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Compatibility of road lighting with star visibility

The influence of city lighting on astronomical observations is analysed by using a single parameter for a whole town based on the upward light from the luminaires. This parameter separates the influence of the upward light on the maximum visible stellar magnitude and on energy saving from the unavoidable reflections from the illuminated surfaces, independently of the way light is scattered in the atmosphere. It is shown that extreme reduction of spill light leads to no appreciable benefit for astronomy and increases installation and energy costs.