From: "Richard F.L.R. Snashall" 
To:  atm{at}shore.net
Reply-To: "Richard F.L.R. Snashall" 




Mark Holm wrote:
>
> 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).
>

The radii tolerance would be tight.  I surmised, though, that the thickness
could be fixed by depositing 1/2 wave of AR coating (or its equivalent 1/2
wave of glue) on spots about the periphery.  Then, push the glue to the
stops.  Of course, there would also be problems with thermal coefficient of
expansion differences if the surfaces were really strong.

> 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.
>

Born & Wolf say sqrt( n1 * n2 ).

                Rick S.

> Mark Holm
> mdholm{at}telerama.com
>
>
>
>

--- BBBS/NT v4.00 MP