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.


Thanks for all the suggestions.
To clarify, it will be a light, but the optics will be similar if not identical
to a solar fire starter, but the light source will be at the focal point of the
mirror. I used that picture as it provided the best illustration of what is
needed as all the picture of searchlights I could find were lacking in detail.

Perhaps I should explain a bit more. If you are not a cyclist then some of the
following probably won't make much sense, but bear with me.

Until about 8 years ago all bike lights were essentially rubbish, then a
revolution in high powered LED technology resulted in a series of 0.5w (half a
watt) and 1w bike lights that were streets ahead in reliability and battery life
compared to their incandescent cousins.

Technology marched on and now we have 3w, 5w and even 10w LEDs powered by
lithium batteries. Unfortunately the optical design hasn't kept pace and so the
minor "spill" (unfocused light directly from the emitter) has become so bright
that these lights are blinding to other oncoming cyclists. This is a particular
problem on cycle paths where the separation between cyclists travelling in
opposite directions can be as little as 6 feet and the combined closing speed
can be 40mph+.

Speaking from experience, it's like looking into the sun. All you can see is a
blinding dot of light coming towards you. You cannot see the road in front which
means that you cannot judge your position or speed. A head on collision is a
real possibility.

There are two possible solutions:

1) Join the arms race by buying one of these super bright lights. I'll be able
to see OK, but everyone else without an equally bright light will be blinded.
Obviously this is not good.

2) Use a bright light that has an ultra-low spill design - like a search light.
I'll be able to see the road and nobody will be blinded.

Finding such a light that will fit on a bike has not been easy. Currently I
only know of 2 manufactures, one has a poor reputation for quality, the other
uses ABS plastic enclosures, which means that prolonged running at 3w+ isn't an
option because of the heat dissipation problems.

So, at the moment it looks like I'll need to build my own. I've ordered a solar
fire starter from Ebay in the hope that the optics will be good enough. If it
doesn't work out then it looks like I have to learn the art of metal spinning.

Mike