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LED Light - Beam Charactoristic?

  1. Mar 15, 2012 #1

    For some of the high powered LED lights, the beam seems to have bright and dimmer sections along its length when projected through air.

    They seem to be regularly spaced rather than random. Some people have described it as looking as though the beam would get thinner and wider along its length.

    It was not observed (noticed) for HID beams for example...just for high powered LED.

    What would be an explanation for that?

    Off hand, I wondered if it were some sort of resonance/harmonic between the emitter frequencies and the media or wavelength of the light?
  2. jcsd
  3. Mar 15, 2012 #2


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    It's probably just the lenses or reflectors in the flashlight making a beam that is convergent rather than divergent.
  4. Mar 15, 2012 #3
    How would that make the beam cross over more than once though?

    IE: If it had one bright spot in the middle somewhere...I could se that...but what would be happening for the beam to have a spot like that every so many meters, etc?
  5. Mar 15, 2012 #4
    Variations in fog density?
  6. Mar 16, 2012 #5

    Yeah, that would be a media issue...but its not visually foggy at least...albeit I'm sure there is plenty of vapor in the air nonetheless.

    It would need to be a regularly spaced interval of vapor to explain people in Australia and NJ and TX describing the same thing...but no one measured it specifically...so it could be a common, habitat phenomenon.

    Why would this affect an LED, but not an HID beam though?
  7. Mar 16, 2012 #6


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    If the reflector of the LED is not well finished (varying curvature across its diameter) then you can expect a number of focal lengths to be involved. It looks that different bits of your beam are focussed at different points. I think you probably get what you pay for.
  8. Mar 16, 2012 #7


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    well it may not be fog, but there is something there else you wouldnt be seeing the beam being reflected back to you as it is. so its water vapor, dust .... something ??
    and variations in that media is going to cause variations in the brightness of the beam along its length as its reflected back to you

    dont know, set up your experiment again so we can see both beams going out side by side

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