Sky Glow calculations
I would recommend the PiNoir Camera. Since it has no IR-filter attached, it's perfectly made for nightly observations. If you need the Raspberry just to calculate Skyglow in general, you could try to collect all light and bundle it to a beam, then measure its intensity.

What also came to mind is: When calculating the rating of the night, or a point in the sky, consider moon phases in your calculations. A bright and full moon will significantly raise average sky-glow and vice versa.