LEDs and Actual LED Lifetime versus Lumen Maintenance|
Article- March 2012 By Larson Electronics.com
Larson Electronics General Area-150 Watt High Bay LED Light Fixture
Although LEDs are quickly making inroads towards potentially becoming the dominant alternative to incandescent light bulbs, there remain serious obstacles. Perhaps one of the biggest remaining issues concerning LEDs is the lack of a clear and easy to understand standard by which to judge their performance. Since LEDs produce light in wholly different manner from traditional forms of lighting, attempting to apply old methods of measuring things like lamp life and lumen output has proven difficult. Manufacturers and developers of course understand this better than anyone, but as yet there exists no single accepted set of standards or regulations by which all LEDs can be compared and or certified. The result has been a confusing LED lighting landscape, with wide variances in the performance of LEDs from one manufacturer compared to the next.
Standards are voluntary guidelines adhered to by developers and manufacturers in order to ensure homogeny between different versions of the same technologies. Although LEDs offer vast improvements over traditional forms of lighting, and thus are attractive to a wide range of lighting sectors, the lack of homogeny among manufacturers means it is difficult to be certain a particular LED product is going to perform to expectations. Without an accepted set of standards to which all developers adhere, there is no way to really be certain that an LED product is going to perform as advertised or to customer expectations.
One of the bigger areas of confusion for consumers and commercial applications is the manner in which LED longevity and output are measured. We often see LEDs advertised as having a “50,000 hour lifespan”, but what does this really mean? For most people, the natural assumption would be that after 50,000 hours, the LED can be expected to soon fail to produce light, in essence, burn out much like a traditional incandescent lamp does at the end of its life cycle. However, this is not exactly the case, and the manner in which actual LED life is measured takes into account not only the expected time of failure, but average lumen maintenance over the life of the LED fixture. As a result, we often see LEDs advertised as having 70% lumen retention after 50,000 hours.
The confusion here is between actual LED life expectancy and LED lumen maintenance. The rated life span of a typical luminaire is defined in ANSI/IES RP-16, as “the life value assigned to a particular type lamp. This is commonly a statistically-determined estimate of median operational life.” What this means is, that for a given sample number of lamps, the percentage of those which fail at certain intervals will determine average lamp life. Rated life is thus a statistical measure and is defined as “Bp”, with p denoting the percentage. If a sampling section achieves a B50 rating at 1,000 hours, it means that half the tested samples operated for 1,000 hours without any failures. In traditional lamps, life is typically measured by the amount of time it takes for 50% of a sample group to actually fail.
LED light fixtures are different from typical incandescent lamps though because they do not tend to simply fail to produce light although this will eventually occur. They can continue to operate far past their actual useful life, which makes understanding the difference between rated life and lumen maintenance critical. Unlike rated life, lumen maintenance denotes the percentage of light produced at a given period of operating time versus initial lumen output. This is important because although an LED may appear to be operating normally after a given period of time, its actual lumen output may have decreased significantly. This is a similar problem encountered with HID lighting, wherein color shifting and reduced output occurs over the life of the lamp. However, with HID lighting, the lumen depreciation is more pronounced, and operating performance also degrades significantly as well with the typical slow starting and on/off cycling associated with HIDs that have reached the end of their useful life becoming pronounced. LEDs thus have a useful life that is better understood as the amount of time before a certain percentage of lumen loss occurs. The problem is that manufacturers and marketing sometimes adhere to the more impressive numbers generated regardless of their origin. You may see an LED advertised as having a 100,000 hours life span rating, but in reality that LED is only producing 40-50% of its original output by that time.
The lack of standardization does not allow an expectation of practical uniformity among LED designs that can be applied on a wide scale. Some of the more reputable manufacturers include lumen maintenance figures along with an expected life rating, which helps a great deal in determining LED suitability for a given task, but this is currently not the norm. Industry groups have been attempting to devise a set of standards and guidelines to address these problems, but the complexity of LED operation makes this a difficult task to say the least. Unlike traditional forms of lighting, LEDs require additional hardware in the form of voltage regulating circuits and heat management in order to operate reliably. They cannot operate natively with normal AC voltages encountered in common applications, and thus require additional drivers to convert and reduce voltage to the low levels they require. The actual performance of an LED is greatly dependent upon such LED driver hardware, and thus this must also be included within the quest for standardization. In some testing, it has been noted that actual operating life was determined more by the failure of LED drivers than the actual failure of the LEDs themselves.
Currently, industry experts are suggesting the use of combined data from both lumen maintenance and actual failure percentages to produce a realistic and effective overall LED lifetime average. This would likely be the most effective route to standardization, but requires addressing not only LEDs but the technology used to power them as well. Until a set of standards is devised, it is likely we will continue to see irregularities among manufacturers and the resultant reluctance on the part of many lighting professionals to utilize LEDs in new designs due to an inability to form practical expected performance parameters. In the meantime, it is probably best for most consumers to use average lumen maintenance figures when evaluating LEDs as this method offers the best way to measure expected LED lifetime performance.