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any way to decrease diffused focal point of fresnel lens?

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Posted by John Roentgen on September 1, 2007, 12:33 pm
 
Recently acquired a fresnel lens from a projection tv.  It is about 36"x36".
My original purpose was going to be to use it for melting metals, etc but
the focal point appears diffused.  It ignites paper and boils water after a
time, but won't melt metals very well.  Since this is a lens, is there any
way I can add an additional fresnel (or even glass) lens at the focal point
to decrease it even further so I get higher melting points?

Thanks in advance,
JR



Posted by Morris Dovey on September 1, 2007, 12:41 pm
 
John Roentgen wrote:

| Since this is a lens, is there any way I can add an additional
| fresnel (or even glass) lens at the focal point to decrease it even
| further so I get higher melting points?

No.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/



Posted by Anthony Matonak on September 1, 2007, 3:02 pm
 John Roentgen wrote:

You can't decrease the image with another lens but you can use
non-imaging optics to concentrate the light more. This could
be in the form of a narrow cone or pyramid of highly reflective
material or a prism shaped in a similar manner.

Anthony

Posted by John Roentgen on September 2, 2007, 8:34 am
 

Thanks..... I'm curious about this approach and not quite sure I understand.
Am I basically reconcentrating the somewhat diffused light into a more
refined focal point?  I am visualizing this perhaps as a concave reflector
placed at the current focal point, or close to it, that captures the light
and then refocuses to a new smaller fp.  Is that similar to what you're
describing?

Thanks,
John


Posted by Anthony Matonak on September 2, 2007, 9:28 am
 John Roentgen wrote:
...

I am not suggesting using another lens to create a more refined focal
point. This won't work. You already have your most refined focal point
from your existing lens.

The only way to concentrate the light more is to use something that
is not a lens and which won't create an image of the sun. You need
something that will deal with the light (optics) but that doesn't
create an image like a lens (non-imaging).

Here are some random examples care of google.com
http://www.grc.nasa.gov/WWW/RT1997/5000/5490donovan.htm
http://www.grc.nasa.gov/WWW/RT2001/5000/5490wong1.html
http://www.shap.it/ENG/solar_tower.htm
http://www.backwoodshome.com/articles/radabaugh30.html

This might not be your problem though, as the other fellow said.
You might just need a better insulated or less reflective target.

Anthony

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