Talking LEDs

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When specifying LED light sources for exterior lighting applications, there are a number of new factors to consider, compared to more traditional light sources. Terry Dean of DW Windsor explains

 

Since LED light sources first entered mainstream lighting we have seen some significant advances in LED technology which have considerably broadened their suitability for a wider range of lighting applications.

As a result, LED light sources are increasingly being considered for exterior lighting projects, where they offer significant benefits. These include much higher energy efficiency, enabling organisations to reduce their energy bills and carbon emissions. This is combined with the opportunity to introduce more sophisticated lighting control, such as reducing light levels on roads during quiet periods, with the option to ramp them up again very quickly if needed. Such enhanced control adds to the energy savings achieved through the efficiency of the light sources.

LED sources will also come on immediately following a power outage, without the extended warm-up period required for high intensity discharge (HID) lamps, helping to improve safety.
They improve visibility for drivers and pedestrians through better lighting quality with improved colour rendering, in comparison to more commonly used HID light sources. In addition, the very long life of LED reduces the maintenance costs associated with re-lamping.

For all of these reasons it’s hardly surprising that many specifiers of exterior lighting are embracing the opportunities that LEDs provide with great enthusiasm. However, we must remember that LED light sources are very different from the more traditional light sources (HID) that we in the lighting industry have been accustomed to. This doesn’t make them any less suitable for exterior lighting but it does necessitate a different approach to the specification process if the best solution is to be achieved.

One of the key factors here is that LEDs from different manufacturers can vary considerably, so we need to be more precise about performance criteria. It’s not like for example, a 70W SON lamp, where everyone understands what to expect. In the latter case, a 70W SON lamp from one manufacturer is very similar to that from another manufacturer. Therefore the light output and lamp life are also very similar and most of the added value comes from the expertise of the luminaire manufacturers in maximising light output ratios (LORs) and optimising light distribution.

With LEDs, the power consumption is no longer a useful indicator of the lumen output. In fact, this can vary considerably between manufacturers so thinking just in terms of lumens and LORs is not helpful when specifying LED lighting. A far more meaningful criterion is ‘useful lumens’ as this gives a clearer indication of the lighting performance that can be expected.

One reason for this is LED light sources are inherently directional as each has a lens. When this is combined with luminaires optimised for LEDs the useful lumens can be maximised so that the light is delivered precisely to where it is needed, with little or no losses through a reflector as per a traditional HID lamp. A well-designed fitting can result in an LED luminaire that has a lower lumen output (with a lower installed load) than another LED luminaire, yet delivers more useful lumens. Clearly this is an important consideration as it highlights the interaction between the light source and the luminaire and how this impacts on overall efficiency.

So while there may be a tendency to think all LED luminaires are efficient, because LED sources are inherently efficient compared to traditional HID light sources, the overall efficiency of LED luminaires can vary considerably. The expertise of the luminaire manufacturer in designing optical systems that take advantage of the characteristics of LED light sources is crucial in achieving maximum performance. This, along with the useful lumens, needs to be taken into account when specifying.

Another issue is the drive current that is used with the LEDs, as this also relates to overall efficiency but is often poorly understood. Early LEDs were designed to run with a drive current of 350mA as, broadly speaking, this achieved the peak of the linear relationship between current and lumen output. Above 350mA the efficiency of these early LEDs tailed off significantly (the droop characteristic). Also, there were, and still are, concerns about thermal management at higher currents, combined with a relatively poor understanding of the longevity of modern LED light sources.

That was the situation with early LEDs. Modern LEDs are now able to operate with higher drive currents and the relationship between lumen output and drive current is almost linear above 350mA - without the droop that used to typify higher drive currents. In fact, with some modern LEDs, only 23% of the capacity is being used at 350mA. Which means that 77% of the LED’s capacity is unused!

Consequently, running above the binning current is no longer a matter of ‘over-driving’ the LED. It is simply taking advantage of the characteristic of modern LEDs and using the available capacity of the light source to provide more efficient and cost-effective lighting.

However, there is also a relationship between drive current and lamp life that needs to be taken into account as well. The ‘useful’ life of LED light sources is denoted by the L70, a measurement of light output introduced by the Illuminating Engineering Society (IES). L70 is a measure of how many hours the LED will run and still deliver 70% of its original lumen output. For instance, a light source that emits 1,000 lumens when new and 700 lumens after 50,000 hours has an L70 of 50,000 hours.

Driving a modern LED light source at 350mA could achieve an L70 of 240,000 hours, equivalent to 54 years (if the lighting was used 12 hours per day). In these circumstances the light sources would probably outlive the luminaires and would certainly be an old and inefficient technology by that time. Furthermore, for the reasons given above, they would not be delivering optimum performance during this time.

An alternative would be to run the LEDs at 1,000mA to achieve an L70 of around 75,000 hours with a life of 17 years if used for 12 hours per day – during which time the lighting is delivering optimum performance, potentially resulting in the need for less LEDs or even less luminaires, to achieve the same lighting result.

Thermal management
Driving LEDs at higher currents results in higher heat generation, so when specifying luminaires it’s important to ensure they incorporate good thermal management. This should include an efficient heat sink to maximise conduction of heat away from the LED chips. In parallel, the heat needs to be dissipated away from the heat sink by maximising airflows to create effective convection currents. Using surfaces and coatings that facilitate radiation also plays an important role in thermal management.

This issue places greater emphasis on the quality of the construction of the luminaires, which can also be combined with innovative designs that address the aesthetic requirements of projects as well as lighting performance. There is no longer a need to compromise on aesthetics when using LED light sources.

Looking to the future
In summary, LEDs are subject to constant and rapid evolution and within considerably shorter periods than we have previously been accustomed to. This makes it important that LED luminaires are easy to upgrade in the future. In this way, end users can continue to take advantage of the benefits of LED throughout the life of the installation, effectively future-proofing their investment.

Given the long life of LED lighting, it also makes sense to work with a manufacturer that is likely to be around in the future when the lighting is upgraded. We’ve all seen companies that enter a market to try to make a quick kill with a new technology and then move onto something else.

There are significant benefits to dealing with a manufacturer that has experience of the technical issues relating to exterior lighting as well as the specific criteria that govern LED performance. The fact LEDs are essentially electronic components has resulted in a number of new entrants to the market whose expertise is very much in the electronics field but have limited experience of lighting, if any. Operating in an outdoor environment is one of the most demanding applications for a luminaire, so it’s important that the manufacturer fully understands what’s involved and has a strong pedigree in exterior lighting.