All lighting technologies available on the market from HID and fluorescent to LED suffer from lumen depreciation, the process by which the lumen output produced by the source reduces over life time. Lumen depreciation is dependent on a number of factors such as the type of source used, the operating temperature of the source and the operating hours undertaken. Lumen depreciation varies widely between technologies and is generally lowest on sources such as fluorescent and LED and highest on sources such as HID.
When consideration is made for lumen depreciation in any lighting design several negatives become apparent. Lighting schemes should always be produced with consideration for the lumen depreciation between maintenance schedules when replacement of the lamps or luminaires is undertaken. This is referred to as the maintenance factor, a percentage by which the estimated initial lux level on the scheme is reduced. As a result most installations are over lit at the outset resulting in increased capital cost of the installation and unnecessary energy consumption. Where maintenance schedules are not adhered to there is a danger that lumen depreciation can reduce lumen output to an extent where the installation no longer complies to the required lux levels as stipulated in CIBSE and HSG38 as a result of which liability can occur in the event of an accident. It is therefore desirable to eliminate or reduce lumen depreciation wherever possible.
Philips produce a range of LEDs and drivers with a “Constant Lumen Output” feature which are incorporated into a variety of Dexeco luminaires. These LEDs and drivers offset lumen depreciation by slowly increasing the output of the driver. As a result lighting designs can be carried out with these systems with minimal maintenance factor allowing installations to be lit correctly at the outset. Whilst it is always advisable to use a small maintenance factor to offset factors such as dirt accumulation on reflectors the use of a “Constant Lumen Output” source can significantly help reduce energy consumption and capital cost of an installation.