{
	These formulas compute the index of refraction and
	normal transmission for a pane of glass given the
	measured normal transmittance and reflectance values,
	Tn and Rn, respectively.

	These formulas are approximate -- the exact solution
	involves a quartic equation and seemed more trouble
	than it was worth.  If you know the index of refraction,
	you are better off with the formula in trans.cal.

	Oct. 2002	Greg Ward
}
				{ formula for normal transmission }
tn = Tn / (1 - Rn);
F = Rn / (1 + tn*tn);
n = (1 + sqrt(F)) / (1 - sqrt(F));