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Collimating incoherent light emitted by a LED

  1. Nov 20, 2012 #1

    What would be the best method to "collimate" light from a LED with an emitting area of 12mm^2 and a maximum divergence angle of 80° with minimal loss of light intensity?

    I want to collimate it into a beam of approximately 5-10mm.

    I have an aspheric condenser lense at hand with an effective focal length of 13.7mm. This lens succeeds in making the beam spherical, with a diameter of approximately 30mm at the focal length, but it still undergoes significant divergence. I have tried adding more positive lenses into the optical system to try, but I can't seem to create such a small beam diameter without losing a great portion of the emitted lighted.

    Is there any particular lense configuration I could use?

  2. jcsd
  3. Nov 21, 2012 #2


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    I think it is hard to keep a significant fraction of light in the beam, unless you add a lense every 2-5 cm or something similar. You cannot reduce the phase-space size of your light, no matter how the setup looks like.
    Last edited: Nov 21, 2012
  4. Nov 21, 2012 #3

    Andy Resnick

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    I agree with mfb- the large size of the emitter restricts your ability to collimate the beam at even a moderate diameter without lossy beam shaping (spatial filtering, for example).

    Over what range does the beam need to be 5-10mm in diameter?
  5. Nov 21, 2012 #4
    there is no limit as to distance, as I have a lot of mirrors available and a lot of workspace. what is the best way to reduce the amount of light lost? I managed to "collimate" it to a diameter of about 10mm, but in order to do so I used a setup similar to a pinhole and I lost a lot of light. but to me at this stage a pinhole seems the way to go...
  6. Nov 22, 2012 #5


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    to ask the obvious.....

    why not just use a laser LED ?? already collimated with built in lens :)

  7. Nov 22, 2012 #6
    i want to try and use a LED instead!
  8. Nov 22, 2012 #7
    You´re perfectly right about the pinhole.
    The standard way of producing a parallel beam used to be:
    Use a lens to produce a (reduced) image of your light source onto/into a pinhole. Try to capture as much of the light as possible.
    On the other side of the pinhole, use a lens to produce your collimated beam. Then you use autocollimation to adjust the position (and tilt) of the lens for minimal divergence (and beam position). Beam size adjustment requires a diaphragm.
    But of course you will lose a lot of light this way. You can vary the pinhole size for the best compromise.
    And you´d better use an optical bench or an optical table for mechanical stability.
  9. Nov 22, 2012 #8
    "no limit as to distance" implies zero power from an incoherent source, even if there were no diffraction.

    A laser diode would be limited by diffraction. You get one diode in each CD or DVD reader-burner.
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