Copyright 2012 Lighting Analysts Inc. 1 Classifying Outdoor Luminaires: The Limits of BUG by Ian Ashdown, P. Eng., FIES Senior Software Engineer Lighting Analysts Inc. August 17 th , 2012 Abstract The IES BUG rating system is useful for classifying outdoor luminaires when considering light trespass, sky glow, and glare in accordance with the Joint IDA-IES Model Lighting Ordinance (MLO). However, there are limits to what can be measured in the laboratory. Luminaires with uplight ratings of U0 or U1 or a glare rating of G0 may require further analysis when comparing manufacturers’ products. Introduction Light pollution, including light trespass, nighttime sky glow, and visual glare, has been a topic of concern since the 1970s. To address this issue, the International Dark-Sky Association (IDA) and Illuminating Engineering Society of North America (IESNA) recently issued the Joint IDA- IES Model Lighting Ordinance (MLO) [IDA-IES 2011]. Written for lighting designers, city officials, engineers, citizen groups, and other interested parties, this document is a template for municipalities interested in developing outdoor lighting regulations. Recognizing that municipalities will have varying levels of expertise in lighting design, the MLO provides two methods for determining compliance. The prescriptive method “contains precise and easily verifiable requirements for luminaire light output and fixture design that limit glare, uplight, light trespass and the amount of light that can be used,” while the performance method is mostly intended for lighting designers and professional engineers using lighting simulation software. Both methods rely on the Backlight, Uplight, and Glare (BUG) rating system as described in IES TM-15-11, Luminaire Classification System for Outdoor Luminaires [IESNA 2011]. In particular, luminaires intended for use in the five MLO lighting zones (LZ-0 to LZ-4) must have the appropriate BUG ratings to ensure that sufficient shielding is used to limit light pollution. The BUG system is quite elegant in that the backlight, uplight, and glare ratings are easily calculated from the luminaire manufacturer’s photometric data reports. Equally important, lighting professionals can easily verify these ratings using commercial lighting design software. If an outdoor luminaire manufacturer provides BUG ratings, everyone from lighting designers and specifiers to municipal engineers and homeowners can be confident that the products will meet the appropriate MLO requirements. Or can they? It may be easy to verify a BUG rating based on the luminaire’s photometric data report, but this leaves open the question of whether the photometric data report itself is sufficiently accurate or even correct. The MLO prohibits any uplight whatsoever (an uplight rating of U0) and glare ratings of 10 lumens or fewer above 60 degrees vertical (a glare rating of G0) for lighting zones LZ-0 and LZ- 1. As the MLO notes, 10 lumens is roughly equivalent to the light output of a 5-watt incandescent lamp. At the same time, the downlight may be on the order of tens of thousands of lumens from high-wattage lamps. It is therefore appropriate to ask how this light is measured in the photometric laboratory.