> But I'm wondering: while you cite the Preetham paper you do use
> different values for the A to C parameters and also for the Perez
> formulation.

I took the values that the Stellarium source code used rather than the 
paper itself, I guess the authors of Stellarium tweaked them a bit. I 
guess you can use whichever set look better for you.

> Also I'm wondering about the multiplication factor of 1000 for the
> zenith luminance. As far as I understand it this formula return the
> luminance in cd/m2 therefore this additional multiplication just makes
> the exposure factor unnecessary small.

The formula in the paper gives the luminance in K cd/m2, that's why the 
1000 factor is in there.

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