/usr/share/radiance/lumdist.cal is in radiance-materials 4R1+20120125-1.
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Conversion from zonal lumens to watts/steradian
}
lum2watt(t0,t1,l) = l / 683 / (omega(t1)-omega(t0)) ;
omega(d) = 2*PI * (1-cos(d*PI/180)) ;
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