Terms like L70, B10, F10 @ 60,000 hrs are often quoted in specifications but what do all these terms mean? Furthermore are the methods used to measure them standardised and universally adopted?
In order to answer these questions, we need to look at how these figures are derived.
LED Lifetime
The lifetime data that we receive for LEDs from the manufacturers is generated by the combination of two standardised processes developed by the Illuminating Engineering Society (IES): an experimental measurement (IES LM-80-08) and a mathematical prediction (IES TM-21-11). A sample quantity of LEDs is tested to the LM-80 standard to determine the relative lumen output over time of the individual samples. This data, gathered over at least 6k hours but preferably over more than 10k hours, is then mathematically projected according to TM-21 to result in an industry standardised lifetime prediction. This lifetime prediction is called the “Rated lumen-maintenance life” and is defined in LM-80 as “the elapsed operating time at which the specified percentage of the lumen depreciation or lumen maintenance is reached, expressed in hours.” This is denoted as Lx where “x” is the specified percentage of the lumen maintenance. A common example is L70 50000h which means that after 50000 hours, the LEDs will emit at least 70% of their initial lumen output on average.
LED Module Lifetime
The lifetime of an LED module is usually expressed in terms of its “useful life” denoted as LxBy. This is defined in IEC 62717 as “The length of time until a percentage y of a population of operating LED modules reaches gradual light output degradation of a percentage x”. For example, a useful life of L70B10 50000h means that after 50000 hours, 10% of modules will have degraded to 70% of their initial lumen output. The “median useful life” denoted as LxB50 indicates the time at which half of the modules have degraded to x% of their initial lumen output.
Where the B-value specifies gradual degradation in the lumen output of modules and luminaires, the C-value deals with abrupt lumen output and is defined as “The length of time until a percentage y of a population of LED modules reaches abrupt light output degradation of a percentage y is called the time to abrupt failure and expressed as Cy” (IEC 62717). For example, C10 of 50000h means that after 50000 hours, 10% of modules will fail to produce any light at all. It is uncommon to see C-values in the LED Lighting industry as it is rare for LEDs to fail catastrophically.
The B and C values can be combined into a single lifetime called the “LED Lamp Life” expressed as “MxFy” and defined as “The length of time until a percentage y of a population of LED lamps reaches combined gradual and abrupt light output degradation, meaning the LED lamps have either parametrically failed, no longer producing at least x % of their initial luminous flux, or abruptly failed”
LED Luminaire Lifetime
The IEC standard for Luminaire performance (IEC 62722-2-1) recommends that the same B, C and F values be specified at a luminaire level.
However, as is stated in the standard: “Lifetime of LED luminaire can be far more than what practically can be verified with testing” – The maximum test time required for compliance with the standard is only 6000h and “Due to this limited test time, the claimed life of a LED luminaire cannot be confirmed nor rejected in most cases”. For this reason, it is necessary to make a mathematical projection of the test data to predict the lumen output of the luminaire at 50000 hours. Using a similar methodology to that of predicting LED lifetime, it is possible for luminaire lumen output to be tested and projected according to the relevant luminaire standards (IES-LM-84-14 and IES-TM-28-14 respectively).
Unfortunately, as is the case with the lumen depreciation projection method for LEDs, the projection method used for luminaires uses an average lumen degradation based on a relatively small sample size. While this sample size is large enough to produce a reliable averaged projection of rated lumen-maintenance life, it is too small to produce a B-value of any statistical significance. And while the IEC standard makes recommendations on what lifetime metrics should be stated it does not define how those lifetime metrics should be calculated, or what lumen degradation projection method should be used.
At LEDwise, we design our luminaires to run the LEDs to at least L70 of 50k hours, but most of our LEDs will perform well above this minimum design requirement. We achieve this through applying our ‘belts and braces’ approach of ensuring excellent thermal performance of our luminaires, never over driving our LEDs, using high-quality LEDs from leading manufacturers and always considering the worst case. Due to the uncertainty in the determination of the B value for luminaires, we don’t believe it is a meaningful metric to compare luminaire lifetime.
For more information on our LED lifetime conundrum, please contact our team of South African engineers or follow LEDwise to stay updated on our latest innovations and industry news.