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Collimating incoherent light emitted by a LED

by jcfutbal
Tags: collimate led laser
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jcfutbal
#1
Nov20-12, 11:47 PM
P: 3
Hello,

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?

Thanks.
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mfb
#2
Nov21-12, 08:47 AM
Mentor
P: 11,925
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.
Andy Resnick
#3
Nov21-12, 09:55 AM
Sci Advisor
P: 5,524
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?

jcfutbal
#4
Nov21-12, 03:47 PM
P: 3
Collimating incoherent light emitted by a LED

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...
davenn
#5
Nov22-12, 04:43 AM
Sci Advisor
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P: 2,667
to ask the obvious.....

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

Dave
jcfutbal
#6
Nov22-12, 07:56 AM
P: 3
i want to try and use a LED instead!
maimonides
#7
Nov22-12, 01:01 PM
P: 129
YouŽre 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 youŽd better use an optical bench or an optical table for mechanical stability.
Enthalpy
#8
Nov22-12, 02:05 PM
P: 661
"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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