LED Lifetime - What you need to know

The latest variable to fall victim to being inaccurately utilized to reflect the perceived quality of a luminaire is ‘LED Rated Lifetime’.

Don’t get me wrong - "LED Rated Lifetime” is a very important variable for understanding the performance of a luminaire, however it should be used, and interpreted, correctly.

Before explaining what LED Rated Lifetime actually is, allow me to highlight what it is not:

• LED Rated Lifetime is not an indication of the expected lifetime of the luminaire.
• LED Rated Lifetime is not the expected guarantee of a luminaire.
• LED Rated Lifetime does not on its own give you an indication of the overall quality of the luminaire.

So, what then is LED Rated Lifetime?

LED Rated Lifetime refers to the amount of time that an LED (not a light fixture!) can be expected to operate before it loses a significant amount of its brightness. This time is usually measured in hours. Further, the variable should be quoted not just as hours but with two variables accompanying it – ‘L’ and ‘B’, where:

• ‘L’ defines the percentage of light at time t as compared with the initial output, and
• ‘B’ defines the percentage of units expected to fail ‘L’.

Thus, in the context of LED rated lifetime, LED Rated Lifetime should be quoted as follows:

LxBy XX,XXXhrs

For example:

L80B10 50,000 hours means that at 50,000 hours, the LEDs have an 80% lumen output as compared to the initial lumen output, and only 10% of the LEDs are expected to fail to reach the 80% output variable.

Similarly L70B20 100,000 hours means that one can expect only 20% of the LEDs to fail to achieve a 70% efficacy at 100,000 hours as compared to the original output.

Essentially:

• the higher the L variable the slower the predicted LED depreciation, and
• the lower the B variable the better the perceived quality for the said amount of hours.

What one needs to keep in mind however is that the rated lifetime of an LED is based on laboratory testing under specific conditions, and it assumes that the LED is used within certain parameters such as temperature, current and voltage. In real-world usage however, the performance of an LED can be affected by many factors including heat, quality of power supply, as well as variations in manufacturing.

(To determine how the LED operates outside of these laboratory conditions, one would need to analyse the behaviour of the specific model of LED as the junction temperature (Tj) of the LED increases, but that is a topic for another day).

In general, the longer the rated lifetime of an LED, one would automatically assume that the more reliable and longer lasting the fixture will be. In an ideal world, that would indeed be the case, however in a world where quoted variables are king, unscrupulous sellers are known to use such figures to portray a picture that is too often not the case.

The truth is that a fixture will always operate at its best if it is designed to operate in the environment in which it is used. Ie if it designed from inception with its ultimate purpose and environment in mind - if it is fit-for-purpose. A fit-for-purpose luminaire design will attempt to deal with the challenges of the environment as best as possible, be it excessive vibration, poor quality power, or increased heat.

So, the rated lifetime of an LED, whilst very important in understanding the quality of LED used in a luminaire, and thus, potentially, the associated quality of the luminaire itself, should not be looked at on its own to determine the quality of the luminaire, nor should it be likened to a guarantee for the performance of the luminaire.

Thus, the LED rated lifetime is not the same as the Guaranteed lifetime of a fixture.

LED rated lifetime is a measure of how long an LED light source will last under ideal conditions, and the Guaranteed lifetime is how long the manufacturer guarantees the fixture to last under normal operating conditions.

In summary:

• The LED rated lifetime is not a rating for the light fixture – It is a rating for the LED light source itself, which is a component of the luminaire.
• The rated lifetime of an LED is based on laboratory testing under specific conditions, and it assumes that the LED is used within certain parameters such as temperature, current and voltage.
• In real-world usage, the performance of an LED can be affected by many factors including heat dissipation of luminaire, power supply quality, and environmental conditions such as ambient temperature (Ta).
• The performance of a luminaire, on the other hand, depends on various factors such as the suitability of the housing, the type of optics used, the quality of the driver, materials and components, and ultimately, workmanship.
• A luminaire's guarantee usually covers the defects in materials and workmanship of the luminaire, which is different from the LED rated lifetime.
• The rated lifetime of an LED is an estimate of how long the LED will last under ideal conditions, whilst the guarantee of the luminaire is how long the manufacturer guarantees the whole fixture to last under normal operating conditions.

In conclusion, whilst LED Rated lifetime is an important variable, it is not the gauge by which the quality of a luminaire is measured, nor is it an indication of the guarantee of the fixture. In truth, no single variable is the ultimate indicator of quality. Quality is an amalgamation of a multitude of variables all working together to create a solution that works for the environment in which a fixture is designed to operate.