[Laser] Re: Fresnel lenses and dispersal angle

[Chris L]

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.