
    fig.1 Colour and energy of photons.

    You may be tempted to think that therefore it would be best to use just blue light as it carries the most energy, it's not quite as simple as that. Our ultimate goal here is to make sure that as many of the photons emitted by the lamp as possible trigger photosynthesis. This means that we want as many photons as possible to be absorbed by the plant and then to contribute to photosynthesis. This is quantified by the "absorption spectrum" and an example can be seen below.
    light color temp?-chlorophyll_a_b_spectrum_3-jpg
    fig.2. Chlorophyl absorption spectra.

    Finally, the photosynthetic system is not a simple one. Absorption of photons can be done by different molecules within the plant and the chance of this leading to photosynthesis is not the same for every case. The chance of photosynthesis taking place after absorption is also related to the colour of the photon. This is quantified by something called the "action spectrum" for photosynthesis(1).
    light color temp?-fig9-png
    fig.3. Photosynthetic action spectra.

    So there you have it. To use your light efficiently it must be of appropriate energy to be absorbed AND of appropriate energy to trigger photosynthesis. From the above spectra it seems that blue and red light are pretty good ad doing both. There are of course other issues to consider, different colours of light may be responsible for different triggers in a plant, e.g. flowering.

    2. Colour temperature can be a little bit of a vague characterisation of a light source as it only tells you about the "effective" colour of light that comes out of your lamps. Sure you know that "cool white" will probably have more blue in it than "warm white" but it is not the case that the consituent colours of this white light will be the same for an LED as they are for a fluorescent lamp. To answer your question it is more informative to look at the actual emission spectra of the lamps you are considering. Since most LED's on the market are made in the same way (blue LED with phosphorescent material on top) you can safely assume that what you find online will match up with the emission of your lamp.
    light color temp?-f4pb8fmhs0czzoy-medium-jpg
    fig.4. LED emission spectra for different colour temperatures.

    You can see that the "colder" you get the more the red emission is favoured and the less blue light there is.

    3. This is the big question. How does all of this add up to your plant growing and being happy? I'm not a biologist and my knowledge in this field is pretty limited but I do know that links have been made between the rate of photosynthesis and rate of plant growth(2). I also know that it is not the whole story. Some wavelengths are needed for triggering different events in a plant's life e.g. flowering.
    From my experience I can tell you that CCT of 6500K is good for growing N. ventricosa x ampullaria, some species of sphagnum moss, cephalotus follicularis and heliamphora minor. These can be treated as case studies as I have not done any real experiments on those plants. I'd love to see a discussion here on the forum relating to this question. For those interested in a little bit of reading those are some of the articles I've found on the matter(3)(4)(5)


    (1) "BIOLOGICAL ACTION SPECTRA" Holly L. Gorton.Department of Biology St. Mary's College of Maryland. http://photobiology.info/Gorton.html
    (2) Does Enhanced Photosynthesis Enhance Growth? Lessons Learned from CO2 Enrichment Studies. Plant Physiology Jan 2011, 155 (1) 117-124; DOI: 10.1104/pp.110.166819
    (3) Journal of Experimental Botany, Volume 58, Issue 12, 1 September 2007, Pages 3099–3111, https://doi.org/10.1093/jxb/erm130
    (4) Journal of Experimental Botany, Volume 48, Issue 7, 1 July 1997, Pages 1407–1413, https://doi.org/10.1093/jxb/48.7.1407
    (5) Journal of Experimental Botany, Volume 58, Issue 12, 1 September 2007, Pages 3071–3077, https://doi.org/10.1093/jxb/erm251
    Last edited by loki89; 10-06-2017 at 06:45 AM.

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    Plant Heathen's Avatar
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    Thank you for the extensive reply. So what I have gathered is a light the is between 5-and 65k would be the best choirs as it is more well rounded in the blue and red spectrum.
    Earth laughs in flowers. ~Ralph Waldo Emerson,

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    nimbulan's Avatar
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    That is indeed quite a lengthy reply. I'm sure we could fill up a whole page in this thread easily with discussion about different aspects of photosynthesis. We could talk about how blue light acts largely as a growth regulator and too much or too little can cause serious problems with certain plant species (not a problem with CPs that I'm aware of.) Lack of blue light is also the primary driver behind etiolation (plants stretching for more light.) We could also talk about how the photosynthetic efficiency of each wavelength changes depending on light intensity - green becomes more efficient the brighter the light is and can actually surpass red.

    I'm sticking with my previous statement though - light intensity is far more important than color temperature.

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    Quote Originally Posted by loki89 View Post
    Sure you know that "cool white" will probably have more blue in it than "warm white" but it is not the case that the consituent colours of this white light will be the same for an LED as they are for a fluorescent lamp. To answer your question it is more informative to look at the actual emission spectra of the lamps you are considering. Since most LED's on the market are made in the same way (blue LED with phosphorescent material on top) you can safely assume that what you find online will match up with the emission of your lamp.
    light color temp?-f4pb8fmhs0czzoy-medium-jpg
    fig.4. LED emission spectra for different colour temperatures.
    Good info, Loki. You switched red vs blue, I think. In the relative spectral power distribution (SPD), shorter wavelength (left) is blue. So cooler color (high K) has more blue and less red. Also, people should note that Y-axis in this graph is relative. For each corrected color temperature (CCT), the peak is set to 100%. So you can't directly compare the amount of red light flux between 3000K vs 5000K. The other caveat is that Y-axis is power, not the number of photons. As you mentioned, photosynthesis is a quantum process (i.e. the number of photons is more important than the energy level of each photon). When you convert the SPD to number of photons, then the blue peaks become quite a bit lower.

    It is good to point out that the absorption spectra by cholophylls aren't so important. A lot of people are confused about it. The action spectrum of photosynthesis (=McCree curve=quantum yield of photosynthesis) is probably more relevant (although there are quite a few caveats about it). From this curve, you can easily see that green light isn't useless, and the efficiency is similar to blue light. One photon of red light is about 30% more efficient than one photon of blue light.

    The other misuse of this McCree curve is that some people think that if the emission spectrum is close to the curve, it will results in the best photosynthesis. This is nonsense. The curve is not showing the shape of optimum emission spectrum, but the curve is telling you that red light is most efficient, so 100% red light should be most efficient. This is too simplistic because of the reason nimbulan mentioned. In addition to photosynthesis, plants change their behavior based on the color spectrum (photomorphogenesis). So the ratio of blue, green, red, far-red etc can influence the shape, color, chemical expression, size of plants.

    I think the reason people believed that 6500K is better is that is marketed as "daylight" white. I measured PAR PPFD from different color fluorescent light (and LEDs). There aren't so much difference between 6500K and 3000K. Note that PPFD is measuring the total number of photon flux between 400-700nm. So in terms of photosynthesis, lower K should be more efficient if you understand McCree curve.

    But in practice, I would agree with nimbulan, that plants seem to grow well in 5000K or 3000K, and total amount (and efficiency if you care about the cost) is probably more interesting. In some Drosera, the color seems to be influenced by CCT, but it might be from a placebo effect or some other factor like total intensity (I haven't matched PPFD).
    Last edited by naoki; 10-06-2017 at 01:21 PM.
    My photos of non-human subjects are licensed under a Creative Commons Attribution-ShareAlike 2.0 Generic. Please use freely with appropriate credit (see here).

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    Plant Heathen's Avatar
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    Ok so color temp plays little in the aspect of growth. Looms are more important. So if you had a strip light with two 3000 loom led bulbs would that = 6000 looms or are we still at 3000?
    Earth laughs in flowers. ~Ralph Waldo Emerson,

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    nimbulan's Avatar
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    Lumens is simply a measurement of the brightness (total light output) of the light, so yes if you have two 3000 lumen lights you would have 6000 lumens total. I like to aim for about 2000 lumens per square foot of grow space and it keeps my sundews nice and healthy.

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