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2 Layer AR Coatings (V-coat)

Benefits:  More material choices for index.  More refinement options.  Easy to
                    manufacture.

Limitations:  Narrow band.

Choosing Layer indices:

A straight forward way of determining if the materials you have on hand, combined, would make a decent AR, is to use a Schuster diagram. (reference 1)

Below, the Schuster diagram has been calculated
for glass (n=1.52) & germanium (n=4.0) in an air medium (nm=1.0).

The calculation requires that of the following three expressions:

All three must be positive
or,
Any two are negative and the third positive.

Layer thicknesses that are equal optical thickness (reference 2)

To calculate the refractive index requirments
for a 2 layer AR coating where both layers
are of equal optical thickness (each are QWOT's, see terminology)
then the use the following: