Lumen Maintenance and Reliability
One of the key benefits of LED technology compared to other lighting technologies is its reliability and high operating lifetime. For example, incandescent light bulbs tend to fail abruptly after approximately 1,500 hours of operation, after which they do not emit any light. The light output of an LED, in contrast, tends to degrade gradually over time and can often be described with an exponential decay model. This stark difference in behavior is graphically shown in Figure 1.
In order to facilitate a comparison of the long term performance between different LEDs, the LED industry developed a standard, IES LM-80-08, which specifies how LED manufacturers and lighting manufacturers can test LED components to determine their performance over time. The LM-80 standard calls for operating a set of LED devices for a minimum of 6,000 hours under well controlled operating conditions (e.g. a constant DC drive current in a thermal chamber where the LED temperature and surrounding air temperature are controlled within specified limits). The LED samples are regularly removed (e.g. every 1,000 hours) from the thermal chamber in order to test relevant electrical and optical parameters for each LED, including forward voltage, light output, and color point. Once the test reaches at least 6,000 hours, all test data is summarized in a comprehensive LM-80 test report.
For many applications it is important to estimate when the light output will drop below 70% of the initial light output. However, given their excellent long-term performance, white LEDs typically exhibit only a small drop in light output after 6,000 hours. Therefore, the LED Industry adopted a method, described in IES TM-21-11, to fit an exponential lumen maintenance model to actual LM-80 data. This model can then be used to project the average lumen maintenance of an LED product beyond the actual number of operating hours tested. Figure 2 shows typical lumen maintenance data, collected according to LM-80, and corresponding lumen maintenance extrapolation curves, according to TM-21.
Lumen maintenance is not the only factor affecting the long term performance of an LED system. In addition to a gradual decrease in light output over time, there is a very small probability of abrupt failures, which cause a LED to stop emitting any light. In general, this probability of failure can be described by the semiconductor “bath-tub” curve which captures three distinct regions of operation. There is an early failure period (e.g. a region where the failure rate is decreasing), the useful life period (e.g. a region where the failure rate is constant), and, finally, a wear-out period (e.g. a region where the failure rate is increasing until all units fail).
LED failures can behave either as an electrical open circuit (e.g. a wire bond inside the package is broken) or an electrical short (e.g. an insulator layer between anode and cathode within the LED chip has broken down). These different types of LED failures may affect the overall LED system reliability differently. For example, some LED systems consist of multiple LEDs connected in a single (series) string. In this configuration, a short failure in one LED still allows current to flow through the string, i.e. the other LEDs in the string still light up. However, an open failure in one LED interrupts the current through all the LEDs in the string. Other LED systems consist of multiple parallel strings, each with one or more LEDs. In a parallel configuration, the input current is split equally between the parallel strings. If an LED fails open in one string, the current which used to flow through that string is redistributed among the other functioning strings. The resulting system light loss is often minimal because the increased light output from the remaining strings partially offsets the light which was originally generated by the failed string. However, a LED which fails short in a parallel string configuration may cause all the current to flow through the string with the shorted LED, bypassing the other fully functional strings in the system, which is typically not desirable.
Lumileds has a long history of providing LEDs to the automotive, consumer electronics, and general illumination industries. Given the stringent reliability requirements imposed by these industries on their suppliers, Lumileds implemented a rigorous reliability test program early on for all its LED products. This program is designed to uncover potential reliability weaknesses early on in the product design cycle and to continually improve the long-term reliability of Lumileds products. Prior to market release, all new Lumileds products are subjected to a variety of reliability tests to ensure that the product is designed and manufactured to achieve a high degree of reliability within the operating conditions specified in the datasheet. These tests typically include a series of highly accelerated reliability tests, environmental stress tests, and mechanical tests. In addition, Lumileds maintains an ISO 17025 accredited test lab, with LM-80 in its scope of accreditation, to assess the lumen maintenance behavior of its (illumination grade) LED products.