Consumers purchasing LED systems, which have no comprehensive performance record in lighting system, usually want to know what they're getting for their money, including an accurate prediction of how long the systems will perform so that they can estimate maintenance frequency and costs. The lifetime projections for today’s white LEDs compare to traditional light sources shows a potentially very long life. Do LEDs really last 50,000 hours?

This article discusses the useful life of LEDs related to lumen depreciation and the LEDs life measurement.

Useful life of LEDs

Unlike other light sources, LEDs don't typically “burn out” they simply get dimmer over time. To measure useful life of an LED, a level of acceptable lumen depreciation must be chosen.

The lumen depreciation is a decrease in the amount of light that a light source emits over time. For LEDs, the primary cause of lumen depreciation is heat generated at the LED junction. As LEDs do not emit heat as infrared radiation (IR), the heat must be removed from the device by conduction or convection. Without adequate heat sinking or ventilation, the device temperature will rise, and continuous high temperature operation will cause permanent reduction in light output. Furthermore, LEDs continue to operate even after their light output has decreased to very low levels. This factor is important in determining the effective useful life of LEDs.

 

For general lighting in an office environment, research has shown that the majority of occupants in a space will accept light level reductions of up to 30% with little notice, particularly if the reduction is gradual. Therefore a level of 70% of initial light level could be considered an appropriate threshold of useful life for general lighting. Based on this research, the Alliance for Solid State Illumination Systems and Technologies (ASSIST), a group led by the Lighting Research Center (LRC), recommends defining useful life as the point at which light output has declined to 70% of initial lumens for general lighting and 50% for LEDs used for decorative purposes.

LEDs life measurement

The lifetimes of traditional light sources are rated through established test procedures. As an example, for compacts fluorescent lamps (CFLs), a statistically valid sample of lamps is tested using an operating cycle of 3 hours ON and 20 minutes OFF. The point at which half the lamps in the sample have failed shows the average life for that lamp. For 10,000 hour lamps, this process takes about 15 months.

For LEDs life, switching is not a determining factor, so there is no need for the on-off cycling. But full life testing for LEDs is impractical due to the long expected lifetimes. Indeed, even with 24/7 operation, testing an LED for 50,000 hours would take 5.7 years, and products would be obsolete by the time they finished life testing.

LRC LED life test laboratory

The IESNA is currently developing a life testing procedure for LED products, based in part on the ASSIST recommends approach. According to a publication from DOE entitled “Lifetime of white LEDs”, the proposed method involves operating the LED component or system at rated current and voltage for 1,000 hours as a “seasoning period.” This is necessary because the light output actually increases during the first 1,000 hours of operation, for most LEDs. Then, the LED is operated for another 5,000 hours. The radiant output of the device is measured at 1,000 hours of operation; this is normalized to 100%. Measurements taken between 1,000 and 6,000 hours are compared to the initial (1,000 hour) level. If, for general lighting applications, 70% of initial lumens have not been reached during the 6,000 hours, the data are used to extrapolate those points.

Most manufacturers of high-power white LEDs estimate a lifetime of around 30,000 hours to the 70% lumen maintenance level, assuming operation at 350 milliamps (mA) constant current and maintaining junction temperature at no higher than 90°C. However, LED durability continues to improve, allowing for higher drive currents and higher operating temperatures and today, some LEDs available are rated for 50,000 hours at 1000 mA with junction temperature up to 120°C.