Darren Ward, Group Technical Director of Dextra Group plc. discusses…
It has long been the case that a lack of standardisation in testing methods and data publication with LED sources has allowed a minority to exploit loopholes and exaggerate the performance of their products. As each loophole is closed another seems to open, the most recent of which seems to be an inclination among some lighting designers to disregard long established rules regarding maintenance factors used in lighting designs. Where strict criteria were laid out regarding HID and fluorescent sources some lighting designers now seem to disregard these rules and apply maintenance factors of 0.95 or even 1 across the board when designing with LED luminaires. We need to question why these changes have occurred as the effect on a lighting design can be dramatic. Do these maintenance factors genuinely reflect the performance of LED luminaires or, more cynically, is it a conscious decision to reduce the number of luminaires required on an installation to help sell their products even though this may not truly reflect the overall lifetime performance of the luminaire?
To help us understand why LEDs have brought about this change lets break down what maintenance factors are and the effects LED have had upon them:
Three elements exist to determine the maintenance factor that should be applied to an internal lighting design; Lamp lumen maintenance factor is an estimate of the depreciation of the source between lamp or luminaire changes. Luminaire maintenance factor is an estimate of the depreciation of optics and dirt accumulation on the luminaire that will reduce the light output ratio of the luminaire between cleaning cycles. Room surface maintenance factor is an estimate of the degradation and dirt accumulation that will occur on floor, walls and ceiling finish in a room between decoration cycles that will reduce reflectances and therefore overall light levels. These three elements are combined to calculate a total maintenance factor to be used on a lighting design to ensure that come the next relamp, redecoration and cleaning cycle the correct lux levels are still met. So how does LED differ from more traditional sources that means some lighting designers now appear to disregard these factors?
With the vast majority of LED luminaires there is no lamp to be changed and the source is designed to last for the design life of the entire luminaire. Contrary to popular lighting myth LED sources do depreciate over lifetime, the critical factor here is LM80 data, a method used by LED manufacturers to determine the failure rate and lumen depreciation of the source over lifetime. LEDs are operated at highest permissible temperature and current for a period of 6000 hours to allow a lifetime estimate to be established, typically over 50,000 operating hours. 70% lumen depreciation and 10% failure rate are commonly quoted over these operating hours by quality manufacturers of LED mid power sources, hence in theory this should be applied to the maintenance factor of a lighting design.
So why are we not seeing maintenance factors of 0.7 and greater applied to LED lighting designs to reflect both LM80 lumen depreciation of the source and degradation of optics and room surfaces? There are some crucial differences with LED sources that must be considered before we apply such maintenance factors across the board. Many luminaire designs do not operate the LED to their maximum permissible current and temperature and as a result their lifetime should be dramatically extended and lumen depreciation reduced compared to the LED manufacturer’s LM80 data. The ambient temperature within which they are installed is also a critical consideration. Some LED drivers incorporate a constant lumen feature which gradually increases the drive current to the chip over the 50,000 hour lifetime offsetting lumen depreciation of the source and therefore allowing the lighting designer to make consideration for luminaire optic and room surface depreciation only, although in this scenario an average power consumption over life time should be used for any return on investment calculation rather than the initial power consumption of the luminaire.
Of course LED technology does not eliminate accumulation of dirt on luminaires, optics and room surfaces and nor do they prevent degradation of plastics used in optics so the maintenance factors that have traditionally been applied to these aspects are still entirely relevant.
Whilst LEDs offer massive benefit over traditional sources in terms of maintenance and energy efficiency it is more critical than ever that consideration is made for the quality of the luminaire design, thermal management and quality of LED as well as how hard the LED is driven to ensure that an accurate estimate of maintenance factor can be made. In most scenarios some consideration should be made for source lumen depreciation even if we don’t go as far as assuming a 30% depreciation and 10% failure rate over lifetime. It is clear however that it’s a nonsense to apply a maintenance factor of 1 to any lighting scheme, no matter how good the chip, driver and luminaire design they can never eliminate the need for us to clean our luminaires and repaint our walls.
