[Laser] Fresnel lens application

[John Schnurer]

	Dear Group,

	This brief communication refers to nearly Any lens.
The key idea here is Applied Science, read, use of the science.  in this
case, use of a lens.

	(A)	In this case the lens is a Fresnel Lens, and will be
abbreviated as FL.   The FL may be constructed to do the same general job
as nearly any simple or single lens.  A series of shallow  cuts, grooves
or molded-in locations are made in a material which is generally
transparent to or in the electromagnetic wavelength of interest.
More abbreviations:
	Electromagnetic wavelength 	=	WL
	Electromagnetic 		=	EM
	Lens				=	LN

	The grooves are generally triangular in shape if you were to look 
at a magnified cross-section.  Very much like a prism in shape and in
function.
	Frequently the grooves are in a series of concentric rings.
	The lens can be the same in general function as a Magnifying
lens, or convex in shape.  Or the lens can be like a Reducing lens, or
concave in shape.   A simple lens may be concave-concave, plano (flat)
concave or many variations.  The properties of BOTH FL and non FL lenses
depend on the intent of the designer, the material[s] used and the
quality of the construction.

	Idea:

	For this Brief discussion we will be referring to a Magnifying or
convex-convex lens.   These lenses are designed to bend light to a central
point or focus.

	The general idea of the Fresnel lens is a given groove which is
made as a prism also bends the light falling on it to a central point or
focus.  The FL is a series of tens or even hundreds of these
prism-shaped-in-cross-section grooves.  Each of these concentric rings has
a slightly different  pitch or angle... and so bends the light in
a manner ever so slightly different than its immediate meighbor.  A ring
toward the outside or nearer the edge of the FL bends the light a lot...
and the ring found half way toward the center bends the light less.... and
the rings near the center of the lens only need to bend the light a
liittle bit....

	Which side to use?


	This depends on the design of the lens!

	The distance?  Also depends on the design... and the use to which
you will put the lens.

	If you are going to use the lens to concentrate light on a
detector, for example, it is a VERY good idea set up the detector so you
can EASILY CHANGE the distance to the face of the lens.   AND:  Even
though there is some loss in ANY lens system, with mirrors being usually
the least lossy .... it is not a bad idea to provide for modification...
you may wish to use an aperature, beam stop or another lens.


	Diagram and some text:


	http://www.lanternroom.com/misc/freslens.htm