Let me preface this by saying that I have a very limited knowledge of optics -- basically, I know enough to be dangerous. So I have a square Fresnel lens that measures about 10.5 inches on a side. The focal length is about 11.5 inches. I'm trying to find a presumably-smaller glass collimating lens that will take the light from the Fresnel lens and convert it to a beam with an indefinite focal length (or at least, a focal length of a few feet). I don't care about image preservation, I just mainly want to transfer some focused light from point A to point B with a minimum of loss. I basically need to know what kind of lens to buy and where to place it. I've tried single and double convex and concave lenses and seemed to have the most luck with single convex, but wasn't able to get much of a beam past a couple of inches out. The attached picture shows kind of what I am looking for (lens on left represents Fresnel lens, lens on right represents collimating lens).

Any help you could give me with this would be great.

enter image description here


1 Answer 1


Since your diagram indicates a point source, you only need one lens to do this. If you place the Fresnel lens at a distance equal to its focal length from the light source you will get a parallel beam of light.

  • $\begingroup$ Let's say my light source is a somewhat more diffuse but very strong faraway source. How would I get a parallel beam of light at that point? $\endgroup$
    – user45073
    Apr 23, 2014 at 23:40
  • $\begingroup$ A diffuse source can not be collimated perfectly. It needs to be from a point source. The only way to do this is add a pinhole into the system, which wastes a lot of light. If the source is however truly far away it will be already parallel. Does it cast a sharp shadow? If so forget about the lens and us a mask with a hole in it. It all depends on what you mean by diffuse and faraway. $\endgroup$
    – docPhil
    Apr 24, 2014 at 13:02

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