[Paper Review] The Artificial Sky Luminance And The Emission Angles Of The Upward Light Flux

P. Cinzano, F. J. Diaz Castro|arXiv (Cornell University)|Nov 19, 1998

Impact of Light on Environment and Health2 references12 citations

TL;DR

This paper models how upward light emissions from cities contribute to artificial sky luminance at different elevation angles, showing that light emitted between 0° and 10° above the horizon contributes disproportionately to skyglow at distant sites. The key finding is that upward emissions below 10° are as impactful as all higher-angle emissions combined, necessitating strict control—limiting such emissions to no more than 10% of total upward light—for effective skyglow mitigation near dark-sky sites.

ABSTRACT

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 $θ_{0}$ and the vertical and the artificial sky luminance $b_{L}$ produced by its upward light emission between the horizontal and the threshold angle $θ_{0}$. 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°~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.

Motivation & Objective

• To quantify the relative contribution of upward light emissions at different elevation angles to artificial sky luminance at various distances from a city.
• To identify which emission angles are most responsible for skyglow at distant observation sites.
• To inform light pollution control strategies by determining the threshold angle below which emissions must be minimized to protect dark-sky sites.

Proposed method

• Developed detailed atmospheric light pollution propagation models to simulate sky luminance at different distances from urban centers.
• Calculated the ratio $ b_H / b_L $, where $ b_H $ is luminance from emissions between $ \theta_0 $ and the vertical, and $ b_L $ is from emissions between the horizontal and $ \theta_0 $.
• Varied the threshold angle $ \theta_0 $ to assess the relative impact of high-angle vs. low-angle upward emissions.
• Assessed the contribution of spill light from luminaires to low-angle emissions (0°–10°), particularly in relation to fixture shielding.
• Used numerical simulations to evaluate how the relative importance of different emission angles changes with distance from the city.
• Evaluated the effectiveness of fully shielded luminaires (e.g., flat glass or asymmetric spotlights) in minimizing low-angle upward emissions.

Experimental results

Research questions

• RQ1 How does the contribution of upward light emissions at different elevation angles to artificial sky luminance vary with distance from a city?
• RQ2 What is the relative impact of light emitted between 0° and 10° above the horizon compared to emissions at higher angles?
• RQ3 To what extent do spill light from luminaires dominate low-angle upward emissions?
• RQ4 What fraction of total upward light emission must be restricted below 10° to effectively reduce skyglow at distant sites?
• RQ5 How effective are fully shielded luminaires in minimizing low-angle upward light emissions?

Key findings

• At distances beyond a few kilometers from a city, light emitted between 0° and 10° above the horizontal contributes as much to artificial sky luminance as all higher-angle emissions combined.
• The ratio $ b_H / b_L $ decreases with increasing distance, indicating that high-angle emissions become less significant relative to low-angle emissions at greater distances.
• Low-angle emissions (0°–10°) are predominantly caused by spill light from improperly shielded luminaires.
• To protect dark-sky sites, upward light emissions below 10° must be limited to no more than 10% of the total upward flux.
• Fully shielded luminaires—such as flat glass fixtures or asymmetric spotlights installed without tilt—are essential to minimize low-angle upward light emissions.
• The study confirms that controlling low-angle emissions is critical for effective light pollution mitigation, even when total upward light is reduced.