[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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