Have you ever seen the satellite image of the US at night? You can tell where all the cities are because of the lights. I suppose you can use that image and compare with the population in each city. The amount of light in each city would be difficult to quantify though.

What are you trying to compare? Are you comparing cities? Then you would need data on each of the cities. If you are compaing towns, I suppose you can drive around to differnt towns at night and look at what is the dimmest star you can make out in the sky (do this on the same night because amount of moisture in the atmosphere will also affect what you can see, not just light pollution)

Using something like light meter would not be good for this because it will be too specific– light conditions will be different at locations just few feet away.

Sorry, I probably haven’ t been that much help…