The lighting industry is undergoing a fundamental change with the advent of LEDs.

Visit any lighting exhibition or read the lighting press and it is full of companies promoting LED luminaires; it is almost as if the traditional discharge lamp and luminaire no longer exists. Major OEM companies are investing heavily in LEDs and are only continuing to develop certain high efficiency HID lamp types such as Cosmopolis.

Discharge lighting still has its place and LED technology may not be suitable for all applications. Many claims are made for LED efficiency; life, lack of heat generation etc and great care must be taken when specifying this new technology.

Considerations for Specifying LEDs

Many factors must be taken into account with LED luminaires, such as LED life, sealing, lens design, efficiency of power supplies and maintenance. Can the LEDS be replaced should early failures occur? What maintenance factor do you use? LEDS are not replaced on a three or four year cycle as with discharge lamps and the LED light output at end of the LED module life must be considered.

Interpreting Datasheets for Comparison

LEDs behave very differently to traditional light sources. With a discharge lamp, the output is given by the manufacturer on a data sheet and, when it can be coupled with a reflector of known efficiency, the luminaire output is easily measured. LEDs also come with data sheets giving figures for luminous flux and lumens per Watt, based on low junction temperatures with the LED only briefly flashed on and off. These figures cannot be used for real life situations with LEDs fitted in luminaires as the junction temperature, or temperature of the LED, with be much higher due to heating effects.

Photometric Distribution & Lighting Standards

The photometric distributions now available using LEDs give very different results when trying to design to lighting standards. It is now much easier to achieve the minimum lighting requirements with much lower averages than with traditional discharge sources. Both types of lighting can meet the standard but the “hot spot” under discharge lamp sources can be avoided with the LED lenses spreading the light much better.

Where’s the Proof?

There are many pitfalls in specifying LED lighting and many claims are made which cannot be backed up with evidence. One measure which should be considered is very basic. Look at the energy put into the system vs. the light obtained on the ground.

For example for a fixed spacing geometry such a roadway, a comparison was made between discharge sources and high efficiency LED luminaries. Figures were calculated for the Watts per Lux per square metre (W/lx/m²) for typical installations. Figures were very comparable for Cosmopolis lamps compared to currently available LED lanterns. Only the very latest LEDs were able to show any real benefits in terms of energy savings. However when cost is factored in, modern HID sources are still hard to beat. Most LED lanterns, even the most efficient ones, are still generally more expensive than their HID counterparts.

In summary, it is only in 2012 that we may be turning the corner where LEDs are the most cost effective and energy efficient choice for outdoor lighting.