Defining LED Useful Life                                          

 To provide an appropriate measure of useful life of an LED, a level of acceptable lumen depreciation must be chosen. At what point is the light level no longer meeting the needs of the application? The answer may differ depending on the application of the product. For a common application such as 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 (abbreviated as L70) for general lighting and 50% (L50) for LEDs used for decorative purposes. For some applications, a level higher than 70% may be required.

The LM-80 test procedure addresses only one factor in the life of an LED luminaire – lumen depreciation of the LED device over the prescribed test period. When LEDs are installed in a luminaire or system, there are many additional factors that can affect the rate of lumen depreciation or the likelihood of catastrophic failure. These include temperature extremes, humidity, moisture incursion, voltage or current fluctuations, failure of the driver or other electrical components, damage or degradation of the encapsulant material covering the LEDs, damage to the wire bonds that connect the LEDs to the fixture, and degradation of the phosphors.
LED performance is directly linked with other components in the luminaire, all critical for understanding product reliability. Current testing efforts and related research have provided some confidence in the reliability of some components. However, other characteristics of the technology, interactions, and application remain untested. Many LED luminaires are newly designed products, increasing the likelihood users will experience unanticipated problems relative to fixtures that have been manufactured and refined for years.

While the lifetime of an LED source is one important indicator of LED luminaire life, it would be misleading to rate the entire LED luminaire based solely on the LED source. There is often a huge gap between the warranted life of a product and the expected life of the LED source in that product. Further, reliability of fixtures that include replaceable LED engines and replaceable components should be assessed differently than reliability of entirely integrated fixtures.

For white light LEDs providing general illumination, the definition of useful life is often given as the hours of operation at which the LED’s light output has decreased to 70% of initial output (abbreviated as L70 or L70). The selection of 70% is based on vision research indicating that in general lighting applications, the “typical” human eye does not detect the decrease in light until it exceeds 30%. LED manufacturers publish lumen depreciation curves based on testing of their products, extrapolating lumen depreciation to the 70% level. Depending on the application, other depreciation levels may be appropriate as end of life limits, such as L50 or L80.

Figure 1. The curves show expected LED life based on drive current indicated by each colored line and target LED junction temperatures. Source: Philips Lumileds.

LED manufacturers make lumen maintenance projections based on extended in-house testing and statistical extrapolation, accounting for the effects of drive current and LED junction temperature of the device in operation. For example, Philips publishes curves shown in Figure 1. This graph shows the projected lifetimes (B50, L70 means 50% of the products have at least 70% lumen maintenance for the projected number of operating hours) for the K2 LED package. According to this graph, 50% of a sample of K2 devices run at 350 mA would be expected to maintain at least 70% of initial lumens for 60,000 hours, if the junction temperature is maintained at about 160C or lower. If the device is run at a higher current, e.g., 700 mA, Tj would have to be kept lower (about 140C) to last 60,000 hours.

At this time, there is not a standard reporting format for LED lifetime or lumen depreciation curves. A test procedure currently in development by the Illuminating Engineering Society of North America (designated LM-80, IESNA Approved Method for Measuring Lumen Maintenance of LED Light Sources) will provide a common procedure for making lumen maintenance measurements at the LED device, array, or module levels.

Measuring Light Source Life

We’ve all heard the small “pop” as an incandescent lamp fails. It’s the sound of the tungsten filament finally breaking as the electric current hits it. This makes it easy to recognize the end of life for an incandescent light source. With fluorescent lamps, end of life may involve flickering or the lamp may simply not activate when the switch is turned on. With LEDs, outright failure of the device is less likely, although it can happen due to component failure. Instead, the LED’s light output slowly declines over time.

The lifetimes of traditional light sources are rated through established test procedures. The life testing procedure for compact fluorescent lamps, for example, is published by the Illuminating Engineering Society (IES) as LM-65. It calls for a statistically valid sample of lamps to be tested at an ambient temperature of 25 degrees Celsius using an operating cycle of 3 hours ON and 20 minutes OFF. The point at which half the lamps in the sample have failed is the rated average life for that lamp. For 10,000 hour lamps, this process takes about 15 months.

How are LED lifetimes rated? Life testing for LEDs is impractical due to the long expected lifetimes. Switching is not a determining factor in LED life, so there is no need for the on-off cycling used with other light sources. But even with 24/7 operation, testing an LED for 50,000 hours would take 5.7 years. Because the technology continues to develop and evolve so quickly, products would be obsolete by the time they finished life testing.

A life testing procedure for LEDs is currently under development by the Illuminating Engineering Society of North America (IESNA). The proposed method is based on the idea of “useful life,” i.e., the operating time in hours at which the device’s light output has declined to a level deemed to no longer meet the needs of the application. For example, for general ambient lighting, the level might be set at 70% of initial lumens. Useful life would be stated as the average number of hours that the LED would operate before depreciating to 70% of initial lumens.

The leading LED manufacturers have begun using the L70 language, stating that their white LEDs “are projected” to have lumen maintenance of greater than 70% on average after 50,000 hours when used in accordance with published guidelines.

Electrical and thermal design of the LED system or fixture determine how long LEDs will last and how much light they will provide. Driving the LED at higher than rated current will increase relative light output but decrease useful life. Operating the LED at higher than design temperature will also decrease useful life significantly.

How do the lifetime projections for LEDs compare to traditional light sources?

Color Rendering Index

Reprinted courtesy of the Illuminating Engineering Society of North America.

Another important measure of color quality used by the lighting industry is the color rendering index (CRI). CRI indicates how well a light source renders colors, on a scale of 0 to 100, compared to a reference light source of similar color temperature.

The test procedure established by the International Commission on Illumination (CIE) involves measuring the extent to which a series of eight standardized color samples differ in appearance when illuminated under a given light source, relative to the reference source.

The average “shift” in those eight color samples is reported as Ra or CRI. In addition to the eight color samples used by convention, some lighting manufacturers report an “R9″ score, which indicates how well the light source renders a saturated deep red color.

    

Thermal Management of White LEDs

LEDs won’t burn your hand like some light sources, but they do produce heat. In fact, thermal management is arguably the most important aspect of successful LED system design. This section reviews the role of heat in LED performance and methods for managing it.

Why Does Thermal Management Matter?

Excess heat directly affects both short-term and long-term LED performance. The short-term (reversible) effects are color shift and reduced light output while the long-term effect is accelerated lumen depreciation and thus shortened useful life.

The light output of different colored LEDs responds differently to temperature changes, with amber and red the most sensitive, and blue the least. (See graph below.) These unique temperature response rates can result in noticeable color shifts in RGB-based white light systems if operating T_j differs from the design parameters. LED manufacturers test and sort (or “bin”) their products for luminous flux and color based on a 15-20 millisecond power pulse, at a fixed T_j of 25°C (77°F). Under constant current operation at room temperatures and with engineered heat mitigation mechanisms, T_j is typically 60°C or greater. Therefore white LEDs will provide at least 10% less light than the manufacturer’s rating, and the reduction in light output for products with inadequate thermal design can be significantly higher.

Continuous operation at elevated temperature dramatically accelerates lumen depreciation resulting in shortened useful life. The chart below shows the light output over time (experimental data to 10,000 hours and extrapolation beyond) for two identical LEDs driven at the same current but with an 11°C difference in T_j. Estimated useful life (defined as 70% of initial lumen output) decreased from ~37,000 hours to ~16,000 hours, a 57% reduction, with the 11°C temperature increase.

However, the industry continues to improve the durability of LEDs at higher operating temperatures. The Luxeon K2, for example, claims 70% lumen maintenance for 50,000 hours at drive currents up to 1000 mA and T_j at or below 120°C.