# Collimating incoherent light emitted by a LED

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.

mfb
Mentor
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:
Andy Resnick
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?

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
Gold Member
2021 Award

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

Dave

i want to try and use a LED instead!

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.

"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.