/usr/share/radiance/trans.cal is in radiance-materials 4R1+20120125-1.
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{
These formulas give exact results from the infinite
series solution for an uncoated pane of glass.
Inputs:
Tn = normal transmittance
n = index of refraction
Outputs:
rn = single-layer normal reflectivity
tn = transmissivity
Rn = normal reflectance
}
Tn = 0.88; { normal transmittance }
n = 1.52; { index of refraction }
sq(x) : x*x;
rn = sq((1-n)/(1+n));
tn = (sqrt(sq(sq(1-rn))+4*sq(rn*Tn))-sq(1-rn)) / (2*sq(rn)*Tn);
Rn = rn + rn*sq((1-rn)*tn)/(1-sq(tn*rn));
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