Reliability and Lifetime of LEDs
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LED LIFETIME EXPLAINED The longevity of led technology is considered a major advantage over traditional lighting. With the increasing complexity of module and individual components the reliability, lifetime and the costs associated with changes and revisions has become increasingly important for the customer. More importantly buyers need accurate information to justify purchases and calculate accurate payback. To answer this both LED and luminaire manufacturers need to accurately define lifetime. We have all come across varying luminaire lifetime figures. These lifetime figures can state anything from 200,000 hours to 50,000 hours, these figures may indicate the longer lifetime is best. However, this is not always the case. At present there is no defined way to quantify the LED lifetime figures using a common denominator. In comparison the automotive industry use mpg to compare efficiencies. Although mpg figures can be easily compared, there can still be confusion between these different indicators whether it be ‘urban’, ‘extra urban’ and ‘combined’ values. When comparing a cars economy it is crucial to compare a like for like figure. This is also true when comparing the lifetime of luminaires.
LED LIFETIME BACKGROUND 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. Due to the fact LEDs don’t ‘fail’ like traditional light sources and in order to facilitate a comparison of the long term performance between different LEDs, the LED industry developed a standard to measure lifespan ratings of LEDs. This method was based on the amount of time it takes the light source to dim below certain levels. For many applications it is important for lighting designers to estimate when the light output will drop below 70% of the initial light output. This is widely recognised as an L70 figure at 50,000 hours.
This L70 is the useful life commonly accepted within the industry, this is 70% of the initial lumen output of the luminaire. Luminaire manufacturers may also state L80 or L90, which is 80% or 90% of the initial luminaire output. The Illuminating Engineering Society of North America (IESNA) developed 2 standards to rate this reduction in lifetime, LM-80 and TM-21. These standards are recognised globally within the lighting industry.
LM-80
Before the introduction of LM80, Led manufacturers each reported lumen maintenance data using their own varied systems. To avoid customer confusion, members of the Illuminating Engineering Society (IES) created a standard methodology that would allow customers to evaluate and compare the lumen maintenance of LED from different companies. LM-80 refers to the method for measuring the lumen depreciation.
TM-21
In August 2011, the Illuminating Engineering Society (IES) published the TM-21 document entitled “Lumen degradation lifetime estimation method for LED light sources.” TM-21 is the IES recommended method for projecting lumen degradation of an LED based on data collected according to LM-80 then extrapolated by using algorithms to predict overall life of an LED. There are 2 types of TM-21 ratings, ‘Reported’ and ‘Calculated
Reported TM-21
Reported TM-21 values have an upper limit of 6-times the number of LM-80 test hours. If the LED chip has been tested for 6,000 hours, its max ‘Reported’ TM-21 lifetime would be 36,000 hours.
Calculated TM-21
Calculated TM-21 lifetimes are the final result of the calculated algorithm from the LM-80 data, which is not limited by the LM-80 data.
TM-21 ADDITIONS
In addition to the theoretical limits of the LEDs there are also realistic differences due to every LED ageing differently. In order to try to gain further clarity the IESNA has further developed the standard to include a ‘B’ value. This B value uses the median % failure rate, different manufacturers will state different B values depending on luminaire. For example: A “B” value of B50, means that 50% of the LEDs will fall below the L70 (useful life) figure. A figure of B10 means that 10% of the LEDs will fall below the L70 figure.
acdc STATEMENT When comparing lifetime data it is essential to understand the parameters that affect the longevity of LEDs. A major factor is heat, the lower the junction temperature of an LED, the longer the lifetime. If a luminaire is quoted at 50,000 hours L70 at an ambient temperature of 25°, this will have a shorter lifetime than a luminaire specified up to 50°C. Finding a common denominator when comparing luminaires is required in order to ensure correct choices are made, as is similar with using MPG when comparing car economies. L70B50 @ 50000 hours (40°C) is a common benchmark used by luminaire manufacturers, this is a good baseline when comparing LED Lifetimes. There are discussions within the lighting industry particularly with Lighting Europe to find a common benchmark for the customers to make comparisons easier. This would involve removing “B”values as there are questions regarding how these are measured/validated. This would provide a common denominator similar to how MPG is compared within the automotive trade.
WORKING EXAMPLE If a manufacturer states 200,000 hours @L70 B10 at 40°C. Using the ‘6 times rule’ stated in the report, this means that if stating ‘Reported data’ physical testing will have to have been completed for 33,333 hours. This equates to 3 Years, 9months, 23 days and 21 hours of continuous running. As we all know the fast changing world of LED development, this would either highlight that these figures are not the reported figures acdc state, or that the LEDs used within the luminaire are a very old generation.