Backlight, Uplight, and Glare (BUG) Ratings for Outdoor Lighting Systems

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Backlight, Uplight, and Glare (BUG) Ratings are used to evaluate luminaire optical performance related to the potential for light trespass, sky glow, and high-angle brightness control. These ratings are based on zonal lumen calculation for secondary solid angles defined in the IES Technical Memorandum—Luminaire Classification System (LCS) for Outdoor Luminaires (TM-15-11). The BUG rating system was developed to replace the previous IES cutoff classifications (noncutoff, semicutoff, cutoff, and full cutoff) which were based only on luminous intensities at or above 80 degrees, rather than on luminaire lumens. The cutoff classification system did not fully describe the light emanating from different angles of a fixture, which often makes it difficult for lighting professionals to identify the lumen distribution within solid angles of specific interest.

The LCS quantifies light distribution in front of the luminaire, behind the luminaire, and above the luminaire. It defines the distribution of light from an outdoor luminaire within three primary solid angles: Forward Light, Back Light, and Uplight. The primary solid angles are further divided into 10 secondary solid angles.

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Forward Light
Forward light describes lumen distribution in front of the luminaire. This primary solid angle is defined between 0 and 90 degrees vertical, and 270 to 90 degrees horizontal in front of the luminaire. There are four secondary solid angles in the forward light solid angle:
  • Forward light low (FL, 0° - 30° vertical) is the light emitted that reaches the ground from directly below the luminaire to 0.6 mounting heights away from luminaire.
  • Forward light mid (FM, 30° - 60° vertical) is the light emitted that reaches the ground from directly 0.6 to 1.7 mounting heights away from luminaire.
  • Forward light high (FH, 60° - 80° vertical) is the light emitted that reaches the ground from directly 1.7 to 5.7 mounting heights away from luminaire.
  • Forward light very high (FVH, 80° - 90° vertical) is the light emitted that reaches the ground beyond 5.7 mounting heights away from luminaire.

FH and FVH secondary solid angles are glare zones in which the lumen output is accounted in assigning the glare ratings.

Back Light
Back light describes lumen distribution in back of the luminaire. This primary solid angle is defined between 0 and 90 degrees vertical, and 270 to 90 degrees horizontal in back of the luminaire. There are four secondary solid angles in the forward light solid angle:
  • Back light low (BL, 0° - 30° vertical) is the light emitted that reaches the ground from directly below the luminaire to 0.6 mounting heights away from luminaire.
  • Back light mid (BM, 30° - 60° vertical) is the light emitted that reaches the ground from directly 0.6 to 1.7 mounting heights away from luminaire.
  • Back light high (BH, 60° - 80° vertical) is the light emitted that reaches the ground from directly 1.7 to 5.7 mounting heights away from luminaire.
  • Back light very high (BVH, 80° - 90° vertical) is the light emitted that reaches the ground beyond 5.7 mounting heights away from luminaire.

BH and BVH secondary solid angles are glare zones in which the lumen output is accounted in assigning the glare ratings. The lumen output in BL, BM, and BH secondary solid angles is accounted in assigning the backlight ratings.

Uplight
Uplight describes the lumen distributed above the luminaire, between 90° and 180° vertical, and 0° - 360° horizontal. This primary solid angle is further refined into 2 vertical secondary solid angles:
  • Uplight low (UL): Lumen output between 90° and 100° vertical, 360° around the luminaire
  • Uplight high (UH): Lumen output between 100° and 180° vertical, 360° around the luminaire
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By comparing the lumen output in the various Backlight, Uplight and Glare zones to the maximum amount of lumens allowable on tables established within the TM-15-11, an outdoor luminaire is assigned a BUG rating. The BUG rating system allows lighting designers to evaluate fixtures for the amount of light emanating in all directions that may produce nuisance light.
 
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