Lighting ordinances enacted across the United States in recent years have sought to reduce light pollution, such as skyglow, which is light emitted into the sky that obscures a view of the stars, and light trespass, which is light emitted onto neighboring properties.

To be able to specify outdoor lighting that does not produce skyglow and/or light trespass, lighting designers require tools they can use to quickly evaluate products. Years ago, the Illuminating Engineering Society (IES) produced a classification system for outdoor light fixtures based on “cutoff,” which addresses uplight that can produce skyglow.

Imagine an area fixture with a horizontal line running through it. A full cutoff fixture emits no light at or above the horizontal line. In other words, no light is emitted up. A cutoff fixture emits light at or above the horizontal at an intensity of light less than 2.5 percent. A semi-cutoff fixture emits light at or above the horizontal at an intensity of light less than 5 percent. And noncutoff has no limit.

This system addresses uplight, but not backlight, which can cause light trespass if uncontrolled. It can also cause glare or excessive brightness in the field of view, which can be irritating and even disabling.

As a result, the cutoff system is being supplanted by a new system developed jointly between the IES and the International Dark-Sky Association (IDA) as part of the Model Lighting Ordinance (MLO), an outdoor lighting rule that can be used by communities seeking guidance based on input by both the astronomical community and lighting professionals. Called BUG, it goes beyond the cutoff system by addressing backlight (B), uplight (U) and glare (G). BUG ratings allow designers to assess how well a given fixture controls stray light and then compare products based on these values.

Imagine a sphere enclosing a lighting fixture, which covers all potential directions that light can be emitted. The BUG system divides this sphere into three areas in which light may be emitted: frontlight, backlight and uplight. Each of these areas is further divided into angles, enabling more detailed analysis. The frontlight and backlight areas, for example, are divided into very high, high, medium and low angles, enabling a glare rating to be produced based on the amount of light emitted in the high and very high angles.

How is BUG used?
The rating system includes a table of maximum light output (in lumens) by angle of light emission for five lighting zones in the MLO. These zones, LZ0 through LZ4, cover areas with no ambient lighting (requiring a higher degree of stray lighting control) through areas with high ambient lighting, which accept a lower degree of stray light control. If a fixture exceeds the maximum lumens for frontlight, backlight or uplight based on its photometric testing, the fixture is bumped up to the next zone, gradually limiting its application to areas that do not require good control of stray light. The BUG rating, in other words, expresses the lowest level lighting zone in which the fixture can be used and still comply with the MLO. A good BUG rating indicates a higher amount of control of stray light and resulting higher suitability for applications requiring that control.

This use of BUG is actually relatively simple: Once the lighting zone is determined, the designer checks the BUG rating to determine whether the product is suitable for that zone.

Another application for BUG is more detailed analysis and comparison that can aid in product selection. It goes beyond uplight and works well for lamp-integrated fixtures, such as light-emitting diode (LED) fixtures (for which the previous cutoff system does not apply). Using the BUG rating, a designer gains a snapshot of each aspect of fixture performance for evaluation and comparison that goes beyond the traditional metric of overall fixture efficiency. While fixture efficiency tells us how much light is being emitted in all directions, BUG indicates how much light is coming out of the fixture in each direction and at specific angles. This data can help contractors recommend products that are appropriate for specific applications.

While BUG provides a more complete description of the product, designers should not regard the rating as an absolute predictor of suitability. It does not cover variables, such as color temperature, mounting heights, etc. Plus every application is different; how well a given product will perform in that application will depend on the application characteristics and how the fixture is installed. What’s more, it still has limited use. For example, residential products are not getting BUG ratings because their manufacturers typically do not conduct photometric testing.

The bottom line is BUG is more complex but also more complete. As outdoor lighting increasingly features lamp-integrated LED fixtures and the IES/IDA MLO becomes adopted by jurisdictions, its use is almost certain to grow.

Thanks to Architectural Area Lighting, BetaLED and Cooper Lighting for their assistance in developing this article.


DILOUIE, a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at www.zinginc.com.