The Artificial Sky Luminance and the Emission Angles of the Upward Light Flux

The direction of the upward light emission has different polluting effects on the sky depending on the distance of the observation site. We studied with detailed models for light pollution propagation the ratio bH bL , at given distances from a city, between the artificial sky luminance bH produced by its upward light emission between a given threshold angle θ0 and the vertical and the artificial sky luminance bL produced by its upward light emission between the horizontal and the threshold angle θ0. Our results show that as the distance from the city increases the effects of the emission at high angles above the horizontal decrease relative to the effects of emission at lower angles above the horizontal. Outside some kilometers from cities or towns the light emitted between the horizontal and 10 is as important as the light emitted at all the other angles in producing the artificial sky luminance. Therefore the protection of a site requires also a careful control of this emission which needs to be reduced to at most 1/10 of the remaining emission. The emission between the horizontal and 10◦ is mostly produced by spill light from luminaires, so fully shielded fixtures (e.g. flat glass luminaires or asymmetric spot-lights installed without any tilt) are needed for this purpose. An adequate protection of the sky with the aim to save citizen’s capability to see the heavens near or inside cities and towns requires the limitation of the upward emission at all emission angles. Nevertheless a specific limitation of light emitted at low angles over the horizontal is useful also in highly urbanized areas where an important fraction of the artificial sky luminance, even inside meansize cities, is produced by the sum of the contributions of a big number of sources in the surrounding land. The use of fully shielded fixtures can greatly reduce this contribution from outside.