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Calculate BRTDF of Transparent Insulation Materials
made up of many small tubes packed tightly together.
29 Nov 1991 Greg Ward and Raphael Compagnon
06 Feb 1995 Updated for Radiance 2.4 by Greg Ward
Apply with following BRTDfunc:
mod BRTDfunc tim1
10 0 0 0
stran stran stran
brtdf brtdf brtdf
tim1.cal
0
12 0 0 0 0 0 0 0 0 0 R T K
where:
R = diffuse reflectance when Ktan_t is zero
T = total transmittance
K = ratio of tube length to diameter
}
Ktan_t = arg(12) * Sqrt(1-RdotP*RdotP)/RdotP;
stran = if(1-Ktan_t, 2/PI*Acos(Ktan_t) - Ktan_t/PI*Sqrt(1-Ktan_t*Ktan_t), 0);
brtdf(lx,ly,lz) = if(lx*NxP+ly*NyP+lz*NzP, arg(10), arg(11)/(1-arg(10)))
* (1-stran)/PI;
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