How LED Lifetime Is Calculated

LED’s are awesome and one of the key points in their advertising is the 50,000 hour lifetime, but how do they actually calculate this value?

50,000 hours = 5.7 years

That is too long for a real test, and the product would become outdated really fast, so what do they do?

LED’s fade and don’t blow.

LED’s don’t actually blow, instead they gradually dim and reduce brightness with age, so that figure is based on LLMF of 70%. The level of light from a light source at any particular moment in the future is referred to as lamp lumen maintenance factor (LLMF). According to long-established industry standards, the lifetime of LED is calculated as the exact period when its output arrives at 70% of initial output.

LLMF Notation : L70 & L50

• L50 = Operating hours until 50% of the brightness is produced.
• L70 = Operating hours until 70% of the brightness is produced.

A standard home bulb would be represented by the equation L 70 = Minimum 50,000 hours.

Why 70%?

As researched by The Alliance for Solid-State Illumination Systems and Technologies (ASSIST), this is the point when the reduction becomes visibly different to the human eye, above this, it is more difficult to noticeably see the difference in brightness.

It is important to note that the output derived from all light sources, LED included usually decreases over the passage of time.

Predicting LED lifetime : Step Stress accelerated testing with Weibull distribution

Many brands such as Phillips use the Weibull distribution method to estimate the lifetime.

This method predicts the future based on the previous behaviour of the objects in question. The full instructions can be found here.

Thousands of units are placed in a few different testing environments and are turned on for a few thousand hours. The change in brightness is measured. This data is collected and then used with the equation to predict the future behaviour.

The more units tested  and the longer the test runs the more accurate the equation becomes.

Usually, this still takes a few  months to be at an accurate level of prediction but that’s still a lot faster than a 5-year real test.

This equation is also used to calculate the weather forecast.

Extending the lifetime

If these standard lifetimes are still not long enough, these methods are used by some manufacturers to give even more runtime till L70 is reached.

• Reducing the supplied current and running them at below-rated output you can extend the lifetime considerably.
• Good thermal design that ensures the LED stays cool will also extend the LED lifetime.

What other things can cause LED failure?

For the most part, LED failure is normally associated with gradual lumen degradation. However, what is known as catastrophic failure can occur from time to time in the lifetime of these light sources. When subjected to very extreme operating conditions, these light sources can fail at some time or the other in their lifetimes. In some poorly designed LED lights or torches, bad heat sinking causes overheating, this could totally burn out the led or increase the rate of decline in brightness. The LED driver that provides the power supply can also be unreliable and cause premature failure, in these cases, it is often the driver failing and the LED remaining operational.

Finally, the exact quality of the material, which make up the LED modules has a strong link to the lifecycle and indeed their overall performance. Therefore, the larger the number of faults in such material, the higher the probability that electron-hole recombination will trigger non radiative decay. Basically, in these situations, the material is sub-standard.

LED’s are the way of the future and we hope this has provided some information on how the lifetime of the LED is estimated by manufacturers. Just like the weather, it is all based on predictions.