Advanced LED Lights for determining Effective Useful Life

LED technology for general lighting has reached a point where it is widely available in all major market segments: residential, commercial, industrial, and outdoor. Throughout each of these segments, LED technology has matured to provide an efficient replacement for a variety of lighting technologies, including incandescent, halogen, compact fluorescent, linear fluorescent, high-pressured sodium, metal halide, and more. Furthermore, the cost of LED products has declined significantly over the past decade. With initial costs falling significantly due to more efficient manufacturing as well as utility incentives, consumers are beginning to embrace the new technology. However, many questions still remain around LED technology for general lighting. Of primary concern is the question, “Will an LED fixture last as long as the manufacturer claims?” This is an important question because long life allows for greater payback and increased customer satisfaction. Determining the end of lifetime for an LED fixture is a complicated task. Using current lighting standards, such as LM-79 and TM-21, LED lumen decline is calculated using projections after short-term laboratory testing. These tests result in a lifetime projection called the L70 value, which is the point at which the lumen output has degraded down to 70% of its initial output. Once an LED reaches L70, the diode may continue to operate, but the light level has declined to dimmed state, which may or may not continue to be acceptable for application. For example, a light-intensive application like a hospital operating room or a manufacturing floor may determine that lights need to be replaced at L90 rather than at a lower value. Further complicating the lifetime issue, warranty claim metrics by manufacturers have nothing to do with lumen decline. Most manufacturers offer between 5- to 10-year limited warranties for which the metric for determining product failure is when more than 10% of the diodes fail to illuminate. Greater than 10% failure of diodes is typically caused by driver failures or mechanical failures. Some LED fixtures provide interchangeable drivers which allow for an extension of the fixture life, but in other fixtures the driver is integrated into the design and is not replaceable, constituting a full-product end of life. Hence, there are multiple failure mechanisms to consider when determining the lifetime of an LED product. This leads to the question, “How does one know an LED product has reached its end of useful life?” In most cases, LED technology for general lighting has not been installed long enough for the industry to understand all the potential failure mechanisms and the issues surrounding end of life. Some manufacturers claim lifetime values (100,000-200,000 hours) based on the LED's lumen output. Others understand that more than the lumen output goes into determining the actual lifetime of a product, including driver failures, mechanical failures, and environmental factors such as temperature, moisture, and air flow.