We live in the age of lighting products that last 100,000 hours. Or do we? When it comes to LEDs, questions of product life and failure just aren’t as simple as they used to be.
Lawyer Paul Stone has set out the legal implications of this in an accompanying article. Here, I'm going to explain how to assess a manufacturer's claim about product life, and in particular, what to look for in a warranty.
A failure of a luminaire with a conventional light source was easy to ascertain: if the lamp didn’t emit any light, you tried with a new one. If that didn’t work, you checked the mains supply. If there was still no light, the chances were the luminaire had failed.
"Most luminaire warranties cover ‘failure’ of the light source. But what constitutes failure?"
There was a clear distinction between lamps and luminaires failing, and most luminaires would only contain one or two lamps. Plus, lumen depreciation was easily understood and lamps could be tested to total failure – a fluorescent or HID lamp with a rated life of 16,000 hours requires less than two years of testing.
A world of complexity
With an LED luminaire, the situation is a lot more complicated. Firstly, you can’t just unscrew a failed LED and insert new one (even with Zhaga-compliant modules it’s not that easy). More importantly, most luminaires have many individual LEDs mounted on one or more circuit boards. Several LEDs mounted on a circuit board are normally referred to as an LED module. Both the individual LEDs and the circuit boards may be wired in series, parallel or, more likely, a combination of both. Some high-output luminaires such as a high bays, floodlights or streetlighting lanterns might have 100 individual LEDs on half a dozen modules.
Currently, most luminaire warranties cover ‘failure’ of the light source. But what constitutes failure? Total failure of a few LEDs will hardly affect the overall light output from the fitting. If one LED on a module of a dozen LEDs fails (and there may be several modules in the luminaire) has the module or luminaire failed?
The meaning of life
Since LEDs can emit light over tens of thousands of hours, the industry agreed that the rated life of an LED would be published as the point at which its light output dropped to a certain level. This was generally agreed to be 70 per cent of the initial output, a value designated in technical literature as L70. For example, you might see a rated life of 50,000 hours to L70. There is flexibility, so if you want a very long life unit you can ask for the L value at 100,000 hours instead of 50,000. Conversely, if the light output is critical, you could ask your supplier for the rated life for L90 instead of L70 – the number of hours the LED will last before its output drops to less than 90 per cent of its initial value.
So far, so good, but remember that most luminaires contain many LEDs, and some will perform better than others. This applies equally to the circuit boards on which they are mounted. An LED module of 10 chips where only one chip has failed is obviously a better module than one where five chips have failed, if the working chips on both modules are all emitting 70 per cent of their original light.
The lighting manufacturers, with the best of intentions, have tried to overcome this issue by introducing a second piece of information which represents how likely the product is to fail. You see this on a datasheet as either Fxx or Bxx. For example, L70 F90 or L70 B50. Unfortunately, F and B are different ways of representing failure, which you only stand a chance of understanding if you’re the kind of person who throws expressions like ‘median’, ‘standard deviation’ and ‘skewed bell curve’ into everyday chit-chat. The good news is that the IEC panel that deals with LED life and failure is revising and simplifying its definitions, so things should become clearer soon.
There is one value that has meaning to designers and engineers who want to understand how the light output of the luminaire depreciates over time and that is B50 (not F50, but B50). Ask your luminaire supplier for the rated median life at Lxx B50. As mentioned earlier, L is usually 70 but you can choose other values depending on the application. This will then tell you how much light you can expect from the complete luminaire after a given period of time. How else can you do a maintenance factor calculation?
Also, make sure the data are produced in accordance with IEC PAS 62717 and 62722-2-1 test methods. Most are, but it’s worth checking. Conformance with the US standards IES LM79, LM80 and LM21 is also acceptable. If the luminaire fails to perform to this data, most major manufacturers will honour their warranty in terms of the definition of ‘failure’.
Most warranties demand that you demonstrate burning hours. This could be found from the building management system or a simpler installation might use a time clock. A photocell can be used for dusk to dawn outdoor applications.
There is just one more qualification: the rated life of a luminaire depends on the ambient temperature during operation. This must be stated in the manufacturer’s data. Streetlights obviously operate in much lower ambient temperatures than industrial high bays and, as a consequence, have a longer rated life even if they use identical LED modules. Ambient temperature during operation is harder to verify but shouldn’t really be a problem as long as there is honesty and goodwill on both sides.
Getting it right
Most manufacturers have a ‘standard’ warranty. This covers everything at a fairly basic level and, as a consequence, is for a fairly short period of time. Most manufacturers will offer an extended warranty and this will depend on factors including ambient temperature, switching cycle, dimming regime, operating hours and light/power output. But buyers shouldn't let the supplier dictate what's in the warranty. As Simon Waldron of Sainsbury's puts it, 'the warranty terms are there to protect the manufacturer more than the buyer'. You must sit down with your supplier and discuss the performance you need.
If you are an end user writing a specification and need a guarantee of performance, think carefully about what you want the luminaire to do or what would constitute a failure. I’ve heard of tenders that stated the manufacturer must offer a 15-year warranty, but stipulated nothing about what the warranty should cover – that’s not good. I also know of manufacturers who have had to turn down major contracts because their warranties did not extend to 25 years. And ironically, it is the least responsible manufacturers who are most likely to claim the longest lives. Good luck.