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Thread: kelvin rating vs colour spectrum ANSWERED!

  1. #1
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    Thumbs up kelvin rating vs colour spectrum ANSWERED!

    thought i'd make a post regarding light spectrum vs kelvin rating as it seems to have confused a lot of forum members. this post is not the final law on the matter but what i've recollected from my colour science labs back in uni and as such given as personal advice only.


    Kelvin rating is a colour index which reflects what a black body emits at that particular temperature. at lower temperatures (3,000K) it looks red, at a higher temperature (10,000K) it looks white and higher still (20,000K) it looks blue.
    a simple example is when you heat a piece of metal, it glows red. keep heating it and it glows "white hot". keep heating it further and it goes blue.
    Kelvin rating is negligible when it comes to growing plants. it is the overall colour and a matter of personal taste or preference.


    The visible (colour) spectrum refers to the wave lengths across the visible light range where 650nm is red, 580nm is yellow, 550nm is green and 500nm is blue.
    This is what is important when it comes to plant growth.
    plants use chlorophyll which is a green pigment during photosynthesis. chlorophyll absorbs mostly blue and red wavelengths hence why most plants and algae look green.

    while i dont know about MH lights (never covered it in class though i'd guess the principles would be the same) i do know that with flourescent tubes they produce light by exciting phosphors - different phosphors produce a different spectrum of colour.

    "cool white" phosphor tubes emit little in the red and blue regions but more in the yellow region because this is what the human eye picks up the best - it looks brighter to our eyes.
    but the catch is, we want red and blue for plant growth! hence the best flourescent tubes to use in aquaria are triphosphor (3 different phosphor coating) tubes. we want:
    blue because it penetrates the deepest under water
    red because it is the best range for photosynthesis
    yellow for visibility/clarity
    these wavelengths are where you want the peaks on the triphosphor tube.


    i hope people made it through that post without their eyes glazing over as mine did when professor Postle (i cant believe i still remember his name) taught our class those many many years ago!
    hopefully this all makes sense and people can make their own decisions based on the advice.
    if i've confused you even further... questions to clarify are welcome
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  2. #2
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    Can you add to this by describing the differences between these and Lux/Lumens?
    From memory it has something to do with how our eyes perceive brightness?

    What do we look for when we try to work out what lamp has the best intensity with the right colour rendition?

  3. #3
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    G'day PiL,

    Great post. I would have thought that the Kelvin rating would have an effect on plant growth. For example if the Kelvin rating is low, doesn't that mean only red light is emitted and not the other important shorter wavelengths such as the blues??

    Cheers

    Gavin

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    Having studied and worked in imagery from satellites for over ten years prior to being a "mum" I can confirm the colour spectrum information regarding plants is correct. The satellites are programmed to capture images of the earth in particular light spectrums so we can analyse land cover types for instance.

    Vegetation actually shows up (reflects light) best with Near Infrared (around 0.76-0.90 ┬Ám) and you actually start detecting the health of type of plants. We can only see the visible spectrum with our eyes though.

    Jola

  5. #5
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    Thanks for that PiL. SOme good information there.

    While I was on Plantedtank the other night I came across this thread. There is some good information there for anyone that likes a read.

    Also PiL I have some T8 tubes I bought from Bunnings that are Quadphosphor (I am assuming there are 4 different phosphor coatings). What would the 4th coating be ?
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  6. #6
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    Quadphosphor? I wonder how that will go. I may try one myself...
    What brand shake?

  7. #7
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    4th coating be green.
    What The pseudomugil Furcatus was that!?

  8. #8
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    Quote Originally Posted by MrFish View Post
    Quadphosphor? I wonder how that will go. I may try one myself...
    What brand shake?
    NEC I think MrFish. What ever Bunnings carry, and apparently 30% brighter than triphosphor.
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  9. #9
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    hey Mr_Docfish, lumens and brightness is a whole 'nother kettle of fish so to speak and i dont know much about it. some plants need more light intensity than others, but light spectrum-wise it's the same. i know that some kelvin ratings "look" brighter than others but in actual fact lumens are the same.

    hey partrg, no that is incorrect. an example is the common incandescent bulb. it is full spectrum but because of it's low kelvin rating, approx 3000K it looks yellow. same for warm white flourescents which try to mimick incandescent light at 3000K share the same spectrum peak of yellow as cool white but due to it's higher 4100K rating, cool white looks whiter.

    and shake, yes quadphosphors are on the market, but as i told Mjay, (he's always lurking) you need to look at the specs. these should either be listed on the tube or on the cardboard packaging. if they aren't, you should be able to find this information on the company's website or some kind of colour chart that may be available at the store of purchase. i'll see if i can find it for your particular tube. just because a tube is tri or quadphosphor doesn't necessarily mean that the peaks are where you want them to be!
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  10. #10
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    Found an old post from Perth Cichlid Forum that I posted up:

    Quote Originally Posted by Mr_docfish
    Plant chlorophyll absorbs all light at wavelengths of 400 to 700 nm
    Green light is around 500 nm
    Chlorophyll prefers 400-500 and 600-650 but requires all wavelengths in order to synthesize correctly.
    Also there are other pigments involved in different plants like xanthophylls that give you the red/brown colour - they prefer a more even intensity from 420 - 480 nm.
    Quote Originally Posted by Mr_docfish
    Factors in Choosing a Light

    There are many choices for lighting a planted tank these days, ranging vastly in price (though high price is not necessarily correlated with high quality). Each form of lighting has several factors to consider, summarized below.

    1. Total light output: how bright. The unit for this is Lumens. Plants require a certain amount of lumens to grow.
    2. Efficiency: how much light can be produced for a given amount of electrical power (lumens per watt). Higher efficiency bulbs are going to cost you less to operate, and may offset the initial purchase cost after a few years.
    3. Color rendering index (CRI) and Color Temperature: How close the light approximates the sun. This can be complicated to explain, but I'll try to keep it simple. The sun's light is produced in all colors (red to violet), the combination (spectral curve) of each appears to us as white. The CRI refers to how close the spectral curve of the light approximates that of the sun, with 100 being the closest. In the crude graphs below, you can see that the sun is a nice gradually sloping curve with components from the entire spectrum, while a full-spectrum fluorescent bulb (CRI of 70-90) merely approximates this with "spikes" of different colors. A bulb with a very bad CRI, such as a "cool white" consists mostly of a big green spike.



    The Color Temperature, measured in Kelvin, refers to the color where the spectral curve is ``centered'' or ``balanced''. A higher temperature means that the curve is shifted blue, and a lower temperature means the curve is shifted red. Midday sun has a temperature of 5700 Kelvin, so most plantkeepers try to get lights of similar color temperature. The problem is that many lights are only available in lower temperatures, because people are used to incandescent bulbs of 3200 Kelvin (and before that, fire) for lighting their houses.



    Extracted from thekrib.com
    by Erik Olson
    June 1997


    Thought it might be of help to understand the other aspects of lighting.
    Note that the 'Midday sun has a kelvin rating of 5700'....so why consider the more expensive lamps that produce 10000+ K??


    Problem I have is that I was told that Lumens is a measure of light intensity within the 'visible' spectrum of the human eye. So would this be relevant to plants?

  11. #11
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    those graphs representing the SPECTRUM of cool white and triphosphor are great but the one that represents the sun is misleading, as are the kelvin graphs.

    here's why: the one respresenting the sun should not be next to the flourescent graphs (it is drawn on a different scale), it should be with the low/high kelvin graphs as it represents the overal black body temperature COLOUR, not the SPECTRUM.
    if you were to see a SPECTRUM graph of the sun, it is actually VERY spikey. so much so that it is usually represented as a colour bar with black stripes (the black stripes being the colours that the sun DOESNT produce).

    let me try clarify with your sun example. for all intents and purposes, the sun is FULL spectrum (white light). yet it looks yellow because it's kelvin rating is approximately 5,600K.
    what you are trying to do is compare length and mass, they are 2 completely different scales... but often misread as the same. (prof. Postle drilled this into us) ie, you are trying to compare the representative colour of kelvin temperature with the representative colour of electromagnetic radiation (light) measured in nanometres on the same scale...


    here's a neat representation of kelvin colour using stars which is better than my heated metal example the sun is represented here as a 5,600k approximation:


    this post should answer the last part of your post regarding why choose 10K over something similar to the sun's 5,700K - it makes no difference, it's purely your taste in colour. both are artificial and do not contain the full spectrum that the sun does. assuming that both are triphosphor, they will both look like the triphophor graph you have posted regardless of their kelvin rating.


    and finally, like i mentioned before, lumens is the intensity of lighting and has nothing to do with spectrum or kelvin. again you are trying to compare totally different scales. simply put, you get higher lumens in full sun in the tropics, and lower lumens in the subtropics in the shade.

    hope this helps!
    Last edited by PiL; 05-04-08 at 07:48 AM.
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  12. #12
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    good info - explains alot

    does anyone know what the spectrum is of MH bulbs?
    i know many people use them (including myself) so they must be good, but i'm wondering if they have a 'better' or 'worse' spectrum than fluros in regards to plant needs.
    Last edited by Jezza; 05-04-08 at 11:51 AM. Reason: bad grammer (me no good at english)

  13. #13
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    Quote Originally Posted by PiL View Post
    simply put, you get higher lumens in full sun in the tropics, and lower lumens in the subtropics in the shade.

    lol doesn't matter whether your in the tropics or not, if your in the shade theres going to be less light
    He's not the messiah, he's a very naughty boy!

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    Quote Originally Posted by Jezza View Post
    good info - explains alot

    does anyone know what the spectrum is of MH bulbs?
    i know many people use them (including myself) so they must be good, but i'm wondering if they have a 'better' or 'worse' spectrum than fluros in regards to plant needs.
    Really depends on the manufacturer. Different spectrum readings could be different for the same kelvin rating globes.
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  15. #15
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    Pil,
    Thanks for that, the colour graphs are old ones from thekrib.com, so you are right, not very accurate.

    What you are saying is that in general, Kelvin has nothing to do with what plants require, it is the Colour Spectrum and Intensity/Brightness.

    If this is the case, why are we in the hobby, so hell bent on the measurement of lighting by the Kelvin rating? Why don't lamps have a Colour Rendition Index on them? And what in your opinion is the best way of measuring the Intensity/Brightness of lamps in relation to what plants require.

    The reason I am on the topic of Lumens and such, is that there is very little concise detail on what aquatic plants need, in particular "where everyone can agree on it". I just want to find out what the learned people out there think is the best way of determining what the best lamps are for the purpose of aquatic plant growth.
    In the field of home theatre for example, they work on ANSI lumens, in the field of UV sterilization they use Watts per sq cm. What should we in the planted aquarium industry work on?

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