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RE: Fresnel lenses again

To Brian,

>>Another problem that we haven't thought of when using mirrors or lenses is
>>that they absorb some of the energy of the beam, this changes the density 
>>the lens/mirror. The result is that unwanted bumps arise and that the
>>focus-distance changes.
>Density change?  Why would the lense/mirror, have a change in density.  Or 
>are you talking about changing the density that the design calls for?  Why 
>would there be bumps?

When some of the energy of the beam is absorbed, that means the lens/mirror
get a bit warmer (or hotter). When things get warm their density decreases
and their shape changes. Chances are that the material where the lens is
constructed from is not equal everywhere and probably neither is the density
of the beam.
So unequal amounts of expansion will occur making the problem even worse.