Controlling light has become increasingly important as lighting ordinances across the United States seek to address all sources of light pollution, including skyglow and light trespass. A three-year collaboration between the Illuminating Engineering Society (IES) and the International Dark-Sky Association took on the challenge of holistically measuring light outputs. The result is the BUG rating, a luminaire classification system that replaces existing cutoff and full cutoff terminology to more comprehensively address light pollution from all directions, not just up into the night sky.

BUG stands for backlight, uplight, and glare. Backlight is the light directed behind the fixture; uplight is any light directed upward above the horizontal plane of the luminaire; and glare is the amount of light emitted from the luminaire at high angles. “In short, the BUG system looks at the amount of light in certain angle ranges when it leaves the fixture,” explains Landscape Forms’ Lighting Team. “These angle ranges are broken into zones. The backlight, uplight, and glare ratings are assigned a value between 0 and 5 depending on the maximum amount of light in these zones based on thresholds defined by the IES. The rating gives specifiers, designers, and lighting professionals a snapshot of where the light is going after it leaves the fixture and provides them with a useful tool when first selecting or comparing fixtures for a project.”

“LEDs allow us to control light much better than old legacy lighting technology. The BUG rating system can help people understand in detail what that LED configuration and fixture design actually do in terms of light output in different directions.”

Specifiers can typically find BUG ratings for outdoor fixtures in the product specifications from lighting manufacturers. Landscape Forms includes BUG ratings on all lighting product data sheets.

How BUG Ratings are Assigned

Backlight is the amount of light emitted behind the fixture. Often used to help evaluate light trespass.

BH: Backlight High (60-80 degrees)
BM: Backlight Mid (30-60 degrees)
BL: Backlight Low (0-30 degrees)

Landscape Forms provides scenarios where backlight level is important to know. “Imagine a project where a fixture is to be located near a property line. If the criteria require little-to-no light at that property line, you may want a fixture with a low ‘B’ backlight rating. Conversely, a streetscape project where the fixture must light the street and also provide enough light to illuminate the sidewalk behind it may require a fixture with a higher ‘B’ backlight rating.”

Uplight is the amount of light directed above the horizontal plane of the luminaire. Often used to help evaluate skyglow.

UH: Uplight High (100-180 degrees)
UL: Uplight Low (90-100 degrees)

“When a fixture has a ‘U0’ rating, it tells us the fixture emits zero light up into the night sky and could be a candidate for projects requiring a Dark Sky-compliant fixture. This is one the most common uses of the uplight rating.”

Glare is the amount of light emitted from luminaires at high angles known to cause glare.

FVH: Forward Very High (80-90 degrees)
BVH: Backlight Very High (80-90 degrees)
FH: Forward High (60-80 degrees)
BH: Backlight High (60-80 degrees)

“A luminaire with a high ‘G’ rating has more light in these high-angle ranges than a fixture with a low ‘G’ rating. A lower ‘G’ rating means reduced high-angle brightness and possible reduction of glare, but reducing high-angle brightness can also impact pole spacing. A fixture with a high ‘G’ rating has the ability to throw light a greater distance than a fixture with a low ‘G’ rating, but visual comfort may be negatively impacted.”

The lumens in the zones defined by this system are intended to be just one of the metrics used to evaluate luminaire distribution and the potential for light pollution and obtrusive light. Detailed evaluation of the lighting should also take into account the overall system design, including luminaire locations, utilization of light where it is needed, lighting quality, visual tasks, aesthetics, safety requirements, and security issues.

Lighting Support Services

The Lighting Support Services team at Landscape Forms uses BUG ratings as they work with clients to help them select the best products for their specific projects and project goals. “As with lighting specifiers, we consider the BUG rating one tool in our toolkit that helps us select the proper fixture for a client’s application,” says Gleesing. “Our team partners with clients to review the site plan, works with them to understand the project objectives, and then develops photometrics that achieve an efficient and aesthetically appealing lighting layout.”