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 $(b_{H})/(b_{L})$, at given distances from a city, between the artificial sky luminance $b_{H}$ produced by its upward light emission between a given threshold angle $\theta_{0}$ and the vertical and the artificial sky luminance $b_{L}$ produced by its upward light emission between the horizontal and the threshold angle $\theta_{0}$. 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\deg ~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\deg ~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. 
P. Cinzano (1), F.J. Diaz Castro (2)((1) Dipartimento di Astronomia,   Padova, Italy, (2) Oficina Tecnica para la Proteccion...