The team at ROBUS offers advice on how electricians and electrical professionals should respond to the age-old conundrum of how long LEDs really last.
The question around the length of time that LEDs will last is one that you will almost certainly be asked – whether it comes from a client, a colleague, an apprentice, or perhaps it might be the million-pound question if you one day end up on Who Wants to be a Millionaire?
The introduction of LED light sources into the lighting market resulted in many exaggerated claims about the amount of light they emit and their longevity. But, with respect to the facts, let’s dig into the real measurement of the LED lifecycle.
LEDs don’t last forever
The graph (pictured below) shows how different light sources perform over time.
Although each technology experiences losses at a differing rate, all of them experience a decrease in efficiency. How can we measure and define this for LED luminaires as a standard throughout the lighting market? That is the very purpose of TM-21.
TM-21: the long-term lumen maintenance method
TM-21 is the globally accepted method for projecting the long-term lumen maintenance of an LED light source. In other words, how long LEDs last. The TM-21 method uses lumen depreciation data, also called LM-80, to make these projections. That’s quite a collection of abbreviations!
To begin with, we need to clarify what LM-80 represents.
How to measure lumen depreciation
LM-80 is the collection of lumen depreciation data for at least 6,000 hours, but how does it work in practice? LED suppliers test their LEDs (in batches of 20/30 units) for 6,000 hours and measure the lumen depreciation every 1,000 hours.
For example: a sample quantity of 20 units, tested for 9,000 hours at a temperature of 25°C can have the results extrapolated to 5.5 times their value. The same test, when run on 30 units allows for an extrapolation to 6 times their value. For instance, 6 x 9,000 hours = 54,000 hours. At ROBUS, we always round this down to 50,000 hours.
The above data establishes the benchmark used to define the useable life of the LED product in hours.
This data is put into the TM-21 system, and this is how the expected lifetime of an LED can be deduced. The method produces several useful values that provide insights into how long the LEDs should last. These values are known as L-values, B-values, C-values, and F-values.
L-value: measures luminous flux and colour maintenance
The L-value (lumen maintenance value) indicates the expected decline in light efficiency over a specific duration. Humans can discern changes in light levels when the light output is reduced to 70% of the initial lumens, which is why at ROBUS we use L70 as our standard.
For example: a typical 1,000 Lm product, rated at 50,000 hours, could produce as little as 700 Lm by the time 50,000 hours have elapsed. Therefore, it retains 70% of its initial brightness and loses 30% of its light output. The L-value in this example would be labelled as L70.
One action you may take would be to increase the number of LEDs by 30% after 50,000 hours to ensure 1,000 Lm are maintained.
B-value: the failure percentage
B-value indicates the variation of the expected lumen level at the specified hours. This value is directly related to the L-value. When the B-value is B50 and the L-value is L70, it means that we can anticipate that 50% of the LEDs will not meet the L70 threshold.
How do we assess B-values? Generally, is it not tested because the product would need to run for the full duration as results can’t be reliably extrapolated i.e. a 50,000 hour product would require a six-year test!
Therefore, B-values often go untested. Instead, the industry typically defines them based on data from TM-21. However, this makes them easy to manipulate. It is important to only rely on trusted suppliers for this information. ROBUS aims to be fair and reasonable by maintaining a realistic B50 rating, which you can confidently trust.
C-value: percentage of catastrophic failures
The C-value represents the anticipated percentage of catastrophic failures during the LED’s lifetime. A catastrophic failure is when the LED ceases to produce any light. It is worth noting, the failure of a single or several LEDs in a cluster isn’t included in this value as these are accounted for by the B-value.
For instance, the C-value can be C10 or lower. C10 would mean that after the specified duration, 10% of the LEDs might have experienced catastrophic failure. The C-value for standard LEDs is often much lower and in the range of 1%, and as such, it is often disregarded and not disclosed for indoor products.
F-value: combination of B and C
Some manufacturers of LED luminaires may use the F-value instead of B and C values. The F-value is a sum of the B and C values:
Summary
So, how long do LEDs really last? The quickest answer can be found in the technical specifications of the product, but it is worth noting that when a specifications sheet mentions XX,000 hours it isn’t the true lifespan – it’s what LEDs are tested to.
The key is to choose a supplier whose information you can trust as, unfortunately, it is possible to manipulate these numbers. These specifications typically include the hours of lamp life, and for reputable suppliers like ROBUS, this information is obtained through the TM-21 method.
But, for your winning Who Wants to be a Millionaire? answer, you can rightly say that it depends on its daily usage!
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