/usr/share/radiance/blinds.cal is in radiance-materials 4R1+20120125-1.1.
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Compute Venetian Blinds angle to exclude direct sun.
3/19/93
Formula by Kostantinos Papamichael,
Translated by Greg Ward and Saba Rofchaei
}
{
Input variables are solar altitude and azimuth in degrees:
SALT - solar altitude (degrees)
SAZI - solar azimuth (degrees from South, West is positive)
}
L = .625; {slat's width}
D = .45833; {distance between slats}
ORIENT = 0.0; { Window orientation }
DEGREE : PI/180.0;
tan_lambda = tan(SALT*DEGREE) / cos((SAZI+ORIENT)*DEGREE);
lambda = atan(tan_lambda) / DEGREE;
a = tan_lambda*tan_lambda + 1.0;
b = -2.0*(D/L);
c = (D/L)*(D/L) - tan_lambda*tan_lambda;
det = sqrt(b*b - 4.0*a*c);
{calculating slats' angle corresponding to solar position}
POS1 = asin((-b+det)/(2.0*a)) / DEGREE;
POS2 = asin((-b-det)/(2.0*a)) / DEGREE;
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