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