The September 2023 issue of LD+A has an interesting article arguing that a 180-degree cut-off for street and area lights is an inadequate requirement. The article is written by Thomas Paterson, who lays out the following arguments for a tighter beam cut-off angle, such as 150 or 120 degrees:
Tolerance – A 180-degree cut-off doesn’t allow any tolerance for street pole angle or deformity, variation in luminaire leveling, and the practice of intentional tilting of luminaires up toward the roadway.
Wasted Horizon Light – Light at angles just below 90 degrees from vertically down is wasted light, in most cases. This light shines great distances before hitting the ground, is not on target, and adds insignificant light at the ground. Worse, it creates significant glare and light pollution for humans and wildlife. The author suggests the light should be kept to hitting the ground within a distance of 5X mounting height.
Reducing Wildlife Impacts – Most wildlife exists at heights below street and area lights. Reducing cut-off angles below 180 degrees will reduce near-horizontal light impacting wildlife, whether it be birds, bats, turtles, insects, plants, or other wildlife.
Improves City Skylines– Tighter cut-offs than 180 degrees will reduce glare ‘dots’ when viewing urban panoramas, improving the attractiveness of city viewing at night.
The complete LD+A article can be found here.
