By: Gloria Kilpatrick IES, LC

Exterior lighting layout requests for Leadership in Energy Efficient Design (LEED) compliance are becoming more frequent. LEED v.4 allows for two methods of determining compliance. The first method uses backlight-uplight-glare (BUG) ratings of luminaires; the second uses calculations. We are asked to provide these calculations by our clients.

Obtrusive light is defined as unwanted light and may include property line spill, (light trespass) or glare (light that hinders vision). Any light that causes annoyance, discomfort, distraction, or hinders the ability to see essential information, constitutes obtrusive light. Therefore, LEED v.4 is mainly concerned with vertical calculations measured in a plane perpendicular to the property line. Another part of obtrusive light is the Upward Waste Light Ratio (UWLR). This ratio is defined as the luminous flux emitted by the luminaire above the horizontal in the installed position. UWLR considers the total lumen and total upward lumen output for each luminaire. Limits for both vertical calculations and UWLR are determined by the Lighting Zone (LZ) assigned to the site.

Target horizontal light levels, desired mounting heights, correct LZ, and accurate lighting boundary for the project must be provided by the client before any studies can be conducted. The LEED professional must identify which luminaires will be exempt from the study and obtrusive light calculations. As specified in LEED v.4, calculation points must be no more than 5 feet (1.5 meters) apart, extend from grade level to 33 feet (10 meters) above the height of the highest luminaire. All points must be parallel to the property line and oriented toward the property in a vertical plane. The obtrusive light study will include property line vertical information as well as UWLR. Our layouts show only those areas that have out-of-compliance points or the highest points in compliant layouts; all points need not be shown. Our office always includes BUG rating, luminaire lumens, and wattage in the luminaire schedule. Per LEED v.4, light loss factor (LLF) for all luminaires should be 1.0. After calculations are complete, a report can be generated by the lighting software.

Factors that affect compliance include mounting height, BUG rating, and luminaire proximity to the property line. Mounting height can be easily changed but will affect horizontal levels as well. Horizontal targets should be made available when these adjustments are requested. BUG rating can be addressed by the selection of a different luminaire.

As always, complete information is key to an accurate layout. LEED professionals are not necessarily lighting professionals; they will rely on the knowledge and expertise of the application specialist. It is always advantageous to know the details of the report and be able to intelligently discuss them with the client and LEED professional.

This article is meant only as an overview and should not be used as a reference to either procedures or concepts. More information can be found in LEED v.4 literature and the AGi32 help section.

References:

LEEDv4_SSc8, Light Pollution Reduction

AGi32 v.16.8 Help Section