Compatibility of road lightingwith 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. List of symbols a ratio LA/LB between artificial and natural sky luminances CO,CS contrast of O and S CTH limiting contrast for a just detectable star ES, ER illuminancesontheentrancepupildue to stars S and R (lx) F fraction of the luminous flux not screened in town cavities T,U,R luminous fluxes installedandemitted/ reflected upward by a town assimi-lated to a diffusing source (lm) iU,i T luminous flux emitted upwards and in the full space by the ith luminaire (lm) emission angle in the CIE (C, ) coordinate system (8) LB natural background luminance of the sky (cd/m2) LA artificial sky luminance generated by external lighting (cd/m2) LR, LU artificial sky luminances generated by the reflections of the lit surfaces and the upwards emissions by the lumi-naires (cd/m2) LO luminance of an object (cd/m 2) MTH limiting magnitude for a just detectable star (mag) MTH decrease in limiting magnitude due to the light emitted upwards (mag) O object MS,MR magnitudes of stars S and R (mag) average reflection factor of the all lit surfaces in a town j average reflection factor of a lit surface j R reference star Rn average ratio of upward emission of all luminaires in a town Ri ratio of upward emission of luminaire j S a star 1