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any way to decrease diffused focal point of fresnel lens? - Page 2

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Posted by jan siepelstad on September 1, 2007, 7:03 pm

Is it realy the temperature that causes your problem, or is it a lack of

Posted by teraniageoff on September 3, 2007, 12:06 am
Hi all,
I think the suggestion to limit the re-radiation by using a small
capture aperture, and using a blackened absorber surface is about all
you can do now. The fault is probably casting errors in the lens. Get
a lighthouse lens?

 The cone will give you a smaller aperture and so less re-radiation to
space from your "hotbox, but less "power"  will be collected due to
your large diffused focal disc.
The Fresnel shouldn't absorb much, having little mass...the very
purpose of a fresnel... but the acid test is put your hand on it. Is
it remotely warm?
I'd expect about 10%-ish losses as  thermal during transmission. Even
a glass reflector will absorb this much after two passes of the light
to and from the mirror backing. Reflectors like MIRO4 might be  an
answer with 97%reflectivity and high specularity, but it rarely
remains stable and true to a parabola in my experience... so you're
left with that big ball focus.
The 36" ackack gun searchlight mirror we play with, cuts steel
strapping in about 3 seconds. That the steel is black (blued), and has
poor thermal conductivity, helps heaps. Also black "lava" results from
dolerite (blue metal/roadmetal) at the focus, both being very IR-
absorptive surfaces.
However it is an optical device, having in its past life had a carbon
arc at the focus, and has a focal ball of about 3mm. We have melted
AlOx jingboard (~2050?). It stays cool all day at the reflective
surface, a small surprise I point out at every demo.

Intensely focused  solar thermal certainly is an amazing , pure
research tool....  and I think we have only scratched the
surface...I'm used to oxy, and balancing the cone for reduction-
oxidation-neutral applications...but here's a pure, dead neutral, free
source. Even the burns are interesting. My finger burned in ~0.3secs,
and took a month to heal, and I noted that the third skin layer was
cooked before the first two...
Perhaps you should get a searchlight  ex-army, if you really want to
get that tight focus. You can do party hire on  Saturday nights to
finance the repayments...

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