Understanding the difference between LED rated life and lumen-maintenance life (MAGAZINE)
The rated life of an LED light source is different from
the lumen-maintenance life, and is an essential reliability value that is
required by luminaire makers and end users, as JIANZHONG JIAO explains.
With the completion of the IES test method TM-21-11 (see page 9),
the SSL industry now has a standard method of obtaining long-term
lumen-maintenance information for LED light sources. The method is made up of
two steps. First, the LED light sources must be tested per LM-80. The new TM-21
method is then applied to the collected measurement data to make
lumen-maintenance projections, including in-situ temperature calculations.
However, there is still one measure that is missing: the rated life for
LED light sources. Rated life is an essential reliability property for LED
integrators that design LED luminaires, providing luminaire users with warranty
and usage information
Rated life
The rated life of a lamp or
light source is defined, per 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.” The rated life in hours of an LED lamp or light
source, specified by the manufacturer, applies under certain operational
conditions and for defined failure criteria. The statistical measure for the
rated life is designated Bp and is measured in hours, where p is a percentage.
For example, a B50 rated life of 1,000 hours means that 50% of the
tested products have lasted 1,000 hours without failure. B50 is also known as
the products’ rated average life.
If a product has a B50 rated life of
1,000 hours, this means that only 10% of tested products failed within 1,000
hours, so the product should last much longer than a product with a B₅₀ rated
life of 1,000 hours.
Lumen-maintenance life For LED light sources,
LM-80 defines lumen-maintenance life as “the elapsed operating time at which the
specified percentage of the lumen depreciation or lumen maintenance is reached,
expressed in hours.” Different from rated life, the rated lumen-maintenance life
is defined as “the elapsed operating time over which an LED light source will
maintain the percentage (p) of its initial light output.”
Rated
lumen-maintenance life is measured in hours with associated percentage of light
output, noted as Lp. In other words, L<SUB<70< sub> of 30,000 hours
means that the tested LEDs produce 70% of the initial light output at 30,000
hours. If an LED has L50 of 30,000 hours, its lumen output decays faster than
one with L70 of 30,000 hours.
While Bp life is a statistical measure,
Lp life is a defined durability measure. When testing for Bp life, a large
statistically-meaningful sample size is required. When testing Lp life, there is
no sample size requirement. However, when LM-80 test data is utilized to make
lumen-maintenance projections (per TM-21), the sample size will affect the
uncertainty of the projection. As a consequence, a smaller sample size will lead
to shorter projected life in order to increase the statistical certainty.
Failure
For LED light sources, one can define failure as when
the LED can no longer produce a certain percentage of the initial light-output
value. For example, failure might be defined as when the light output of an LED
reaches 70% or lower of the initial light output (including if the LED’s light
output is zero). In other words, for a given period of time, if an LED produces
insufficient light or no light, the LED is considered at failure. Using this
definition of failure criteria, the statistical measure can be combined with the
defined durability measure. The combination of lumen-maintenance life (Lp) with
statistically-measured failures (Bp) is the LED light source’s rated life, or
BpLp value. For example, if an LED light source is claimed to have B₅₀L₇₀ of
30,000 hours, then 50% of tested samples should have a lumen-maintenance life of
30,000 hours.
Ideally, to obtain the rated life for LEDs, the statistical failure
measurement can be integrated with lumen-maintenance measurements during the
life test. One can use a large LED sample size, large enough to be statistically
meaningful as when measuring traditional lamps, and then track and record the
sample behaviors including light-output change and failures during the life
test. When 50% of the tested samples reach a light output equal to 70% of
initial lumens, including the samples that failed to produce light, then B₅₀L₇₀
(in hours) is obtained.
Obviously, as is the case with lumen
maintenance, it is not practical to conduct real-life tests to get B₅₀L₇₀ values
when such a value can be as long as 30,000 hours, or nearly three and a half
years. The challenge is how to make a projection using the data obtained in a
shorter testing period.
Projections
LED manufacturers have been conducting studies and establishing practices
for reliable approaches to project the rated life for LEDs; in general, there
are two approaches.
The first approach is to conduct LM-80 testing with
a large sample size. The test data are collected for both light-output changes
and failures. The data is then fitted into a mathematical model with a
statistical-certainty band. In addition to the lumen-maintenance projection
curve, the associated sample distribution bandwidth is also plotted. By
analyzing the curve and bandwidth, an estimated B₅₀L₇₀ life is projected.
The second approach is to conduct the lumen-maintenance (LM-80) test
separately from the accelerated-failure-modes test. Using TM-21, the
lumen-maintenance projection can be established. The data collected in the
accelerated-failure-modes test are modeled with a different mathematical
expression. The rated life is then projected by mathematically combining both
models.
There are some discussions in standardization organizations
regarding development of a document or recommendation to address LEDs’ rated
life. To help the LED lighting industry to properly use LED light-source
information, it is necessary to clarify that rated life is not lumen-maintenance
life.
Before the industry establishes a recommendation for a standard
practice, LED integrators may need to request more testing and modeling
information from the manufacturers in regards to the statistical failures of LED
light sources.
About the Author
JIANZHONG JIAO, Director of Regulations and Emerging Technologies at
OSRAM Opto Semiconductors, Inc., is actively involved in LED and SSL standard
development activities. He is Chairman of the SAE Lighting Committee, past
Chairman of NGLIA and past Chairman of the NEMA SSL Technical Committee. He is
an active member of IESNA’s Testing Procedure Committee and Roadway Lighting
Committee, as well as ANSI SSL Working Groups, the Standard Technical Panel of
UL8750, and standards committees within IEEE, CIE USA, SEMI, JEDEC and other
organizations.
With the completion of the IES test method TM-21-11 (see page 9), the SSL industry now has a standard method of obtaining long-term lumen-maintenance information for LED light sources. The method is made up of two steps. First, the LED light sources must be tested per LM-80. The new TM-21 method is then applied to the collected measurement data to make lumen-maintenance projections, including in-situ temperature calculations.
However, there is still one measure that is missing: the rated life for LED light sources. Rated life is an essential reliability property for LED integrators that design LED luminaires, providing luminaire users with warranty and usage information
Rated life
The rated life of a lamp or light source is defined, per 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.” The rated life in hours of an LED lamp or light source, specified by the manufacturer, applies under certain operational conditions and for defined failure criteria. The statistical measure for the rated life is designated Bp and is measured in hours, where p is a percentage.
For example, a B50 rated life of 1,000 hours means that 50% of the tested products have lasted 1,000 hours without failure. B50 is also known as the products’ rated average life.
If a product has a B50 rated life of 1,000 hours, this means that only 10% of tested products failed within 1,000 hours, so the product should last much longer than a product with a B₅₀ rated life of 1,000 hours.
Lumen-maintenance life For LED light sources, LM-80 defines lumen-maintenance life as “the elapsed operating time at which the specified percentage of the lumen depreciation or lumen maintenance is reached, expressed in hours.” Different from rated life, the rated lumen-maintenance life is defined as “the elapsed operating time over which an LED light source will maintain the percentage (p) of its initial light output.”
Rated lumen-maintenance life is measured in hours with associated percentage of light output, noted as Lp. In other words, L<SUB<70< sub> of 30,000 hours means that the tested LEDs produce 70% of the initial light output at 30,000 hours. If an LED has L50 of 30,000 hours, its lumen output decays faster than one with L70 of 30,000 hours.
While Bp life is a statistical measure, Lp life is a defined durability measure. When testing for Bp life, a large statistically-meaningful sample size is required. When testing Lp life, there is no sample size requirement. However, when LM-80 test data is utilized to make lumen-maintenance projections (per TM-21), the sample size will affect the uncertainty of the projection. As a consequence, a smaller sample size will lead to shorter projected life in order to increase the statistical certainty.
Failure
For LED light sources, one can define failure as when the LED can no longer produce a certain percentage of the initial light-output value. For example, failure might be defined as when the light output of an LED reaches 70% or lower of the initial light output (including if the LED’s light output is zero). In other words, for a given period of time, if an LED produces insufficient light or no light, the LED is considered at failure. Using this definition of failure criteria, the statistical measure can be combined with the defined durability measure. The combination of lumen-maintenance life (Lp) with statistically-measured failures (Bp) is the LED light source’s rated life, or BpLp value. For example, if an LED light source is claimed to have B₅₀L₇₀ of 30,000 hours, then 50% of tested samples should have a lumen-maintenance life of 30,000 hours.
Ideally, to obtain the rated life for LEDs, the statistical failure measurement can be integrated with lumen-maintenance measurements during the life test. One can use a large LED sample size, large enough to be statistically meaningful as when measuring traditional lamps, and then track and record the sample behaviors including light-output change and failures during the life test. When 50% of the tested samples reach a light output equal to 70% of initial lumens, including the samples that failed to produce light, then B₅₀L₇₀ (in hours) is obtained.
Obviously, as is the case with lumen maintenance, it is not practical to conduct real-life tests to get B₅₀L₇₀ values when such a value can be as long as 30,000 hours, or nearly three and a half years. The challenge is how to make a projection using the data obtained in a shorter testing period.
Projections
LED manufacturers have been conducting studies and establishing practices for reliable approaches to project the rated life for LEDs; in general, there are two approaches.
The first approach is to conduct LM-80 testing with a large sample size. The test data are collected for both light-output changes and failures. The data is then fitted into a mathematical model with a statistical-certainty band. In addition to the lumen-maintenance projection curve, the associated sample distribution bandwidth is also plotted. By analyzing the curve and bandwidth, an estimated B₅₀L₇₀ life is projected.
The second approach is to conduct the lumen-maintenance (LM-80) test separately from the accelerated-failure-modes test. Using TM-21, the lumen-maintenance projection can be established. The data collected in the accelerated-failure-modes test are modeled with a different mathematical expression. The rated life is then projected by mathematically combining both models.
There are some discussions in standardization organizations regarding development of a document or recommendation to address LEDs’ rated life. To help the LED lighting industry to properly use LED light-source information, it is necessary to clarify that rated life is not lumen-maintenance life.
Before the industry establishes a recommendation for a standard practice, LED integrators may need to request more testing and modeling information from the manufacturers in regards to the statistical failures of LED light sources.
About the Author
JIANZHONG JIAO, Director of Regulations and Emerging Technologies at OSRAM Opto Semiconductors, Inc., is actively involved in LED and SSL standard development activities. He is Chairman of the SAE Lighting Committee, past Chairman of NGLIA and past Chairman of the NEMA SSL Technical Committee. He is an active member of IESNA’s Testing Procedure Committee and Roadway Lighting Committee, as well as ANSI SSL Working Groups, the Standard Technical Panel of UL8750, and standards committees within IEEE, CIE USA, SEMI, JEDEC and other organizations.
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