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White LED technology is moving fast to conquer new lighting applications. Though LED-based general lighting fixtures are just beginning to make their way, the number of commercial, replacement-type LED lamps and light engines for decorative lighting applications is rising.

To address this growing market, the Alliance for Solid-State Illumination Systems and Technologies (ASSIST) has published a new volume in its ASSIST recommends series:

Recommendations for Testing and Evaluating White LED Light Engines and Integrated LED Lamps Used in Decorative Lighting Luminaires

The new volume provides recommended test methods and measurement procedures for LED light engines and integrated lamps used in decorative lighting fixtures such as chandeliers, wall sconces, pendants, table and floor lamps.

The volume is available for free download from the ASSIST Web site: www.lrc.rpi.edu/programs/solidstate/assist/lightengine.asp

Estimating performance for a variety of LED-based decorative lighting products

The goal of the new recommendation is to provide fixture manufacturers with a method of estimating the performance of fully assembled LED-based light engines or integrated lamps, which may include one or more LEDs, heat sink, driver, and other optical, thermal, and electrical components. For reasons of simplicity and cost, fixture manufacturers may use the same LED light engine or integrated lamp in a family of complementary decorative lighting fixtures with different designs and installation environments, says N. Narendran, Ph.D., director of research at the Lighting Research Center, which organizes ASSIST.

“Heat is a critical factor in LED performance, and the amount of heat that accumulates can vary greatly from one fixture to another depending on the fixture design and installation,” says Dr. Narendran. Therefore, a given LED light engine or integrated lamp may perform differently inside different types of fixtures, he says. In terms of decorative lighting, differences can be found in the materials used for housing, the color of optics and shades, and the primary installation type, such as a ceiling-mounted fixture versus a hanging pendant or chandelier. Photometric testing of a complete decorative fixture may not provide useful information regarding the flux, efficacy, and color properties of the light distributed into the environment, says Narendran, because the cover glass or the shade used for creating the style can alter the properties of the light emitted by the light source. Furthermore, it is not practical for manufacturers to test the performance of every complete decorative fixture. Therefore, the method recommended by ASSIST provides an estimate of light source performance that accounts for potential differences in the thermal environment.

Method explained

The recommended method calls for characterizing the performance of the LED light engine or integrated lamp as a function of LED board temperature, which has a direct relationship to LED junction temperature. “While traditional light sources and fixtures are tested at a reference ambient temperature,” says Narendran, “LED light sources and fixtures are better tested using board temperature, which we know to be a good predictor of life and performance.”

The LED light engine or integrated lamp is placed inside a heated thermal chamber, and data on light output, efficacy, color, and life are gathered at three temperatures: 40%, 60%, and 80% of the maximum junction temperature recommended by the LED manufacturer. This range of temperatures covers the possible operating temperatures of a fixture in realistic conditions and provides a baseline for manufacturers interested in using a given LED lighting product in their fixtures.

The next step is to measure the board temperature of the LED light engine or integrated lamp while operating inside a given fixture in its application environment. Using the baseline information gathered previously and the measured board temperature of the LED light engine or lamp as used in the fixture, manufacturers can estimate performance for the fixture by interpolation. ASSIST also recommends measuring board temperature in a worst-case scenario, typically a completely enclosed design.

ASSIST’s top priority in developing this new ASSIST recommends volume, according to Narendran, was to encourage common, consistent methods of testing and data presentation for ease of interpretation and comparison. In turn, this will help fixture manufacturers in selecting suitable LED light engines and integrated LED lamps for their family of products.

Test results for a sample commercial light engine

The Lighting Research Center has tested the performance of a number of commercial light engines for decorative lighting fixtures according to ASSIST’s published recommendations. Data for one sample commercial light engine are plotted in the figures below. The data points show the performance at each of the three tested board temperatures [40%, 60%, and 80% of maximum-recommended junction temperature (TJmax)]. The vertical dashed line shows the board temperature measured with the light engine placed inside a decorative pendant fixture, 76.6°C, and the horizontal dashed line shows the light engine’s performance at this temperature.

The results show that, as expected, light output and luminous efficacy of the light engine decrease and correlated color temperature changes with increases in board temperature. By measuring the board temperature of the light engine while operating inside a decorative lighting fixture, the light engine’s performance within the luminaire can be estimated easily. Since the light engine’s performance is characterized, its performance in any lighting fixture can be estimated by simply measuring the board temperature. 

Test Condition		LED Board Temperature (Ts)
In fixture		76.6°C
40% TJmax		48.6°C
60% TJmax		70.9°C
80% TJmax		97.0°C

About ASSIST

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 Bridgelux; China Solid State Lighting Alliance; Cree; 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 Lighting/Color Kinetics, now Philips Solid-State Lighting Solutions; Photonics Cluster (UK)/The Lighting Association; Seoul Semiconductor; United States Environmental Protection Agency; and USG.

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.