One of the major advantages of LEDs over more traditional light sources is their superior lifetime characteristics. In addition, LEDs display a gradual degradation in light output over time under correct operating conditions. For this reason, the lighting industry’s MTTF (Mean Time to Failure) approach to product lifetime must be replaced by new lifetime metrics for lumen maintenance.

The Lighting Research Centre at Rensselaer Polytechnic Institute, together with the major LED manufacturers has defined more suitable lifetime metrics for LEDs through the ASSIST (Alliance for Solid State Illumination Systems and Technologies) program.

L70% (hours): time to 70% lumen maintenance
L50% (hours): time to 50% lumen maintenance

The L70% figure represents the time taken for the LED to reach 70% of its original light output. Research has shown that a 30% drop in light output over the lifetime of a light source is acceptable for most lighting applications.

Similarly, the L50% figure represents the time taken for the LED to reach 50% of its original light output. This 50% drop in light output is considered acceptable for decorative lighting applications.

Source: http://www.lrc.rpi.edu/programs/solidstate/

LED Lifetime Measurement

At NuaLight, we have a dedicated facility for the validation of product lifetimes. Reliability tests are run on all NuaLight products under typical operating conditions, including freezer, cooler and room temperature environments. Ambient temperature and humidity are well controlled along with the LED current to reduce the number of variables associated with the measurement.

The LED characteristics are monitored consistently using calibrated instrumentation and results are correlated to produce estimates of product lifetime.

LED lifetime is dependent on a number of factors that must all be taken into account when producing a figure for LED lifetime:

• LED current, I
• LED junction temperature, Tj
• Ambient operating conditions (relative humidity and temperature)

LED Lifetime Results

NuaLight has accumulated in excess of 5000 hours of test data on the CryoLED2 product which facilitates an extrapolation of lifetime. From the measurement results, CryoLED2 has a lifetime of L70% >50,000 hours in a freezer environment.