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Kelvin and Watts

  1. Jul 18, 2008 #1

    I have a question regarding light, specifically artificial light. Please forgive me I am not a physics guru by any means and my terminology may be completely wrong, but hopefully someone will be able understand what I am trying to ask.

    First, the only way I know how to ask is in basic terms, I know that there is much more to it than that. So, I am asking in terms that I feel that I understand.

    Ok here it goes.

    Lets assume you have two lights, one light is 5000k and the other is 2700k. Considering that both lights are 23 watts each. Which light has the energy to penetrate...lets say a plant canopy better? The blue or red?

    Or another another way to ask could be..
    A 23watt 5000k bulb could produce light that would travel 2 miles. A 23watt 2700k bulb produces a light that would travel 1 mile. In order to to get the red light to reach 2 miles I would have to increase the 2700k bulb to 40 watts????

    Here is what I think. I feel that the blue light will penetrate deeper than the red given that they are the same wattage. Now, if I am correct in my thinking would it be fair to say that if I should want the red light to penetrate to the level of the blue light at 23watts I would need to increase the wattage of the red bulb to lets say 40 watts? I know the numbers may not be correct, this is all generally speaking of course. I am just trying to keep it on a basic understanding level at this moment. TIA.
    Last edited: Jul 18, 2008
  2. jcsd
  3. Jul 18, 2008 #2
    First we should make a distinction between the radiant flux and the luminous flux of a light source.

    The radiant flux is a measure of the total power emitted, and so a 23W red bulb and a 23W blue bulb have the same radiant flux, because they are emitting the same power.

    The luminous flux is a measure of the total brightness perceived by a human eye. This depends on the radiant flux but also on the frequency of the light, since our eyes are more sensitive to some kinds of light then others.

    When you say:

    First, replace the words "can travel" and "reach" by "can be seen by the naked eye", since I suspect this is closer to what you actually mean. From your question I infer that you are interested in luminous flux, measured in lumens, so I think that the question is asking how much by which we should increase the power of the red lamp to match the lumens from the blue lamp.

    As a side note, the reason that luminous flux is relevant to growing plants, even though it involves the sensitivity of the human eye, is because humans and plants evolved in the same spectrum of daylight.

    With a little bit of work I would be able to calculate the answer to your question, but unfortunately I cannot give the answer as a simple formula. This is because a 5700k bulb corresponds to an entire spectrum of colors, and so the answer requires using calculus to (numerically) integrate over a region of the spectrum. The luminosity data for the human eye can be found here:

    http://www.cie.co.at/main/freepubs.html [Broken]

    If you give me the specifics of your question, then I might do the calculation (it would be a bit of work to write the computer program, but I also have a vested interest in growing plants under red and blue light).
    Last edited by a moderator: May 3, 2017
  4. Jul 18, 2008 #3

    Very well, then we can at least be on the same page. Here is the scenario.

    Where my question derives from is this. When I first started indoor gardening I was using the 23watt 5000k bulbs and 2700k bulbs. The 23 watt 5000k bulbs did an excellent job at maturing the plants growth during the vegetative stage. But during the fruit/flowering stage where the 2700k lights are used, the outcome was less than desired. However, when I switched to the 40watt 2700k bulbs the outcome was excellent. From my observations of the two scenarios, the increase in power on the red light was able to penetrate the plants and do a better job at producing the fruit. What aspect am I missing and not understanding as to what the increase in power, intensity, or whatever the correct term is called.
    Last edited by a moderator: May 3, 2017
  5. Jul 18, 2008 #4
    It appears to me that you are making an assumption that, because you achieved the desired results by doubling the wattage of light during the flowering phase of your plant growth, that the light used during the previous stage of plant growth was somehow more "powerful" due to it's different spectrum. Perhaps your plants simply require more light energy during the flowering phase? They probably smell better during that phase also. :cool:
  6. Jul 18, 2008 #5
    Yes, the missing ingredient is the luminosity of the light bulb, which takes into account the frequency (color) and the power of the light. The red lamp is less luminous then the blue lamp, even though they are at the same wattage.

    In the future I would recommend paying more attention to the luminosity rating of your lamps then to the wattage. Look for something like '50,000 lumens' as the brightness of the light.

    Sometimes inexpensive lights with more wattage are wasting a lot of energy in the form of heat and not producing enough lumens.
  7. Jul 18, 2008 #6
    Well, in my mind I dont pay any attention to lumens, simply because from what I can tell lumens is based on the way humans perceive the light, not plants. I tend to focus on the color temp, wattage, nanometer frequency(if available) and PAR(if available). Does the lumen rating also contain the spectrum of light outside of the visible light for humans? Man, the more and more I look at this the more confused I get in trying to understand the dynamics of this. I cant explain what I am doing now scientifically, but whatever it is my results are pretty good.

    To add to how I use the lights is this. For the vegetative stage I use 4 23watt 5000k and 4 40watt 2700k. For the fruit/flower stage I use 2 23watt 5000k and 6 40watt 2700k. I cant explain it, but it works for me.

    This article here is what I am basing my ideas on. http://www.sunmastergrowlamps.com/SunmLightandPlants.html
    Last edited: Jul 18, 2008
  8. Jul 18, 2008 #7
    I dont know what the hell I am assuming anymore. I am confused.
  9. Jul 18, 2008 #8
    While this is a fine objection from a logical point of view, there has been a lot of research into the red/blue growing spectrums:


    The reason for this is because some kinds of plants tune their vegetative and flowering stages to the seasons using the long and short day as a guide, and the spectrum of dayloght at dawn and dusk is reddish.

    Don't worry, your experience matches perfectly with what science tells us. Your plants are better at absorbing blue light, so a red lamp of the same power will not get as much energy into the plants.

    As I said earlier, the theoretical explanation is that humans and plants evolved to maximize the use of the sun's spectrum. For practical proof that lumens are the way to measure how much light your plants are getting, just look around at guides for indoor growing e.g.

  10. Jul 18, 2008 #9
    Not sure if you saw my edit, but I base my confused knowledge from this article:
  11. Jul 18, 2008 #10
    That is a very informative article, and all I have to say is that the majority of the indoor growing industry does not seem to be caught up with the science that's in there. Most of the sites that sell and review grow lights rate their products in lumens, even though the PAR watts as suggested in the article are a more sensible measure. Lumens are still a better measure of brightness then watts, but these PAR watts are even more suited for rating lights that will be used to grow plants.

    Now all we would need to quantitatively answer your question is a table of data giving the relative sensitivity spectrum that is shown in the article you linked.
  12. Jul 18, 2008 #11


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    Staff: Mentor

    So what plant are you growing? :rolleyes:
  13. Jul 18, 2008 #12
    Here is a partial list of plants that are sometimes grown with a blue light in their vegetative phase and a red light in their blooming phase:


    I think it is more uncommon to find exceptions to this scheme.
  14. Jul 18, 2008 #13


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    Staff: Mentor

    Yeah, but I was asking Skewed specifically what he is growing.

  15. Jul 18, 2008 #14
    Tomatos. Cant imagine why people would be thinking I would be growing something illegal.
  16. Jul 18, 2008 #15
    I really appreciate everyones responses to this, but right now I am not sure where my understanding level is. Guess I need to go eat a tomato and think about it more.
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