I am currently using a broadband light source (300-1100 nm spectrum) and my dispersion at a few feet is too large for me to receive any feedback. I understand that I cannot increase brightness but I need a decent beam size so I receive some feedback. What is the best technique for doing this?
You need a focusing optics which could be metal coated reflector, regular of Fresnel lens. The brightness law implies that the product of the optical aperture and the divergence angle is constant and related to the source emitting dimensions and emission solid angle. For a few feet, I would try the use a Fresnel lens (you can get a pretty big one from Edmundoptics for example), since this is the cheapest option. Using a larger lens located further from the source, you get a smaller focused spot.
This is true in case your white source is a larger halogen lamp filament. Consider using a high pressure Xenon arc lamp (Perkin Elmer Cermax or USHIO UXR-300) having much higher brightness (smaller emitting area) and are better focusable.
Good luck
For beam collimation off-axis parabolic mirrors are commonly used but it would work well only for a source of small dimensions. For extended light sources it would more interesting to use large field of view optical systems.
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