Definitions of IESNA Luminaire Classification System (LCS) and BUG Ratings

The Illuminating Engineering Society of North America (IESNA) defines the light distribution and optical control of roadway and area lighting luminaires by the number of zonal lumens expressed as a percentage of the total lamp lumens. These classifications allow designers to choose the proper product to control spill light, light trespass, and sky glow. The LCS replaces the older IESNA Cutoff Classification System.

The BUG Rating system categorizes luminaires according to the amount of Backlight, Uplight and Glare that they have, and is calculated based on the number of lumens in the various LCS secondary zones. A luminaire's BUG Rating may be used to evaluate its optical performance related to light trespass,sky glow, and high-angle brightness control.

There are three major zones that designers need to reference when creating an outdoor lighting design. These three zones are the Front Lighting Zone, Back Lighting Zone and Upper Lighting Zone. These three areas are further divided into secondary zones to allow the designer to control unwanted light while selecting luminaires with the proper distribution to put light where it is needed. These zones are described below:


•FL (Forward Low) – This zone ranges from nadir (0) to 30 degrees vertical and counter-clockwise from 270 to 90 degrees horizontal (in front of the luminaire). The light emitted in this zone reaches from directly below the luminaire to 0.6 mounting heights from the luminaire.

•FM (Forward Medium) – This zone ranges from 30 to 60 degrees vertical and counter-clockwise from 270 to 90 degrees horizontal (in front of the luminaire). The light emitted in this zone reaches from 0.6 to 1.7 mounting heights from the luminaire.

•FH (Forward High) – This zone ranges from 60 to 80 degrees vertical and counter-clockwise from 270 to 90 degrees horizontal (in front of the luminaire). The FH can contribute to light trespass. However, it can be used to illuminate larger areas. The light emitted in this zone reaches from 1.7 to 5.7 mounting heights from the luminaire.

•FVH (Forward Very High) – This zone ranges from 80 to 90 degrees vertical and counter-clockwise from 270 to 90 degrees horizontal (in front of the luminaire). The light emitted in this zone reaches beyond 5.7 mounting heights from the luminaire. The FVH can contribute to light trespass if near the site perimeter. This is also the angle range most responsible for glare.

•BL (Back Low) – This zone ranges from nadir (0) to 30 degrees vertical and counter-clockwise from 90 to 270 degrees horizontal (behind the luminaire). The light emitted in this zone reaches from directly below the luminaire to 0.6 mounting heights from the luminaire.

•BM (Back Medium) – This zone ranges from 30 to 60 degrees vertical and counter-clockwise from 90 to 270 degrees horizontal (behind the luminaire). The light emitted in this zone reaches from 0.6 to 1.7 mounting heights from the luminaire.

•BH (Back High) – This zone ranges from 60 to 80 degrees vertical and counter-clockwise from 90 to 270 degrees horizontal (behind the luminaire). The BH can contribute to light trespass especially from perimeter fixtures. However it can be used to illuminate larger areas. The light emitted in this zone reaches from 1.7 to 5.7 mounting heights from the luminaire.

•BVH (Back Very High) – This zone ranges from 80 to 90 degrees vertical and counter-clockwise from 90 to 270 degrees horizontal (behind the luminaire). The light emitted in this zone reaches beyond 5.7 mounting heights from the luminaire. The BVH can contribute to light trespass, especially from perimeter fixtures. This is also the angle range most responsible for glare.

•UL (Up Low) – This zone ranges from 90 to 100 degrees vertical and 360 degrees around the luminaire. The UL is responsible for contributing the most to sky glow, especially as observed from great distances.

•UH(Up High) – This is the highest uplight value and ranges from 100 to 180 degrees vertical and 360 degrees around the luminaire. Light emitted more directly upward affects sky glow directly above a city.


Backlight, Uplight, Glare (BUG) Rating System
The Backlight, Uplight, and Glare ratings may be used to evaluate luminaire optical performance related to light trespass, sky glow, and high-angle brightness control. These ratings are based on zonal lumen calculations for the LCS secondary solid angles. Each rating, B, U & G, has six ranges, numbered 0 - 5. The lowest rating value, 0, is the strictest, and a rating of 5 essentially means no restrictions. For example, a Backlight rating of B0 is very restrictive, while B5 means no restrictions on the backlight emitted from the luminaire. B2-U0-G1 would be an example of a complete luminaire BUG Rating.