The Rated Life, Bp of an LED light source is different from the Lumen Maintenance Life, Lp, and is an essential reliability value that is required by both luminaire manufacturers and end users.
Rated Life, Bp
The rated life of a light source is defined as “The life value assigned to a particular type lamp. This is commonly a statistically determined estimate of median operational life.” (as per ANSI/IES RP-16). 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 50,000 hours means that 50% of the tested products have lasted 50,000 hours without failure. B50 is also known as the products’ rated average life.
If a product has a B10 rated life of 50,000 hours, this means that only 10% of tested products failed within 50,000 hours, so the product should last much longer than a product with a B50 rated life of 50,000 hours.
Rated Lumen Maintenance Life, Lp
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.” It is measured in hours with associated percentage of light output, noted as Lp.
For example, L70 of 50,000 hours means that the tested LEDs produce 70% of the initial light output at 50,000 hours. If an LED has L50 of 50,000 hours, its lumen output depreciates faster than one with L70 of 50,000 hours.
Bp Life Statistical Measure vs Lp Life 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, the sample size will affect the uncertainty of the projection. Larger sample size is preferred in order to increase the statistical certainty.
Rated Life B50L70
LED failure can be defined as when the light output of an LED reaches 70% or lower of the initial light output, including no light.
Using this definition of failure criteria, the combination of statistically measured failures (Bp) with lumen maintenance life (Lp) is the LED light source’s rated life, i.e. BpLp value.
For example, if an LED light source is claimed to have B50L70 of 50,000 hours, then 50% of tested samples should have a lumen maintenance life of 50,000 hours. B50L70 is the industry specification for lifetime of LED products.