Re: ATM Determining the index of refraction of glass

[Anthony Stillman <atmer@flash.net>]

Doug Miller wrote:
>...the glass manufacturers melt data (often not accurate enough in any
>case) or to
>fashion a prism from a piece of the glass and send it to a glass
>manufacturer or
>independent lab for measurement.  If you don't have the glass paperwork to
>trace it
>to the exact melt, it is not possible to use the manufacturers melt data.
>Cutting a
>prism for the measurements may be feasible, but will cost you aperture.


By in large I agree with everything Doug has written and I would like to
add that it is not necessary to cut and fashion a prism.  Certainly this is
the perferred method and it is in fact how melt data is usually created.
An alternative for percision measurement is the use of a V-block made from
the hypotenuse of two right angle prisms.  The sample to be tested should
be polished on at least one face and the edge.  Also the edge must be
perpendicular to the polished face.  The sample is then placed in the
V-block and light of various wavelength passed through the assembly.  If
the prisms and sample are all of the same glass the light passes through
undeflected.  If the sample has a higher or lower index than the prisms the
light will be deflected.  This is know as a Hilger-Chance V-block
Refractometer.  It can accurately measure index to four places with even a
short deflection measuring arm.  The nice feature of this method is that
the sample need only have two polished perpendicular faces and only one
needs to be resonably flat.  The other can be round, i.e. the edge of the
lens to be.  Many other methods are well developed and are described in the
various journals.  Most depend on the accurate fabracation of a prism.
Having attempted this a few times, I recommend it to only the most hardened
glass pushers.  Also, an interferometer helps bunches for getting the
angles correct.

Anthony