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I don't have much knowledge on LED lights and find the technology quite fascinating (yes, I know, it is not that new!)

I have read that in order to create "white" light, LEDs actually need to emit light from all spectrums.

My question(s):

  • How do LED light bulbs (cool white or warm) generate white light?
  • What would be the light spectrum for cool white LED light bulbs?
  • What would be the light spectrum for warm white LED light bulbs?

Thanks!

P.S. This is my first question here. If it should be forwarded to another SE site, please let me know!

P.P.S. I have done my research BEFORE asking but could not find a technically accurate answer.

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    \$\begingroup\$ "I have done my research BEFORE asking but could not find a technically accurate answer." what is a technically accurate answer? Your first question has two answers at wikipedia. This search seems to find several candidates for your second question: google.com/… In what way are that not meeting your needs? \$\endgroup\$ – gbulmer Aug 17 '14 at 15:30
  • \$\begingroup\$ gbulmer, 1st those spectrums are for few brands' products. 2nd those advertised light spectrums do not add up. a LED, pushing light of a very narrow spectrum, with half of its energy converted to another and pushed through P filtering should not be able to have that kind of broad spectrum. \$\endgroup\$ – Phil Aug 17 '14 at 16:53
  • \$\begingroup\$ Please add that information to your question. I should add that to me, it makes sense to require a spectrum for each product, and if the manufacturers and labs do not publish, "that's just the way it is". What does "LED, pushing light of a very narrow spectrum" mean? What is very narrow? I do not understand any reason why a phosphor excited by a narrow band can't emit a broad spectrum. Fluorescent light works that way, doesn't it? \$\endgroup\$ – gbulmer Aug 17 '14 at 19:38
  • \$\begingroup\$ That's exactly my point gbulmer. I do not find it to be "narrow spectrum" but that's what the 'net says. So I wanted to ask to people here and requested (kindly) some technical information that was legit and valid. P.S. I would personally classify very narrow as covering 1/6-1/8 of the visible spectrum, if it is even possible at all. \$\endgroup\$ – Phil Aug 18 '14 at 2:13
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gbulmer puts you on the right track. For the most part, "White" LEDs are nothing more than a single color LED with a phosphor on them. The phosphor takes roughly half of the light from the LED and converts it to a second frequency of light. The two frequencies of light combine in our eyes and look to be some variation of white.

A power LED I have emits yellow and purple to attain a "cool white". Warm white has more red in it. In short though, the color spectrum of White LEDs is generally horrible unless you get a really expensive one designed for full spectrum use. In general, white LEDs consist of two-ish spikes of color in the spectrum with everything else very low in comparison. The result of poor color spectrum is that some colors won't even be present even though it appears to us as white. You need 3 separate bands of color to be capable of producing all of the variations in between. With only two bands of color, you could be shining the light at something green and it'll come back looking dark grey.

enter image description here

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  • \$\begingroup\$ Thank you very much horta. Why is this kind of curve called "very narrow", given that it appears to cover a broad area (especially the warm white ones)? \$\endgroup\$ – Phil Aug 17 '14 at 16:54
  • \$\begingroup\$ @Phil compared to an incandescent lightbulb, the two peaks here are very narrow. Obviously, the blue/purple is a narrower peak. \$\endgroup\$ – horta Aug 17 '14 at 17:41
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To extend horta's answer, you might want to have a look at the CREE guide to LED color mixing.

As previously said, the two colors (blue and yellow) mix to create a white. This is shown below on the CIE 1931 color space: CIE color space

The mixed color (white) will be on a line between the two components (blue and yellow). The ratio of the intensities of blue:yellow determines the final color.

Theoretically, you could achieve a white by mixing other colors (eg. cyan and red). One of the advantages of a blue and yellow mix is that many standard "color temperatures" can be achieved.

Color temperatures

As you can see the blue-yellow line is quite close to the line of standard color tempratures (the "Planckian Locus")

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  • \$\begingroup\$ tehwalris, this is brilliant AND amazing, thanks! Do you have a good source (preferably not 1000 pages long) where I can understand about how LED colours are set and how mixing is done on a single diode? (please keep in mind I'm not a scientist so I can not understand a complex white paper, something in simpler terms, if possible) \$\endgroup\$ – Phil Aug 17 '14 at 18:50
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    \$\begingroup\$ This article could be interesting: ledsmagazine.com/articles/print/volume-10/issue-6/features/… \$\endgroup\$ – tehwalris Aug 17 '14 at 19:24
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    \$\begingroup\$ It's important to note that the "averaging" of mixed colors only works when light is being observed directly. Illuminating a green object with a mixture of blue and yellow light will yield very different results from illuminating it with white light. \$\endgroup\$ – supercat Aug 18 '14 at 3:53
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White LEDs are coated with phosphors that glow with the desired color temperature.

"Cool-white" LEDs (more blue) have a color temperature above about 5000K, while "warm-white" (less blue) ones have a color temperature below about 5000K.

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  • \$\begingroup\$ Thank you TDHofstetter. Excellent answer. Can you tell me about the light spectrum though and the effects of the phosphorus coating? Much appreciated! \$\endgroup\$ – Phil Aug 17 '14 at 16:00
  • \$\begingroup\$ The spectral performance of any given LED is very dependent upon the specific LED... some emit over very narrow spectra and some emit over very broad spectra. The spec sheet for a given LED will usually show its spectral performance curves (similar to Horta's picture above). \$\endgroup\$ – TDHofstetter Aug 17 '14 at 16:19
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Google image search for "light spectrum for cool white LED light bulbs" gave e.g.
http://www.enkonn-lighting.com/product/dimmable-led-light-bulbs-ceramic-cool-white/

google image search for "philips light spectrum for cool white LED light bulbs" gave e.g.
http://www.philipslumileds.com/technology/quality-white-light

without further information, I think the technique of searching the web, and looking at the images works. So is this question is complete?

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