# 10k lux overcast sky implementation only yields 9177?

I use gensky to create a 10k lux CIE standard overcast sky with the -B command:

gensky 6 1 14 -B 55.866 -c -g 0.2

I use the following commands to generate a center line of daylight at working height (.85m) in a room:

cnt 11 \

rcalc -e ‘\$1=2;\$2=\$1/2+.5;\$3=.85;\$4=0;\$5=0;\$6=1’ \

rtrace -I -dt 0.03 -ds 0.02 -ab 3 -aa .15 -ar 128 -ad 1024 -as 512 -h -w -oov scene.oct \

rcalc -e ‘\$1=\$2;\$2=179*(.265*\$4+.670*\$5+.065*\$6)’ > lux.dat

But when I remove everything from the scene but the sky, I only get approx 9177 lux on my centerline. According to theory I should be getting approx 10000. What is wrong?

Christian Hviid

Hello Christian,

My suspicions concerning your problem go towards the fact that there apparently are many ways to convert from RGB radiances to Lux (using slightly different RGB weights) ... moreover it is my general experience that Radiance, when looking straight into a light source, produces radiances that are slightly less than the source radiance specified in the scene file. For example when I point the "camera" straight into an RGB = 1 W/(m^2 * sr) sky dome I get radiances of 0.9941 (although that may be due to finite precision in the .hdr file format).

Seeing that you are from Denmark (as well) I took the liberty to call you on the phone (your company switchboard informed me you were away on travel) ... if you feel like discussing some of these things (not that I necessarily have the answer to your problem) we can contact each other when you get back.

Best,
Claus

Christian Anker Hviid wrote:

···

I use gensky to create a 10k lux CIE standard overcast sky with the -B command:
gensky 6 1 14 -B 55.866 -c -g 0.2
I use the following commands to generate a center line of daylight at working height (.85m) in a room:
cnt 11 \
> rcalc -e '\$1=2;\$2=\$1/2+.5;\$3=.85;\$4=0;\$5=0;\$6=1' \
> rtrace -I -dt 0.03 -ds 0.02 -ab 3 -aa .15 -ar 128 -ad 1024 -as 512 -h -w -oov scene.oct \
> rcalc -e '\$1=\$2;\$2=179*(.265*\$4+.670*\$5+.065*\$6)' > lux.dat
But when I remove everything from the scene but the sky, I only get approx 9177 lux on my centerline. According to theory I should be getting approx 10000. What is wrong?