What is Useful Life?
Life testing methods for conventional light sources incandescents, fluorescents, high-intensity discharge lamps, low-pressure sodium lamps, and so on — are well established and well understood. The life testing method for LED light sources is relatively new and less well understood. Today’s Post directly shared to clearify and brief a method for making accurate comparisons between conventional lamps and LED lighting fixtures. Before start to reading post please check the glossary of lighting post shared before.
Rated Lifetime of Conventional Sources
Approved methods for life testing of conventional light sources call for measuring and reporting rated lamp life. These methods are published by the Illuminating Engineering Society (IES) in a variety of official publications. For example, LM-65-01 defines life testing procedures for compact fluorescent lamps (CFLs), while LM-49-01 defines life testing procedures for incandescent filament lamps. Both LM-65-01 and LM-49-01 have been available and in use since 2001, and both methods revise older standards, published in 1991 and 1994 respectively. Both publications establish testing conditions, sample sizes, and methodologies for generalizing test data to arrive at rated life specifications. For CFLs, LM-65 specifies a statistically valid sample to be tested at an ambient temperature of 25° C, in a cycle of three hours on and 20 minutes off (as CFL life is appreciably shortened by the frequency with which the lamp is started). The point at which half the lamps fail is the rated average life for that lamp. For incandescent filament lamps, LM-49 specifies a statistically valid sample to be tested within the manufacturer’s stated operating temperature range and voltage. Lamps are allowed to cool to ambient temperature once a day (usually for 15 to
30 minutes). As with CFLs, rated life for incandescent filament lamps is the total operating time at which half the lamps are still operating.
Lumen Maintenance and Lumen Depreciation
In September 2008, the IES published Measuring Lumen Maintenance of LED Light Sources, publication IES LM-80-08. LM-80 is the LED counterpart of LM-65, LM-49, and other life testing standards for conventional light sources, but it differs from the older standards in a number of important and potentially confusing ways.
Instead of measuring rated lamp life, LM-80 calls for measuring how much an LED source’s lumen output decreases over a certain number of hours of operation. Technically, the term for this decrease is lumen depreciation. The converse of lumen depreciation is lumen maintenance, the industry standard term for the percentage of
initial lumens that a light source maintains over a certain period of time. All electric light sources lose lumen output over time indeed, annexes to both LM-65 and LM-49 address lumen depreciation of CFLs and incandescent
filament lamps. In incandescent lamps, lumen depreciation is caused by depletion of the filament and the build-up of evaporated tungsten particles inside the bulb. Incandescents typically lose 10% – 15% of their initial lumen output over an average lifetime of 1,000 hours. In fluorescent lamps, lumen depreciation is caused by photochemical degradation of the phosphor coating and glass tube, and the build-up of light-absorbing deposits inside the tube. High-quality fluorescent lamps using rare earth phosphors lose only 5% – 10% of initial lumens over 20,000 hours of operation. CFLs depreciate more, but the most well-designed products lose no more than 20% of their initial lumens over an average lifetime of 10,000 hours. In LED sources, factors that cause lumen depreciation include drive current and heat generated within the device itself (technically speaking, at the diode’s p-n junction), which degrades the diode material. Some white-light LEDs may experience degradation of the phosphor coating like that
of fluorescent lamps. Some LEDs can also lose lumen output due to clouding of or impurities in the encapsulant used to cover LED chips. Lumen maintenance measurements take the form Lp, where L is the initial output of a light
source, and p is the percentage maintained by the light source over a certain number of hours. L97 measures how long a light source retains 97% (or loses 3%) of its initial output, L44 measures how long a light source retains 44% (or loses 56%) of its initial output, and so on. Since high-performance LED light sources can produce useful light for tens of thousands of hours, and since they rarely fail outright, lumen maintenance is often used in place of rated life
measurements for LEDs. Measuring the rated life of LED light sources the mean time to failure of a representative sample would require operating the sources continually until they finally faded to darkness, a process which would take many years. Because LED light sources continue to deliver light even after their initial lumen output has decreased by 50% or more, lighting specifiers and designers need to know how long an LED lighting fixture will retain a meaningful percentage of its initial light output, not how long it will take for the light sources to fail.
Useful Life of LED Light Sources
The Alliance for Solid State Illumination Systems and Technologies (ASSIST), a group led by the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, New York, has published a series of recommendations defining the useful life of LED light sources. ASSIST defines useful life as the length of time a light source delivers a
minimum acceptable level of light in a given application. Research performed by ASSIST indicates that changes in general office lighting levels go largely undetected as long as light levels stay above 70% of their initial levels,
especially if the changes are gradual. For general lighting applications, therefore, ASSIST recommends defining useful life as the length of time it takes an LED light source to reach 70% of its initial light output (L70). For decorative and accent applications, ASSIST recommends defining useful life as the length of time it takes an
LED light source to reach 50% of its initial output (L50). L70 and L50 are widely used by the LED lighting community as two important thresholds for useful life, covering a wide range of lighting applications.
Lumen Maintenance Gap
All well and good so far. But there’s a disconnect between the test results typically provided by LM-80 on the one hand and the L70 and L50 thresholds that define useful life on the other. This disconnect, which could be called the
lumen maintenance gap, is the source of a fair amount of confusion among lighting specifiers, designers, and other lighting professionals who need to understand how long an LED lighting system will deliver effective light in a particular application. This understanding is crucial for making valid comparisons between conventional and
LED lighting fixtures, and for accurately calculating installation, maintenance, and replacement costs. Let’s see if we can sort things out.
Lumen Maintenance of LED and Traditional Light Sources
When properly controlled and driven, LED light sources can have useful lives that last considerably longer than the rated lives of conventional sources. The following table compares the typical useful life range of LED light sources with the typical rated life ranges of conventional light sources.
LM-80 requires testing of LED light sources for 6,000 hours, and recommends testing for 10,000 hours. It calls for testing LED sources at three junction temperatures 55° C, 85° C, and a third temperature to be determined by the
manufacturer so that users can see the effects of temperature on light output, and it specifies additional test conditions to ensure consistent and comparable results.
Unfortunately, LM-80 provides no recommendations on how to extrapolate measured data to L70 or L50. Such a methodology, IES Technical Memorandum TM-21 is currently under development. Until TM-21 is published, the only way an LED source manufacturer can claim that their L70 and L50 figures conform to LM-80 is to measure their LED sources until they reach those thresholds. Since a typical L70 number is 50,000 hours, such a test would last longer than five years! Not only would this test be impractical, but LED technology evolves so quickly that a given product would be obsolete by the time the test was completed. In practice, leading LED source manufacturers test their products to the LM-80 minimums of 6,000 or 10,000 hours, then apply their own extrapolation
methodologies to arrive at L70 and L50 figures. Since these methodologies are proprietary, manufacturers can choose to disclose as much or as little of the mathematics and supporting data as they wish.
For example, a leading LED source manufacturer publishes the raw data for a high-performance white-light
LED Their report includes data on a significant sampling of devices, each tested to 6,000 hours in accordance
with LM-80 methods, and L70 extrapolations based on an exponential model. While this set of data is sufficient
to establish the manufacturer’s credibility, users would benefit from more transparency into the model’s
extrapolation formulas and assumptions. Another leading LED source manufacturer bases the lumen maintenance model for a high performance whitelight LED on their interpretation of raw LM-80 test data. According to their published specifications, the data indicate that lumen maintenance is linear after the first 5,000 hours of operation, so they apply a linear model using variables such as the temperature of the thermal pad on the bottom of the LED, junction temperature, ambient temperature, and drive current. While they do not disclose their extrapolation formulas or raw test data, they do clearly explain their approach, and they provide an extensive set of charts to show expected lumen maintenance to L70 at different ambient temperatures and drive currents. Regardless of the extrapolation method used, keep in mind that L70 and L50 figures may be based on LM-80 measurements, but they are not LM-80 measurements.
Useful Life of LED Sources in Lighting Fixtures
The approved method for making photometric measurements of LED lighting products specifically calls for the testing of complete LED lighting fixtures (as spelled out in IES LM-79-08). The approved method for measuring lumen maintenance is just the opposite: It calls for the testing of LED light sources, not complete LED lighting
fixtures. LM-80 explicitly defines light sources as “packages, arrays, and modules only.” This means that LED fixture manufacturers must define their own methods of calculating lumen maintenance for their LED lighting fixtures. As with L70 and L50 figures provided by LED source manufacturers, lumen maintenance figures provided by LED fixture manufacturers may use LM-80 test data and lumen maintenance extrapolations based on them, but they are not LM-80 measurements.
Ambient and internal operating temperatures and drive currents have a significan effect on the lumen maintenance of LED light sources integrated into lighting fixtures, but so do many features of LED lighting fixtures themselves, including lensing, housing color, quality of components, and thermal design. Operational factors such as power surges, static discharge, vibration, and moisture infiltration can also have a significant effect. LM-80 testing for complete LED lighting fixtures would be prohibitively complex and expensive for manufacturers, as they would have to test every different version of their fixtures to account for the effect of each feature or combination of features.
In practice, therefore, reputable LED fixture manufacturers ensure that their fixture drive currents and operating temperatures (especially junction temperatures) fall within the ranges specified by the LED source manufacturers in their lumen maintenance reports. The fixture manufacturers then make their own calculations of the useful life of the LED sources integrated into their lighting fixtures, based on their understanding of the effects of specific physical and operational features.
The useful life and the lifetime of luminaire are completely different things from each other.
ANSI / IESNA RP-16-05, Addenduma. Nomenclature and Definitions for Illuminating Engineering. Illuminating
Engineering Society of North America, 2008.
Design Resource DR03: LM-80 Test Report, White LUXEON Rebel. Philips Lumileds, 2009.
IES LM-80-08. Approved Method: Measuring Lumen Maintenance of LED Light Sources. Illuminating Engineering
Society of North America, 2008.
IES LM-79-08. Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products.
Illuminating Engineering Society of North America, 2008.
IES LM-65-01. IESNA Approved Method for Life Testing of Compact Fluorescent Lamps. Illuminating Engineering Society of North America, 2001.
IES LM-49-01. IESNA Approved Method for Life Testing of Incandescent Filament Lamps. Illuminating Engineering Society of North America, 2001.
LED Life for General Lighting: Recommendations for the Definition and Specification of Useful Life for Lightemitting Diode Light Sources. ASSIST recommends . . . , Volume 1, Issue 7. Alliance for Solid-State Illumination
Systems and Technologies (ASSIST), 2006.
Rea, Mark S., ed. The IESNA Lighting Handbook, Ninth Edition. Illuminating Engineering Society of North America,
U.S. Department of Energy. Lifetime of White LEDs. Building Technologies Program, Publication PNNL-SA-50957,
ColorKinetics Publications, Useful Life Technical Brief Guideline. [you can find the hardcopy of this post by download the document] Colorkinetics.com/support/whitepapers/