[Laser] Re: Fresnel lenses and dispersal angle
TWOSIG at aol.com
TWOSIG at aol.com
Thu Jun 16 19:22:05 EDT 2005
Chris and all.
In the text below you stated that you chose your source LED because "Its
source size matches the size of the fuzzy focal patch on many of the better
fresnels." That stems from the ability of fresnel lenses to focus a point source
at infinity to that "fuzzy focal patch".
I hope that you are not trying to imply that using a fuzzy source will some
how focus at a point at infinity. Quite the contrary. A point source will
project to an "area" at infinity due to the imperfections on any real lens.
Nearby point sources will map to other "areas" at infinity, some of which MAY
overlap. Without knowing the specific errors on the subject lens, I would
have to assume that a fuzzy source would spread your emissions over a larger
target area that a more compact source.
I would agree with you that using ideal lenses and point sources can give
you very misleading results compared to real lenses and real sources. As
always in engineering you need to understand the limits of the errors you ignore
in your calculations.
73
James
N5GUI
In a message dated 6/16/2005 12:28:44 P.M. Central Standard Time,
vocalion1928 at hotmail.com writes:
Well I'm going to throw a spanner in all of this!
I absolutely agree with everything that all of you have said on fresnels
regarding dispersal angles, but there's a PROBLEM....
You're all giving the solution for dispersal through an IDEAL lens, and
moulded fresnel lenses aren't anywhere near ideal. Most of them can focus a
point source at infinity to an area no smaller than 1 or 2 mm in diameter
owing to inevitable surface irregularities. For very small sources, there is
a definite limit to the minimum dispersal angle you can achieve...
So, to your calculated dispersal, you have to ADD the 'inaccuracy factor',
which depends on the quality of your particular lens. I've noticed that
early fresnels, such as those in 1960s 'Overhead' transparency projectors
are not corrected for spherical aberration, in many cases. This would
exacerbate things further.
This is one reason why VK7MJ, VK3KAU and I have opted for the Luxeon III,
rather than lasers, as a modulated source. Its source size matches the size
of the fuzzy focal patch on many of the better fresnels. If you don't use a
source 1 or 2 mm in diameter, you end up with an objective that appears to
be unevenly illuminated when viewed from a great distance - and that drops
the optical gain and increases path scintillation.
Mathematical models are usually a simplification of real situations. This
'dispersal angle' stuff is a case in point.
Hope this doesn't throw too much of a spanner in the works...
All the best,
Chris Long.
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