Reducing the trespass of lighting into areas that are not intended to be lit

Night-time light pollution is exacerbated by poor lighting design, installation and maintenance resulting in light emitted in unnecessary or unwanted directions. Resolving this problem provides a ready means of dramatically decreasing the impacts of artificial light at night at a local scale. Indeed, reduction of light trespass has been shown to reduce the impacts of artificial light at night on organisms.

The direction of light is an important factor in trespass. Horizontal and near-horizontal light emittance produces more skyglow than that emitted upward, and much more than light emitted downward. Horizontal light increases the visibility of light sources from a distance, increasing the potential to disrupt animal navigation, and significantly increases the illuminated area, in turn increasing the encroachment of light into adjacent unlit areas. Horizontal and vertical light may come from illuminated advertisements, architectural lighting and road vehicles, and unintentionally from street lighting.

Reducing light trespass may not only reduce the ecological impacts of artificial light, but also has economic cost-saving benefits in that more focused lighting means a lower luminous flux is necessary to illuminate a given area to a required intensity.