Re: ATM spectacle lenes ep's

[atm@misterg.fsnet.co.uk (Andy Gray)]

On Mon, 04 Aug 2003 19:28:25 GMT, atm@misterg.fsnet.co.uk
(Andy Gray) wrote:

>On Sat, 2 Aug 2003 14:46:56 -0700,  "Bob May"
><bobmay@nethere.com> wrote:
>
>>FWIW, the lower refractive indexes are crown glasses while the higher ones
>>are called flints.
>
><Picky>
>Actually it is dispersion that differentiates crowns and
>flints - Flints have a 'V' value (Abbe number, etc) <55 for
>refractive index (nd) <= 1.60 and <50 for nd>1.60
></Picky>
>
>The distinction is completely arbitrary AFAIK, but it does
>aid discussion
>
>The difficulty in the original proposal to make EPs out of
>ophthalmic glasses is that (with the possible exception of
>bifocal segs) they are all crown glasses (nobody wants
>colour fringing in their eye glasses!). It might therefore
>be difficult to design in any colour correction without
>involving a piece of optical "flint". Some of the
>1.7/1.8/1.9 index ophthalmic crowns have got superb optical
>characteristics, however.
>
>There is next to no economic reason for making EPs, but for
>me, that's missing the point.....
>
>Andy
>
>Andy Gray, N. Wales, UK.