|Philips Lumileds has introduced a new way of looking at and analyzing reliability data that should enable lighting designers to confidently determine lifetime performance of LEDs under different operating conditions.
Using a graphical representation of reliability, Lumileds believes designers will be able to understand and evaluate the impact of temperature and drive current on lumen maintenance and failure rates of LEDs.
Lumileds' approach to providing comprehensive and statistically-relevant reliability performance data is explained on the company's website.
LED reliability performance differentiation can be directly attributed to LED die, materials, and packaging technology. Lumileds claims to have the industry’s largest accumulated database of LED lifetime test data with millions of device-hours of information resulting from tests in a matrix of different environmental and drive conditions for many thousands of hours. The company says that "prudent statistical policies" allow lumen maintenance to be forecast accurately to 60,000 hours.
B and L lifetimes
Most representations of LED lifetime use lumen maintenance i.e. the time taken for the average lumen output to drop to a certain percentage (usually 70% or 50%) of the initial value.
Lumileds now proposes to use two lifetime measurements -- an L value and a B value -- where the L value represents lumen maintenance. A figure of L70 lifetime represents 70% lumen maintenance, and is the time of the average lumen maintenance to drop to 70% of its initial value.
However, in any sample, different LEDs will have outputs both above and below the 70% threshold. This is where the B lifetime comes in. For conventional lamps, where failure is common, a B50 value would denote the time taken for 50% of the population to fail (this figure is also called the average rated life).
For LEDs, failure (mortality) is much less common, so instead the B50 figure is a statistical measure denoting the time taken for 50% of the population to drop below the lumen maintenance threshold figure of interest (note that LEDs exhibiting "lumen maintenance failure" will still provide illumination, but slightly below the threshold).
The graphical format used by Lumileds shows several parameters (lifetime, junction temperature, drive current) for a specific B,L set. Here the graph shows (B50/L70) lifetime values, and would allow a designer to select the appropriate combination of drive current and junction temperature to achieve the required lifetime. (B10/L70) and (B50/L50) lifetimes are also commonly represented.