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Can LED's emit monochromatic light as efficiently as broad spectrum?

  1. Oct 24, 2013 #1
    I read that many high pressure sodium and high pressure mercury street lights are being replaced with much more efficient LED lights, which fortunately point downwards instead of sending light sideways at star gazers like me, or into the sky where it refracts back down and hazes over the stars and galaxies.

    Many people want broad spectrum lighting for good color rendition. I wonder if they would agree to compromise with a three way mix of monochromatic red, green, and blue light, so they still get white light, but astronomers can filter out each of the three peaks. Our filters stand no chance against broad spectrum lighting. What if they had a mix of red, green, and blue LED's, which mix to white light by the time it hits the ground.

    Is there an efficient way to make those colors? I know low pressure sodium makes almost monochromatic yellow light, but it is not as efficient as the broad spectrum LEDs. Does anyone know how LED light is made and if the compromise is efficiently possible?

    Thank you.
  2. jcsd
  3. Oct 24, 2013 #2
    I was told on an astronomy forum that LED's don't make monochromatic light. Well, even if they make a sort of tight bell curve, and we can cut off everything within 60% of the max peak, that would still greatly improve visual astronomy. I photographers can just subtract most of it out with software, but some of us still like to see the light with our own eyes.
  4. Oct 24, 2013 #3

    Simon Bridge

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    LED light is made the same way as any other light - electron transitions between energy states.
    It's just that the energy states in question can be carefully selected at manufacture - this makes single color LEDs quite specific with about 25nm FWHM each - you can filter them pretty effectively.

    There are other ways and this is an area of active research.
    Wikipedia has a set of spectra for the main ways of making white-light LEDs.
    ... you can use that to kick off a more detailed search.

    Note: streetlighting is likely to use the cheapest version possible.
  5. Oct 24, 2013 #4


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    The below pdf document from the International Dark Sky Association discusses the negative effects of light pollution, and specifically addresses the spectral contributions.

    Extract from the Abstract:

    “Outdoor lighting is undergoing a substantial change toward increased use of white
    lighting sources, accelerated most recently by developments in solid-state lighting.
    Though the perceived advantages of this shift (better color rendition, increased “visual
    effectiveness” and efficiency, decreased overall costs, better market acceptance) are
    commonly touted, there has been little discussion of documented or potential
    environmental impacts arising from the change in spectral energy distribution of such
    light sources as compared to the high-pressure sodium technology currently used for most
    area lighting. This paper summarizes atmospheric, visual, health, and environmental
    research into spectral effects of lighting at night.”

    http://www.darksky.org/assets/documents/Reports/IDA-Blue-Rich-Light-White-Paper.pdf [Broken]
    Last edited by a moderator: May 6, 2017
  6. Oct 26, 2013 #5


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    The human eye is most responsive at around 555 nanometers which is green or rather a slightly yellow green. So if you wanted to make a street light with maximum efficiency then guess that would be a reasonable colour to pick (although I can see reasons why a narrow spectrum might not be ideal).

    Perhaps of interest..

  7. Oct 26, 2013 #6


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    It depends on what you want. An environment with monochromatic illumination is disgusting to work in. You have to consider the human factor when lighting a scene.
  8. Oct 26, 2013 #7


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    I don't think most people care about the concerns of astronomers.

    Personally I am much more concerned about seeing what is in front of me when walking or driving at night.
  9. Oct 26, 2013 #8


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    You are probably right, there. Space-borne telescopes don't care too much about our light pollution. There are far more people who need reasonable light levels. It should be said that there are ways of illuminating roads and buildings that are more efficient and produce less light pollution than what we use today.
  10. Oct 26, 2013 #9
    I was not suggesting we use one monochromatic frequency. I was suggesting we use three peaks: red, blue, and green, to give white light to those who need light, but three peaks that ground based astronomers can cut out.

    If there were white light composed of three narrow peaks of red, blue, and greee/yellow, would most people be happy with that? Mercury vapor lights come close to what I'm talking about, but they have two blue peaks instead of one, and they have an orange peak instead of a red one.
  11. Oct 26, 2013 #10


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    On the face of it, that would seem a good solution. However, although the light source ( torch) could be made to look a good white, the light reflected from objects could make them appear very different colours from their colours in broadband white light.
    I guess it might be a compromise to produce a suitable 'white' illuminant with a spread of single wavelengths with a few gaps in. This could give reasonable colour fidelity plus some suitable windows for astronomers.
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