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Math Help - Watts and Kelvin

  1. #1
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    Watts and Kelvin

    Hi,

    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.
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  2. #2
    Grand Panjandrum
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    [quote=Skewed;168217]


    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????
    .
    They both have infinite range other things being equal. Also without absorption the intesities are equal at equal ranges.

    RonL
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  3. #3
    Grand Panjandrum
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    Quote Originally Posted by Skewed View Post
    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.
    I think you had better tell us why you are asking, so that we can answer the real question.

    RonL
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    Quote Originally Posted by CaptainBlack View Post
    I think you had better tell us why you are asking, so that we can answer the real question.

    RonL

    Fair enough.

    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.
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    Grand Panjandrum
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    Quote Originally Posted by Skewed View Post
    Fair enough.

    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.
    Plants have evolved to use sunlight as an energy source in photosysntesis, which means they are (to a hand waving approximation)optimised to use light of with a spectrum similar to that of the daylight. This is close to that of a black body at the same equivalent temprature as the Sun, and corresponds to 5000-6000K.

    So the plants are making better use of the energy from the 5000K bulbs than they are from the 2700K ones. To get the same energy into the plants acceptance band with "cooler" bulbs requires greater brightness and so a higher powered bulb.

    However, ripening may require warmth rather than light for photosynthesis, and the 2700K bulbs will be radiating more energy in the infra-red which may be keeping the ripening fruit warmer.

    RonL
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    Super Member Matt Westwood's Avatar
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    I don't know the full technical details (biology is sketchy with me) but the wavelength may be more crucial than that.

    I presume the 5000k bulb produces a light with a shorter wavelength (and hence by E = h \nu) a higher energy than the 2700k one.

    It may not be the number of photons that arrives on the plant that makes the difference, but the energy of the individual photons that stimulates the chlorophyll into doing what it's supposed to do.

    Check out the photoelectric effect, in which you need light above a certain frequency threshold to kick the electrons free of the atoms. If you need blue light to do it, then it doesn't matter how bright the red light you shine on it, you're not going to see the effect happen.
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    Grand Panjandrum
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    Quote Originally Posted by Matt Westwood View Post
    I presume the 5000k bulb produces a light with a shorter wavelength (and hence by E = h \nu) a higher energy than the 2700k one..

    Black body radiation (that is thermal radiation) like that from the Sun and/ot incandesant light has a continuous spectra, that is all wavelengths are present over a band wider than the optical.

    No one said it was the crude number of photons that was important, but the numbers in the bands that plants were using. Increasing the brightness while maintaining the same colour temorature increases the numbers of photons in all bands.

    RonL
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