I am currently using a simple 8mm focal length lens from Carlco to focus my light from a single small LED into a bundle of fibers that have a total diameter of 9mm. The problem I am having is that I have a high intensity of light at the center of the bundles and the light falls of by at least a factor of 2 at the edge of the bundle.
I make a commercial product and need a lens that is not more than USD10 in cost if possible.
Can any one point me in a good direction please?
Thanks, Michael Groves
Hi,
Since you are already very well aware of your situation, i will add few trivial things. Collimation can be done using two combination of lenses. Even if you end up getting a good collimated beam LED will project its own image on the fiber inputs!!! This image will have its own intensity distribution which your fiber bundle will pickup. One simple approach to solve this problem will be to use a scatterer in front of the LED. This way the fiber inputs will see somewhat uniform intensity distribution at the cost of decrease light intensity at the output of scatterer. To start with you may use a higher power LED than the one you are currently using to compensate for the decrease LED output intensity due to scatterer. But still you have to tinker a little bit for this solution with the setup to get the desire results.
Regards
Dear Michael Groves , the best solution would be, as Amit Morarka pointed, to use a telescope, a combination of two lenses placed at a distance equal to the sum of their focal lengths (if both lenses are positive), the beam at the exit or close to your bundle will be a flat beam, where the intensity will be homogeneous in its section and therefore all the individual fibers will get the same intensity.
Another option implies your 8mm lens and a concave or negative lens, they will be separated by the sum of their focal lengths, but with this set-up the separation between them will be smaller or more compact. In any case you would need one extra lens... and may be a tube to connect them -given you are dealing with a commercial item.
Another alternative is the use of a gradient index lens, that could do the job.
Beam-probes can do the job too, but apparently they are far more expensive:
https://www.newport.com/f/beam-probes
You also could use a phase-plate to flaten your beam at the entrance of the bundle. The advantage here is the compactness, and the disadvantage their prizes.
A cheap solution would be to place your fiber bunch not at the focal distance of your lens, but before (or after) it, say at 7mm or 9mm. In this way you still would have some degree of light variation, but not so strong as you describe.
Hope it helps.
Thanks to both of you for this suggestion. I had to really go back over my earlier physics days (I have a Ph.D. in Biophysics) but am now 70!!
I am using a Carclo plastic molded lens 10356 (f=11mm) which has the attached optical characteristic as shown in the attached beam trace. It is this resultant beam that I would like to make more uniform.
So, as I understand, using a telescope approach might help. So I am wondering if I continue to use the Carclo lens (cost USD 0.64), would a convex lens with a focal length of say 20mm provide a better uniformity of beam? Can you make any suggestions? The diameter of my fiber bundle is 9mm.
By the way, great idea about defocusing the fiber bundle pick-up. In the product I sell I did set up my fiber bundle just beyond the focal plane of this lens (at 12mm) and that improved the uniformity, however, my fiber bundle at 4mm off the centre of it has a significant drop off of intensity by up to 50% (as measured with a photometer).
Thanks for any further suggestions and thanks for your help so far.
Hi Michael,
I read the previous answers and agree with their approach.
I spent some time a while ago trying to get homogeneous irradiation for a group of fibers. I hope my thought can be of use to you.
The ways to get homogeneous irradiation of each fiber in a bundle is still an art and most solutions (light concentrators, light pipes, beam homogenizers, etc.) are exceeding USD10 budget.
You may have time and resources to experiment with the arrangement on your picture:
1. First test your fiber bundle illuminating it with a matt lamp from two distances and also see if the bending of the bundle can affect the uniformity.
2. Bring the bundle close to the lens up to 2mm distance in steps while measuring central and peripheral output from the bundle.
3. Try to block the central spot on the lens. It can be a spot of scattering compound (say alumina ceramic), or just a round piece of aluminum foil temporary attached (glued) to the lens surface (begin from about 2 mm diameter). The distance lens-bundle also should be optimized for the central-spot lens.
4. Bearing in mind unavoidable light losses you might be interested to consider similar lens with LED from Luxeon:
https://www.luxeonstar.com/carclo-20mm-fiber-coupling-optic
https://www.luxeonstar.com/luxeon-rebel-star-leds
5. You might be interested to look at the forum web on the topic:
http://forum.free-track.net/index.php?showtopic=2667
The bottom line is it is a challenge to accomplish high efficiency and uniformity of LED coupling into ha fiber bundle.
I might be of better assistance if you can disclose mode details of your project.
Don’t hesitate to contact me for more details.
Good luck.
Leonid Vesselov, Ph.D.
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