On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

This is for a searchlight application where the light source shines backwards
into the reflector which then reflects the light forwards producing a narrow
beam.
The reflector needs to be about 2 to 3 inches in diameter with a focal point
about 2 to 3 inches in front.

Presumably this was possible in the days before CNC machinery, so how did they
do it?

The following picture gives you an idea of the size, curvature and surface
finish required
http://www.motherearthnews.com/~/med...21_9386858.gif
It's actually a "solar fire starter" so the light is traveling in the opposite
direction, but the optical path is the same.

Not near as much fun as making one from scratch,
http://www.4bobandbob.com/pages/01.html

All the parabolics are sold out and were 4" the non parabolics range from 2
1/2" to 4". Seem to recall they once sold a 6" many years ago... Have not
bought from them in ~25 years, nice to see they are still in business.
--
William