Parking lot and outdoor luminaires are a rapidly growing market for the LED lighting industry, and many LED-based outdoor luminaires have entered the marketplace with claims of significant energy savings and life-cycle cost reductions compared to traditional luminaires. However, some of these LED systems are not realistically able to meet the requirements of good outdoor lighting, and no simple method exists to evaluate and compare the performance of outdoor lighting systems before selection and installation.
To help outdoor lighting designers and specifiers make optimum luminaire choices, the Alliance for Solid-State Illumination Systems and Technologies (ASSIST) has published a new volume in its ASSIST recommends series about parking lot lighting. The issue “Recommendations for Evaluating Parking Lot Luminaires” describes a new, alternative metric for evaluating parking lot luminaires based upon how well they cater to the application’s requirements. The metric was developed through previous research by the Lighting Research Center (LRC) and builds upon the concept of application efficacy.
The volume is available for free download from the ASSIST Web site: www.lrc.rpi.edu/programs/solidstate/assist/parkinglot.asp
Application efficacy and catering to an application’s requirements
The concept of application efficacy1 refers to only the flux leaving the luminaire in a particular direction, specifically toward the task area, rather than the total amount of flux exiting the luminaire or the light source’s luminous efficacy.
“We know that luminous efficacy and luminaire efficacy are not accurate gauges for how appropriately and efficiently a luminaire will meet an application’s lighting needs,” says Yutao Zhou, a research scientist at the Lighting Research Center and one of the designers of ASSIST’s new parking lot luminaire metric. “If a luminaire is not designed well to collect all the flux emitted by the light source and direct it to where it is needed, then the application may not be energy efficient,” he says. For example, a parking lot luminaire that loses some light upward to the sky will not be as efficient as one that directs more flux down to the parking lot’s pavement, from the standpoint of application efficacy.
Zhou notes that ASSIST is not questioning the relevance or appropriateness of existing standards for outdoor and parking lot lighting, but rather accepts the standards as they are and integrates them into ASSIST’s new, recommended metric for parking lot luminaires.
The application’s task area is of central importance to ASSIST’s new metric, and is defined as a function of the luminaire type and its mounting height. Once the task area is defined, it is divided into sections, or cells, and the illuminance reaching each cell is calculated from a given luminaire’s IES file. Cells that have illuminances falling above or below existing application standards (e.g., IESNA’s RP-20-98 Lighting for Parking Facilities) are discounted. The luminous flux of the conforming cells is then used to calculate the total useful luminous flux reaching the task area, which is then multiplied by the percentage of conforming cells in the task area and divided by the input power to obtain the final luminaire system application efficacy. The ASSIST publication provides step-by-step instructions for calculating the luminaire system application efficacy for parking lot luminaires, and may be used to calculate the efficacy of one luminaire or for an entire parking lot.
ASSIST is a collaboration between researchers, manufacturers, and government organizations. Its goal is to identify and reduce major technical hurdles currently facing solid-state lighting. The Lighting Research Center conducts research, demonstration, and educational activities on behalf of ASSIST.
ASSIST is sponsored by Acuity Brands Lighting; Bridgelux; China Solid State Lighting Alliance; Cree; Everlight Electronics Co., Ltd.; Federal Aviation Administration; GE Lumination; ITRI, Industrial Technology Research Institute; Lighting Science Group; Lite-On; NeoPac Lighting; New York State Energy Research and Development Authority (NYSERDA); OSRAM SYLVANIA/OSRAM Opto Semiconductors; Permlight; Philips Color Kinetics; Photonics Cluster (UK)/The Lighting Association; Seoul Semiconductor; United States Environmental Protection Agency; USG; WAC Lighting.
About ASSIST recommends
ASSIST has developed a publication program called ASSIST recommends to provide a set of formal recommendations to the LED and lighting communities about issues important for the reliable performance of LED lighting and its comparison to other light source technologies. The publications include recommendations for LED life definition, testing and measurement, best practice guides for different lighting applications, and recommendations for selecting LED lighting. Unlike traditional test procedures that require products to be tested under standardized, ideal conditions, ASSIST recommends methods call for testing products under conditions similar to those found in the application environment, where the light source could experience many different temperatures and may perform poorly as a result. Testing products by intended application also allows for apples-to-apples comparisons of product performance because test methods have been developed from a technology-neutral standpoint. ASSIST recommends publications are developed under the guidance of ASSIST sponsors using research conducted by the Lighting Research Center (LRC). Each publication undergoes internal review, first by LRC researchers and then by ASSIST sponsors. Industry input also is gathered during the writing process through one or more roundtable sessions hosted by ASSIST and the LRC. Based upon this industry input, the publications are revised and then published online for free download.
As warranted, the publications are updated from time to time to reflect new research, technologies, methods, and equipment.
Rea MS, Bullough JD. Application efficacy. Journal of the Illuminating Engineering Society 2001; 30(2): 73–96