Every year in April, we recognize International Dark Sky Week— a worldwide celebration of the night! It’s also a great time to build awareness about how light pollution affects our nighttime environments, one of the most pressing topics in our industry when designing for outdoor spaces.

A critical question for specifiers is, “How can we produce sufficient light where we want it while minimizing the amount of light anywhere else?”

In our industry, the Dark Sky specification is a set of criteria used to measure the quality of a building’s exterior lighting to protect the night sky. It is based on limiting the amount of light emitted from outdoor luminaires, shielding light sources from the sky, and directing light downward.

This is achieved using luminaires with full cut-off shields to prevent light from escaping above the horizontal plane into the night sky above.

Unwanted uplight is just part of the picture, as additional components of light pollution include glare, skyglow, light trespass, and clutter. These are lighting terms you’ve likely often heard in this context, but here’s a refresher just in case:

• Glare: excessive brightness that causes visual discomfort
• Skyglow: brightening of the night sky over inhabited areas
• Light Trespass: light falling where it is not intended or needed
• Clutter: bright, confusing, and excessive groupings of light sources

Graphic from Selux Lighting illustrating elements of light trespass, glare, backlight, and uplight outside a home.

Dark Sky-compliant luminaires use shielding to prevent light from spilling out to the sides in unwanted directions and are designed to use the minimum amount of light necessary for the task at hand. Additionally, the fixtures are designed to use warm-colored light, which is less intrusive than cool-colored light. They use energy-efficient LED bulbs that produce less heat and use less energy than traditional lighting fixtures.

DarkSky-Approved Luminaires

DarkSky International, formerly known as the International Dark-Sky Association (IDA), is considered to be the leading organization tackling light pollution. The nonprofit organization has over 193,000 supporters, members, and advocates in more than 70 countries. It provides leadership, tools, and resources for individuals, policymakers, and industry to reduce light pollution and promote responsible outdoor lighting that is beautiful, healthy, and functional.

DarkSky also certifies commercial, industrial, and residential outdoor lighting that reduces light pollution.

Reducing light pollution is a core environmental commitment of our friends at Selux, who make a variety of DarkSky luminaires approved by DarkSky International—many of which have an uplight rating of U0. The Selux team compiled this great resource brochure for individuals wanting to learn more about Dark Sky and light pollution.

The DarkSky Approved program provides objective, third-party certification for products. To be DarkSky approved, lighting products must:

• Photometric files are needed to validate performance characteristics.
• There is an uplight allowance of 0.5% of total output or 50 lumens; whichever is lower, with no more than 10 lumens in 90-100 degree UL zone. This is allowable uplight as a byproduct of the structure and not the source.
• Roadway & Area luminaires must have a pre- or post-installation shielding option.
• Luminaires are required to have fixed mounting to ensure they are installed as tested. It is allowable to have +/- 10-degree adjustment for leveling if needed.
• Luminaires must have dimming capability to 10% of full rating.
• Light sources shall have a maximum Correlated Color Temperature of
• Luminaires must have Safety Certification by an independent laboratory.

You can search for products and companies with the DarkSky-approved seal here.

The BUG Rating System

Robert Hawkins, our resident lumen counter and Manager of Application Engineering, would say that talking about Dark Sky is meaningless without the context of the IES BUG rating system.

The Illuminating Engineering Society (IES) developed the BUG ratings, short for Backlight, Uplight, and Glare (BUG), as a tool to measure the amount of light in a given area or Lighting Zone.

This luminaire classification system allows specifiers to understand whether a light fixture suits different outdoor applications based on what’s set by a jurisdiction or lighting ordinance.

For a more thorough definition of the system, you can refer to the current ANSI/IES RP-8-22: Roadway and Parking Facility Lighting Standard, an essential source that covers all aspects of driving at night, including roadway lighting, including streets and roadways, parking lots, intersections, toll plazas, tunnels, and work zones.

Then, these zones are further divided into front, back, and uplight portions according to the following detailed views, which show how we divide the light into angular components:

Figure 2-26: Plan view (top) and section view (bottom) for forward solid angle. Source: ANSI/IES RP-8-18: Roadway and Parking Facility Lighting Standard

Pictured above,”Figure 2-28: Plan view (top) and section view (bottom) for uplight solid angle. Source: ANSI/IES RP-8-18: Roadway and Parking Facility Lighting Standard

Pictured above,”Figure 2-27: Plan view (top) and section view (bottom) for back light solid angle. Source: ANSI/IES RP-8-18: Roadway and Parking Facility Lighting Standard

With those zones in mind, we can look at the following chart to understand acceptable lumens for each rating:

Pictured above, “ANSI/IES RP-8-18+AI Design and Maintenance of Roadyway and Parking Facility Lighting.” Source: ANSI/IES RP-8-18: Roadway and Parking Facility Lighting Standard

So, the B-U-G rating system, when combined with values allowed and/or set by a jurisdiction or a document like the MLO (Model Lighting Ordinance), can guide you in selecting a proper fixture for your project.

Upcoming Dark Sky Lookbook from TLA

This barely touches the topic’s surface, so if you’re working on a project and need help with more detailed calculations like backlight and uplight ratings, glare ratings, and U-zero/full cutoff requirements, we’re here to help.

In fact, TLA is about to release a brand-new Dark Sky Lookbook (available May 1, 2024), which includes many products and ideas for outdoor fixtures that can help you meet your challenges. Talk to your TLA Sales Rep or email us (getinfo@thelightingagency.com) to learn more and request a copy.

Robert Hawkins, our resident lumen counter and Manager of Application Engineering, had this to add for anyone starting an outdoor lighting project:

“Make sure to check in with your local planning authority for their take on their own requirements. That will guide you in choosing appropriate fixture performance characteristics for your application.”

What’s the big picture?

According to the International Dark-Sky Association, there are over 1,000 certified Dark Sky Places worldwide, with more than 200 located in the US. We want to keep them that way and grow that number. The U.S. Department of Energy estimates that over 1 million outdoor lighting fixtures have been retrofitted with Dark Sky-compliant lighting in the U.S.

It’s all about the big picture. Dark Sky specifications help to preserve the natural night sky and protect wildlife from the effects of artificial light for generations to come.

It’s a way of showing respect for the environment and the natural world we love and cherish. Light pollution affects us all and can hurt human health, wildlife, and the environment. By following these specifications, we are taking a step towards preserving the natural beauty of the night sky and protecting the environment for future generations.