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From: T.L.G.vanderLinden
To: KellySt; stevev; jim; zkulpa; hous0042; rddesign; David; lparker; 
Subject: Re: MARS HYBRID DESIGN II (First Draft)
Date: Tuesday, March 05, 1996 11:49PM

Brian 5:15 PM CT 3/5/96

> How about putting a Fresnel lens in the path to TC to refocus our beam. 
 The >lens could be of the thin variety, although I believe we would have 
some >power losses through the lens.

>As far as I know Fresnel lenses are not really lenses, that means they 
>magnify or bundle lightrays.
>These "lenses" are constructed in such a way that all outgoing light has 
>same phase at a predetermined distance (the "focusing" distance). This way
>the cancellation of two waves with a 180 degree phase difference is 
>So this method is only valid when normally INTERFERENCE makes de total
>brightness different.

All I know about Fresnel lenses is what my textbook (now in front of me) 
says.  Since this is a non-calculus based book, it may be leaving important 
detail out.  The following is a partial excerpt of its treatment of Fresnel 

"To focus or to produce a large beam of parallel light rays, a sizable 
converging lens is necessary.  The large mass of glass necessary to form 
such a lens is bulky and heavy; moreover, the thick lens absorbs some of the 
light, and is likely to show aberrations . . . .  Fresnel recognized that 
the refraction of light takes place at the surfaces of a lens.  Hence, a 
lens could be made thinner -- even flat -- by removing glass from the 
interior as long as this was done without changing the refracting properties 
of the surfaces."

Note they are "easily molded from plastic."