Seeking the perfect parabolic reflector


Oct 21, 2010
I’m designing a bike light and need to find a suitable reflector for my needs. I’m working on the principal of not wasting lumens lighting up the sky and blinding other road users, so instead of mounting the LED in its usual place I’ll be cutting a parabolic reflector in two lengthwise and mounting the LED near the focal point aimed down into the half parabola, like so:


My problem is finding a suitable reflector as it needs to be as complete a parabola as possible, i.e. with the smallest opening at the rear (ideally none). It can’t be a reflector for LED use as these are normally cut off at the focal point with the LED mounted there (no need for parabola behind the LED as it doesn’t project that way). An incandescent reflector may work but on many of these the opening is substantial and I’d lose lumens out the back. A Mag C/D reflector is a good example (I’ve got one on order to play about with) – probably a perfectly good reflector but with a 15mm opening at the rear.

I’d imagine it needs to be aluminium (there will be two of them and I’ll mount an XM-L in both) and with a focal length sufficient for me to mount the LED with the front edge round about the focal point and the back edge towards the back of the reflector. Thus some light will be focussed into the spot with the remainder forming a wider-focussed floody beam below – perfect. I don’t think I could mount the LED in front of the focal point as light would then reflect up above the cut-off line of the beam.

And finally coating – smooth I would think, to give the best cut-off. The fact that the LED will be emitting at various distances from the focal length back will spread out the light nicely so I shouldn’t need OP or stippling, especially as that would again direct unwanted light upwards above the cut-off line.