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Tungsten halogen vs LED: what's the difference? Kevin Armstrong from ATO airports discusses the technology choice facing airport managers.

The demand for LED technology on airfields is increasing. The recent atg airports global LED airfield ground lighting (AGL) survey forecasts an increase in the number of LED AGL projects taking place in the next two to three years as airports strive to reduce operating costs and find solutions to reductions in carbon emissions, However, the question remains, are LEDs the universal replacement for tungsten halogen lamps in airfield lighting circuits?

The idea of the electric light bulb, which has been used at some time in all airfield lighting systems, has been around since Sir Humphrey Davey invented the first electric carbon are lamp in 1801. These early 'light bulbs' didn't last for any length of time but, over the following years, inventors refined the development using a metal filament inside a glass bulb containing a vacuum. It was found that heating the filament with electric current to around 1,000K caused it to glow and produce light. These early lamps went through much iteration to deliver improvements which, today, provide a useable light output with a reasonable life expectancy of the filament.

The tungsten halogen lights in common use in airfield lighting fittings today have been around since the early 1970s. Lamps used in many airfield lighting applications are generally high-powered units ranging from 45W in taxiway applications to 105 or 150W in runway lighting services, with the high light output luminaires taking up to 300W using multiple lamps. The life expectancy of the lamps is typically around 1,500 hours, although this varies substantially with the level of current flowing through the filament.

The architecture of the airfield lighting circuit has co-evolved with the development of the lamps. Standards have determined that today the primary current in an airfield lighting circuit will be either 6.6A or in some cases 12A. The traditional circuit design used on airfields around the globe consists of circuits powered by constant current regulators (CCRs) providing 6.6A at a generally high level of output voltage into a circuit using standard 6.0mm2 cables connected via a series isolating transformer in the range 30 to 300W which are sized to suit the power requirement of the tungsten halogen lamp used to produce the requisite photometric output.

An important feature of lights in an airfield, which the tungsten halogen lamp is suited to, is the ability of the control circuit to modify the brilliancy of the lights in relation to the operating conditions; day or night, good visibility or poor. This change in output brilliancy is achieved quite simply in tungsten halogen lights by varying the current flowing through the lamp filament. Units, such as series circuit rectifiers (SCRs) and CCRs, were designed using electronic technology to easily alter the output current level by controlling the conduction of thyristors and the light output from the fittings.

A further issue to consider regarding tungsten halogen technology involves the mechanical and electrical properties of the lamp and whether they are robust enough to be used in the airfield environment. The filament within the tungsten halogen lamp is the weak link for mechanical strength, as filament breakage can occur as a result of the vibration created by landing aircraft. Thermal shock of the filament created by energising the AGL fittings from a cold start to maximum current is another source of failure in tungsten halogen lamps, hence CCRs used to power the circuit have a start-up current ramp to gradually increase the current level over a short time period and prevent breakage from sudden surges of current through the filament. Once illuminated, the filament in the tungsten halogen lamp becomes a fairly robust component, able to withstand excessive levels of peak current of the order of five times the rated level for a limited period.

Mindful of environmental issues in this modem age and the limitations being placed by governments on the aviation industry regarding carbon emissions, airports have started to give consideration to the use of lower-energy forms of lighting. Airports are bolting to electronic sources of light, in particular LED technolog which is providing airports with a number of options for replacing lighting in the future. LEDs provide airports with benefits that include reduction in whole lifecycle costs due to lower energy consumption, reduction in an airport's carbon emissions, an increase in lifetime of the order of 50,000 hours and an improved mechanical robustness of the LED module.

A major concern is whether or not this new technology can easily integrate into the existing airfield lighting infrastructure. Tungsten lighting has been around for many years and the airfield lighting circuit has co-evolved to be perfectly suited to this mature technology. With tungsten halogen lighting products we take certain features for granted, whereas with LEDs we need to give further consideration to the fact that LEDs are delicate electronic devices and the benefits outlined above are achievable as long as specific factors are managed.

Unlike tungsten halogen devices that operate at a high temperature and high current levels, electronic items, such as LEDs, fail when operated at excessive currents beyond their rated value for extended periods of time or if subjected to high operating temperatures. This means that to enable the use of LEDs in the airfield lighting circuit we need to find ways of managing these parameters within the light fitting so that those containing LEDs can be immediate plug-and-play replacements for the existing tungsten halogen units in the airfield environment.

One of the principal factors determining LED service life is thermal management. LEDs run at lower temperatures than tungsten halogen devices, but it would be a misunderstanding to say that they run cold. Heat produced within the fitting by the LED modules and also the external heat gained from solar effects has to be carefully managed to maintain the electronic components well below their maximum operating temperature if the benefits of a long operational lifetime are to be achieved.

A second important factor in the desire to use LEDs is to understand how they are different from tungsten halogen when it comes to power supply considerations. Airfield lighting lamps operate directly from the 6.6A supply, the waveform is AC and they are robust enough to withstand current overload for limited periods and continue to illuminate even when the off time of the supply waveform is greater than the conduction period. The LED is an electronic unit that requires a DC supply, has a maximum input current requirement usually of less than 1 amp, is poor at withstanding excessive current levels for any length of time and requires a stabilised power supply to provide the LEDs with energy to illuminate during the conduction off time of the supply waveform. For this reason modem airfield lighting LED luminaires contain an electronic power supply to provide the LED modules power requirements, and enable the connection of the LED lighting fixtures into the existing series circuit architecture.

Two final considerations relate to the fact that LEDs are a relatively new technolog when compared with the history of incandescent lamps. This new technology is rapidly changing as the manufacturers invest in producing better units that provide increased light output for lower energy consumption. No sooner is the unit that you have selected to use bright enough to give you the output required, then it becomes obsolete and its replacement is too bright! Management of the rapidly changing LED technology is the other important factor in the design of the internal electronics in an LED light fitting. The electronics need to control and monitor LED performance to produce the required light output at all times, both with current LED modules and future replacements.

So, are LEDs the answer to replacing tungsten halogen lamps in airfield lighting? LEDs are still a new technology however they do have benefits which airports want access to; lower energy consumption, longer life expectancy better colour output without the need for filters and slower degradation of light output from the LED module than a tungsten halogen filament. However tungsten halogen is a simple proven technology and the architecture of the airfield lighting circuit has evolved alongside the lamps. Finding a plug-and-play replacement using electronic LED modules presents challenges in ensuring the LEDs can deal not only with the standards imposed by the existing circuit design but the environmental operating conditions faced on an airfield. Products such as atg's IRIS range of high-intensity LED AGL light fittings provide airports with a real alternative to tungsten halogen.

LEDs represent the first major step change in airfield lighting design since standards were laid down. However, reaping the total benefits from this exciting new technology requires modem thinking to re-develop the airfield lighting circuit. Only then can we truly maximise the LED's power saving benefits, whilst maintaining the same level of customer choice between vendors that the industry currently has with simple tungsten halogen lamps.
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Publication:Airports International
Date:Oct 1, 2013
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