From: Mark Holm 
To: atm{at}shore.net
Reply-To: Mark Holm 


Richard F.L.R. Snashall


 >That brings up a question.  In AR coatings, I believe it is a half-wave
 >of material that gives minimal reflection.  What if you made the
 >cement a half-wave thick?  Would that reduce the reflections even
 >further?

I think you are right, but getting the cement a half wave thick, and
uniformly so across the whole aperture, is not a trivial problem.  As a
first condition, the radii of the surfaces would have to match to a pretty
tight tolerance, much tighter than for ordinary cementing.  BTW, the half
wave is measured with respect to the index of refraction in the coating
(glue).

There is a formula to use for finding the optimum index of refraction for
the glue.  It depends on the indices of both glasses.  It is approximate of
course, since the glasses also have different color dispersions.  Getting a
cement that exactly matches the optimum refractive index may not be
possible, but it is good
to be close.  I don't remember the formula but it is in one or more of the
ATM books and undoubtedly in many other optics books.

Mark Holm
mdholm{at}telerama.com

--- BBBS/NT v4.00 MP