[Radiance-general] question about sky.mat

[imenslama at voila.fr]

Hi community,
In radiance tutorial, for simulation of daylighting, we must create a file named sky.rad and an other file named sky_overcast.mat. But I can't find a description of the second file, I mean sky_overcast.mat. I know that is a material for an overcast sky but what should we write to define this file? Please, can you gives me an example?
thank you
imène
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