Larry Jaques fired this volley in
:

On Wed, 21 Aug 2013 14:07:53 +0100, David Billington
wrote:

On 21/08/13 14:02, Spehro Pefhany wrote:
On Wed, 21 Aug 2013 08:27:43 -0400, the renowned Ed Huntress
wrote:

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...torial/Blogs/H
omesteading%20and%20Livestock/Solar%20Fire%20Starter/sundancesolar_
2021_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.


Mike
Mike, are you sure that you can start a fire with a 2" or 3"
reflector? My experience tells me you need at least 6".

When I was in high school I had access to rejects from several
phisics-lab projects at Princeton University (including a nice
ruby-rod laser, but that's another story). One was aluminized glass
parabolic mirrors. They were great toys.

I tried starting fires with most of them. g The smallest ones
that worked at all were around 6" diameter, and for that I needed a
little ball of red cedar inner bark, like you'd use with
flint-and-steel fire starters.

BTW, the best solar fire starters I've ever used are the $10, 8" x
10" plastic fresnel magnifiers you can get at any office-supply
store. The cut (lenticular)sside goes toward the sun.

Good luck. I don't know how to help you machine them, BTW. I made a
parabolic microphone that was 5 feet in diameter when I was 18 or
19, using chicken wire, plywood, Plaster of Paris and a cut piece
of tempered Masonite for a sweep. The plaster became a male mold
for a fiberglass mat reflector. It worked great but it wasn't
accurate enough for optical work.
Years ago I had some premiums that were folding aluminized parabolic
mirrors that could allegedly be used to light a cigarette. They were
at most 3" diameter when opened. Never tried it myself that I can
remember. I've probably still got one around somewhere, I'll have to
try it. I didn't spend the $40 to get medically "evaluated" last
time I was on Venice beach so I guess it will have to be a tobacco
product or paper.

This is a similar idea without the folding part:
http://www.alibaba.com/product-gs/11...olar_cigarette
_lighter.html

I guess it's aimed at the green consitutuent of the smoking public
who mostly lights up a fag at midday- likely an exceedingly narrow
sliver of the Venn diagram.



Best regards,
Spehro Pefhany
I can remember RadioShack had ones like those back in the mid 1970s,
at least the Huntington LI, NY branch did. Definitely smaller than 6".

I had one of those. IIRC, it was an elliptical shape about 3x5".
The diabolical elliptical parabolical lighter.

--
Truth loves to go naked.
--Dr. Thomas Fuller, Gnomologia, 1732


Yeah... way smaller than 6". Consider -- you can light a fire with a 2"
magnifying glass. They pass less than 80% of the heat-producing
wavelengths.

A properly aluminized mirror reflects over 97% of all visible
wavelengths, and most IR.

That means it's more efficient at lighting fires than a magnifying glass
of the same size.

Those dumb Radio Shack lighters were just polished stainless steel -- not
hardly reflective at all, in general optical terms.

LLoyd