/usr/share/radiance/trans2.cal is in radiance-materials 4R0+20110410-1build1.
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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));
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