Calculated sky brightness at night, without clouds

The sky brightness is a measure of how dark it is at night. It concerns the luminance (luminous intensity per unit area) in the point in the sky when you look straight up (the so-called Zenit). The luminance varies greatly at a location. In this map you can see the calculated sky brightness, expressed in milicandela per m2 for the situation:
[1] in the evening without clouds.
[2] in the night without clouds.
[3] in the evening with clouds.
[4] in the night with clouds.
N.B. Only Dutch sources are included in the calculation. Sources from abroad are missing. As a result, the calculated sky brightness in the boundary region is probably an underestimation of the actual sky brightness.