When designing LED-based lighting systems, engineers need to understand LED lumen maintenance and mortality in similar terms to those used when designing with conventional light sources. However, comparable data has been nearly impossible to find. In addition, designers need extra information to predict the lifetime of LEDs under a variety of operating conditions. A number of techniques to predict LED lifetimes have been proposed, but these have not been sufficient to generate the clear and unambiguous data that lighting engineers can use easily. This white paper provides lighting designers with an understanding of a new tool introduced by Philips Lumileds Lighting that simplifies the process, allowing flexibility in design options. This one tool provides designers with information that they need to make decisions about product lifetimes, driver constraints, number of LEDs required, and thermal management.