Light pollution can result in significant negative impacts, especially to wildlife and vegetation.
‘Tis the holiday season, which means homes and business are festooned with holiday lights, many of which contain LEDs. These decorations light up the night for just a few weeks, but what about all of the permanent lighting installations that light up the night year-round?
As anyone who has ever flown over a city at night knows, the sight of all those lights is captivating, but unfortunately, they contribute to sky glow, which can result in significant negative impacts, especially to wildlife and vegetation.
As the lighting industry continues to transition away from legacy light sources, such as high pressure sodium or metal halide for outdoor lighting, there has been increasing attention paid on ways to minimize the light pollution caused by outdoor LED installations.
A recently-published study of municipal street lighting in Tucson, AZ by the GFZ German Research Centre for Geosciences provides insight into how good LED luminaire design can minimize light pollution. Tucson is at the forefront of the transition to LEDs as nearly all of its street lighting has been upgraded and includes both wireless communication and dimming capabilities. The study, conducted over 10 days in March and April of 2019, used the US Suomi National Polar-orbiting Partnership (NPP) satellite to acquire images of the Tucson area at night. Light levels for approximately 14,000 of a total of 19,500 streetlights were set at full brightness for some nights and 30% of full brightness for others.
These images, captured by NASA satellite Suomi NPP, show night-time lighting for streetlights in Tucson set at full brightness and 30% brightness.
The research team concluded that approximately 20% of night-time lighting in Tucson is attributable to street lighting, with the remainder generated by signs and facades, shop windows, sports fields, etc. These conclusions, if transferable to other cities, demonstrates the degree to which LED lighting design can potentially address light pollution concerns.
Arguably, the most important tool in the luminaire designer’s arsenal in terms of minimizing light pollution, or stray light, is known as the “BUG” rating, which stands for back light, uplight, and glare. The BUG rating system was developed by the Illuminating Engineering Society (IES) in conjunction with the International Dark Sky Association (IDA) to provide a consistent methodology for assessing the amount of stray light escaping from outdoor lighting.
Stray light in outdoor settings is undesirable not only because it contributes to sky glow, but also because it results in unnecessary energy consumption. Examples of stray light include light trespass (i.e., light cast in unintended locations, such as from a streetlight entering a nearby window), uplight that contributes to sky glow, and glare (i.e., excessive light at eye level leading to discomfort or visual impairment).
The BUG rating is derived based on measured lumen output in each of the three zones pictured in the diagram below.
The BUG rating is derived based on measured lumen output in each of these zones.
Back light, as its name implies, refers to light emitted behind the luminaire. The back light region is further divided into four subzones corresponding to increasing angles of emittance from 0 degrees (i.e., light is directed downward toward the ground), to 90 degrees (i.e., light is mostly directed outward parallel to the ground). Similarly, the glare zone is also divided into four subzones corresponding to increasing emittance angles from low glare to high glare. The uplight zone includes just two subzones, one for light that is mostly directed upward vs. light that is mostly directed outward.
The BUG rating is determined based on the measured lumens for each of the subzones described above as referenced in look-up tables provided in IES TM-15-11, Luminaire Classification System for Outdoor Luminaires, with an assignment of 0–5 for each parameter. As an example, a BUG rating of B0 U0 G0 would be assigned to a luminaire with highly-directional downward light output. A BUG rating of B2 U0 G2 would be assigned to a luminaire that emits most of its light output both forward and backward, but not upward. Note that BUG ratings are just one of the factors in determining suitability of a luminaire for a specific lighting application, but in general, the lower the BUG rating, the lower the potential for light pollution.
This article was originally published on EDN.
—Yoelit Hiebert has worked in the field of LED lighting for over 10 years and has experience in both the manufacturing and end-user sides of the industry.