When will we know it is time to replace an LED product that is way past its intended design life? Gordon Routledge takes inspiration from the past.
Last week was a trip down memory lane in the form of a trip to Manchester and the rigours of some judging for the Lux Awards. It seems that Manchester has upped it’s game this year, with a raft of entries across all sectors. The government's much-touted northern powerhouse is this year's Lux Awards northern lighthouse. Manchester is a city I am very familiar with as I went to university there. After graduation, I booked a holiday in a travel agents, yes before you could do such things online, which the week after booking was in the epicentre of an IRA terrorist bomb attack. Luckily, nobody was killed, the only casualty being the 1960s architecture in that part of the city; no great loss, some would say. Little did I know at the time that this terrorist event would lead over the coming years to me picking up a significant amount of lighting business as the whole area was redeveloped.
So what has this tale to do with my missive? Well, right at the front of Marks & Spencer, above the door, is one of the first LED products I ever developed in the shape of an edgelit exit sign. A quick check on wikipedia confirmed that the store had been open since 1999. Being an exit sign on a central battery system, there is a good chance that the product has been lit continuously for 16 years or around 140,000 hours. Not quite enough to rival the Livermore light bulb, but it is an achievement I’m quite proud of given the state of white LED technology at that time.
To get the required light output, the LEDs had to be driven quite hard, and they had some inherent degradation problems in the shape of epoxy resin which yellowed under the very light they produced. So that is the good news - a well-engineered LED product can last the distance. Don’t let anyone tell you that they can’t get a driver to last for 15 years; you can, but you may need to pay more, and the same goes for the LEDs, in this instance from Nichia.
The bad news, as you can see from the picture of the sign above, is that the LEDs are not very bright, probably don’t meet the light levels required for the emergency lighting standards and, in truth, should probably have been replaced five years ago. This brings up some challenging issues which we will face over the coming decades.
If an LED product is well designed, the LEDs don’t die, they simply fade away. So will that street be lit to the required light levels in 10 years' time? At what point do we start to change out a LED panel in an office?"
If an LED product is well designed, the LEDs don’t die, they simply fade away. So will that street be lit to the required light levels in 10 years' time? At what point do we start to change out a LED panel in an office? Will someone measure the light levels or do we simply wait until the complaints and optician's bills reach an untenable level?
Lumen degradation is an issue for all applications. The best LED systems can compensate for this in the driver by gradually increasing output over time to compensate, or doing the same on a bigger scale using light level measurement in the control system. The alternative is to compensate in the lighting design which results in spaces being over-lit for most of their lives. Sadly, cheap is the route too many lighting installations choose, with no smart drivers or controls. Let's just hope that they are so cheap that the LED driver fails, forcing a change out, before we are working in the dark!