Optically, if you want a tightly focused beam you basically need to start with a light source that approaches a point source. The most important part is the size of the optics in relation to the size of the light source. This is why lights with large light sources, e.g., 5W Luxeon LEDs, and small reflectors often cannot be made to focus tightly. The Surefire L2 is a floodish light for this reason.
If you start with a source/reflector combination that is already somewhat diffuse, you would need a really large optic (reflector or lens) to straighten all that out.
Strictly speaking, if your ray traces are clean enough, a lens or reflector could be designed on a case-by-case basis. But you would have to have a situation in which, for any given area on the plane of the lens, light rays were only passing through in one direction. If you have light rays passing through any given point in lots of directions, then forget it -- you need a huge optic to do it then.
By "optic" I mean a lens, a reflector, or some combination of these.
Scott
