What is Useful Life for White Light LEDs

The goal of this paper is to initiate a discussion within the lighting community regarding standardized measurement procedures and a definition for useful life for light emitting diode (LED) technology. In general, LEDs do not fail catastrophically, but instead their light output slowly decreases over their operating period. Presently, some manufacturers use a 50% light output level as the criterion for LED life. Although 50% light loss might be acceptable for noncritical signage applications using monochromatic LEDs, it might not be acceptable for general lighting applications. It is important to develop a method for rating lamp life and a definition of “useful life” for LEDs so that when reported by manufacturers, the lighting community can compare LEDs to traditional light sources. The “useful life” definition for LEDs should consider light loss and color shift. Therefore, an experimental study was conducted to investigate light loss and color shift patterns of white LEDs as a function of operating time. The 5-mm type white InGaN +YAG LEDs evaluated in this experiment, representing technology commercially available in 1999, exhibited high light output degradation rates and color shifts as a function of operating time. It is further shown that using a simple mathematical fit to the data gathered during a short life-test study, and extrapolating it to predict the life of white LEDs, depends on the initial data collection period. Therefore, an alternate method for projecting LED life is investigated by overdriving the LEDs at different currents. Using their degradation patterns at higher drive currents, the life of these LEDs was predicted at normal drive current values. The results show excellent correlations between predicted light loss and actual measured losses at 20 and 30 mA drive currents for the LEDs tested. The authors believe that this technique is applicable for accurately predicting life of any type of LED and hope to verify this using future configurations. This study adds information to the knowledge needed for the lighting community to develop standardized measurement procedures and a definition for useful life for LED technology. INTRODUCTION Light emitting diodes (LEDs) were first developed over three decades ago. Most of the early LEDs were narrow wavelength band emitters with light output predominantly in the red to yellow region. During the 1990s Nakamura and colleagues (1994, 1997, 1998, 1999) demonstrated a blue LED based on gallium nitride (GaN). The development of the blue LED made the creation of the broad band white LED possible. Presently, white light is generated by combining the GaN-based blue LED and Y3Al5O12 (yttrium aluminum garnet or YAG) phosphor or by grouping red, green and blue LEDs in the correct proportions. [Refer to Stringfellow and Craford (1997) for an indepth discussion of LED technology. ] The potential for significant energy savings and the potential for long life are the two major factors that have attracted this technology to the general lighting community. Over the past few years the technology has advanced significantly and some white LEDs presently available in the marketplace are rated at 10 to 15 lumens per watt (lm/W). White-light LEDs are among the first signs of an evolving solid-state technology for architectural lighting applications. Many industry experts are optimistic that solid-state technology will revolutionize the architectural lighting industry. Although luminous efficacies have been steadily growing for these LEDs, the amount of light generated by a single device is still low, usually under 1 lm. Manufacturers are actively working towards developing larger light output LED devices. Some of the methods presently used for achieving this goal include, grouping several smaller LED devices together, increasing the size of the semiconductor device, enabling the device to be driven at higher drive currents, and improving the light extraction efficiencies by shaping or modifying the emitting surfaces of the semiconductor device to prevent the light from being trapped within the cavity by total internal reflection (Ochiai-Holcomb et al., 2000; Windisch et al., 2000). The development of LED technology is fueled by the electronics industry, and as a result, the advances of this technology have been much faster than most lighting technologies. In anticipation of its widespread use for architectural lighting applications, many original equipment manufacturers (OEMs) have begun to develop light sources using white LEDs for the marketplace. Developing a new light source for general lighting and achieving high levels of application depends upon industry's success at standardizing product performance and at designing cost-effective products that reliably produce light of acceptable color. Other promising lighting technologies,