PS I found a nice presentation from the NIST group explaining the theory
behind the cubic birefringence effect:
http://math.nist.gov/mcsd/Seminars/2004/2004-06-10-shirley-presentation.
pdf . Slide 33 has some nice graphics summarising the birefringence
effect in all crystal systems.
-- Ian
> -----Original Message-----
> From: [log in to unmask]
> [mailto:[log in to unmask]] On Behalf Of Ian Tickle
> Sent: 13 June 2008 11:20
> To: Ethan Merritt
> Cc: [log in to unmask]
> Subject: RE: [ccp4bb] birefringent spacegroups
>
>
> > -----Original Message-----
> > From: [log in to unmask]
> > [mailto:[log in to unmask]] On Behalf Of Ethan Merritt
> > Sent: 12 June 2008 19:41
> > To: Multiple recipients
> > Cc: [log in to unmask]
> > Subject: Re: [ccp4bb] birefringent spacegroups
> >
> > But the ellipsoid is only a convenient approximation based on
> > properties
> > evaluated at 3 orthogonal axes. It is not a complete
> > description, it is
> > simple model. More complex models may describe more complex
> > properties,
> > as is the case in the paper that Ian refers to.
> >
> > If "birefringence" is formally defined in terms of the
> approximating
> > ellipsoid, this is correct. I do not know if this is the
> case or not.
> > But if birefringence is generalized to mean "optical index
> varies with
> > incident illumination vector" then I believe all protein crystals
> > are birefringent.
> >
> > I may be talking nonsense here. If so, I welcome the opportunity to
> > learn better.
> >
> > Ethan
> >
>
> No I think you're quite right, birefringence clearly refers
> to any case
> where the refractive index is anisotropic for any general pair of
> directions, not just in specific orthogonal directions. In
> any case the
> birefringent direction [110] in cubic is orthogonal to the
> non-birefringent direction [001] so the old definition is still
> applicable anyway. BTW here's a patent issued for a method to get
> around the problems in lithographic fabrication of
> semiconductors caused
> by optical elements made from birefringent cubic crystals, so it's a
> very real problem, not just one of theoretical interest!
>
> http://www.patentstorm.us/patents/6844972/fulltext.html
>
> Cheers
>
> -- Ian
>
>
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