RTI International Develops SSL Luminaire Reliability Model

Photo of LED luminaires and individual LEDs mounted on heat sinks in test chamber" as alt text.

LED luminaires, along with individual LEDs mounted on heat sinks, await accelerated life testing in an RTI International test chamber.

With the help of DOE funding, RTI International is developing and validating accelerated life testing (ALT) methodologies and a reliability model for predicting the lifetime of integrated solid-state lighting luminaires. Since data is essential to building this model, the technical literature in the electronics and lighting communities has been extensively utilized. However, the general lack of certified information on luminaire failure modes (for both SSL luminaires and older technologies) has required that a majority of the effort during this work focus on understanding what happens during the luminaire aging process. This information is being used to identify potential failure modes, understand variations across a given product line, and ultimately determine how long testing will take.

RTI has performed many thousands of hours of ALT testing on entire luminaires and luminaire components under extreme conditions of temperature, humidity, and voltage. In addition, comprehensive photometric testing has been performed in combination with ALT tests. Most of the testing has been conducted on 6" downlights, since these products have been widely available for more than three years and thus allow for a systematic examination of numerous luminaire design options (e.g., remote phosphors, hybrid LEDs, optical mixing cavities). The ALT testing protocols used by RTI were chosen to stress each element of the SSL luminaires, from solder joints to electrical components to the LEDs themselves.

The results to date clearly demonstrate that (1) LED lumen maintenance is not a proxy for luminaire reliability, and that a systems-level approach (encompassing all luminaire components) is required to understand the long-term performance of SSL luminaires; (2) LEDs are an extremely robust light source, even under extreme temperature and humidity conditions; and (3) well-designed SSL luminaires are also robust and can withstand harsh environments for prolonged periods. The luminaire reliability model is being built using existing industry-standard test data (e.g., LM-80) combined with the new findings from RTI's extensive testing. When available in 2014, the model is intended to provide the lighting industry with a tool to understand the benefits and tradeoffs in choosing an advanced lighting technology such as SSL.