ambigousphoton

Hi everyone, I have an optics question that I hope someone could help a poor chemist out with. I have a Xenon lamp that directs most of the white light in a liquid light guide that then comes out and is collimated at 1" with a lens of the same size. If I wanted to collimate the beam at say 1/4", what is the best way of selecting the appropriate lens to use? There seem to be a bunch of lens equations and I'm not sure which to use for a collimating application. For what its worth my point source is like 0.125" and I think my beam divergence is something like 25.5 degrees. (0.125" goes to 4" beam when viewed 4" away.) Thanks so much for your help!