LEDs in a Halogen World:
Adapting Solid-State Lighting Technology to Provide Reduced Downtime in Airfield Signalling

by Peter Baasch, Technical Solutions Manager (Denmark)

READ THIS TO FIND OUT ABOUT: The established protocols of airfield-signalling systems and how these impact solid-state lighting designs. Technologies for leveraging the reliability and efficiency of LEDs in airfield signalling.

Solid-state lighting is challenging traditional incandescent light sources in consumer and automotive applications. Most notably this is occurring in road-traffic signals,where power LEDs are now widely used as the light source in new installations, especially in North America. Peter Baasch, Technical Solutions Manager (Denmark), Future Lighting Solutions outlines the potential for using power LEDs in aircraft signalling applications.

Long life, high reliability and improved efficiency are leading to rapid adoption of LEDs in road-traffic signals and the same reasoning can apply to runway and taxiway lighting in airports. Power LEDs are being used to provide coloured signal lighting, and some white lighting, in airfields today. Beyond this, the technology roadmaps of power-LED manufacturers indicate that all signalling in airfields will be within the remit of solid-state lighting in the foreseeable future. But it is true that the decision to replace incandescent lamps with solid-state lighting is not trivial. The purchase costs of solid-state light sources are thought to be high, and the method for calculating maintenance and replacement costs is different from that used for traditional incandescent lighting.

In the case of airfield signalling, there are also peripheral factors which complicate the evaluation of power LEDs, resulting from the way in which airport lighting is powered and regulated. This article, however, sets out solutions to these challenges, which will help to ensure that the advantages of power LEDs can be brought to airfield lighting.

For taxiways in particular, the reasons for using power LEDs are compelling. In airports the reliability and operating life of power LEDs reduces repair and maintenance costs, and helps eliminate downtime. In airfields, mechanical vibration is a big threat to the halogen signal lamps typically used. Power LEDs themselves are virtually immune to such damage, although the fixture in which they are housed must be carefully designed to withstand vibration. In addition, the efficiency of power LEDs reduces energy costs and provides environmental benefits.

Also, blinking lights are widely used in airfields. This functionality shortens a halogen lamp’s lifetime but has no effect on a power LED.

 

 

Deploying power LEDs in airports: handling the challenges

Power LEDs are suitable candidates today for many airfield-signalling applications. Power LEDs are not yet powerful enough, however, for the main runway lighting.

Runway lights are only permitted to be white, and regulations stipulate a main beam intensity of 5,000cd. Even assuming a 100%-effective lens and prism arrangement (which is practically impossible), this would require an LED with an output of approximately 200 lumens. At first glance this seems to be within reach of the latest LEDs. The highest flux bin of the Philips Lumileds LUXEON® K2 series, for instance, has a typical output of 275lm at 1,500mA. This figure, however, assumes a 25°C junction temperature, which is not readily achievable in this application: thermal design issues are discussed in detail later. In practice, then, a 200-lumen output from a single device is difficult to achieve.

Another way to obtain higher flux values is to use multiple LEDs. One drawback of using multiple LEDs to power a lens-and-prism system such as that required by in-ground signals is that mixing the light from multiple sources has, in the past, been a tough optical challenge.

With the introduction of the LUXEON Rebel LED from Philips Lumileds, however, this task has become much easier, as these miniature power LEDs can be placed as little as 3.5mm apart. System designers will also be able to benefit from the experience of Future Lighting Solutions, which can draw on solutions previously developed by manufacturers of optical equipment.

In-ground taxiway lighting presents far less of an optical challenge to the system designer than main runway lights. Most taxiway signals use coloured light, and to produce this with white halogen lamps requires a filter, which typically absorbs 80% of the light output. Also, placing a prism in front of a lamp with such a broad spectrum creates a rainbow effect at certain viewing angles. This phenomenon does not occur with power LEDs, since they emit saturated, narrow-spectrum colours.

The requirements for in-ground runway and airport lighting are governed by international regulations. These are IEC61827, ICAO annex 14 and FAA AC150/5345. These regulations were drafted before power LEDs were a viable light source. In fact, many of their requirements are specifically based on the assumption that the source of light will be a halogen lamp. As a result, these parameters create difficulties for any manufacturer wanting to substitute existing light sources with power LEDs.

The biggest of these difficulties results from power-supply regulations. Power cables in airports typically stretch over long distances, and the power system therefore uses current loops to combat the voltage drop associated with long cables. To stop a single faulty lamp from cutting off power to all other lamps, current transformers are deployed at every single light unit. The current in the loop is regulated at 6.6A and must be controlled by a dedicated power supply.

 

 

In order to retrofit power LEDs into such an in-ground light, an additional circuit between the current loop and the LED is required to make the LED behave like a halogen lamp.

There are six levels of intensity of light output required, and this is defined by six different levels of input current in the current loop. In halogen lamps the relationship between input current and dimming is far from linear: for instance, at 2.8A or 42% of peak current, a halogen lamp will produce 1% of its peak output. So an LED implementation cannot directly use the input currents applied to a halogen lamp. The dedicated supply circuit for the power LED needs to be designed to provide the light output expected from the specified current inputs.

At the same time, the system engineer must be aware of regulations on colour output. The colours used in airports are white, blue, green, amber and red. LED manufacturers usually specify colours by dominant wavelength, whereas the airport authorities specify them in x/y coordinates. FLS is able to supply tools that perform the conversion of values from one domain to the other.

Difficulties can also arise when white lights are dimmed. The colour temperature of halogen lamps changes significantly when they are dimmed – their white output becomes more yellow. While the colour temperature under dimming is not specified in regulations they do assume the use of halogen lamps.

The colour and Correlated Colour Temperature (CCT) output for white power LEDs, unlike halogen lamps, is stable during dimming operations. Simply dimming a taxiway LED without some form of colour control therefore threatens to invalidate the device for use in airport signalling. On the other hand, the fact that the dimmed white light from LEDs stays white might turn out to be an advantage. Greater contrast with coloured lights reduces the risk of pilots mistaking signals. From a technical standpoint, colour control of power LEDs is straightforward, but the issue here is the uncertainty over how the regulations will be applied to power-LED implementations.

All of the above issues are connected to compliance with international signalling regulations. A successful LED implementation must also handle physical challenges such as temperature. The typical housing for an in-ground taxiway light is an aluminium box measuring 8 or 12 inches in diameter. In many cases, this large 2kg to 4kg aluminium casting offers the perfect heatsink for power LEDs. In hot countries, however, this aluminium housing, embedded in asphalt, could reach a daytime temperature of 90°C, even before the LEDs and their associated electronics are powered up.

The same fixture could be mounted in Arctic regions with ambient temperatures that are 100°C lower. In a halogen-based fixture this difference does not affect the filament, since it is already running at 3,000°C or more.

In LEDs, however, high temperatures shorten lifetime, and temperature variations affect light intensity and CCT. This makes effective thermal design essential, since a long operating lifetime and high reliability are principal reasons for using solid-state lighting. It is important to understand that LED manufacturers specify lifetime in different ways, and designers must be able to compare different devices correctly.

Philips Lumileds has pioneered the statistical modelling of the reliability of power LEDs, and its data sets the standard to which LED manufacturers can aspire. Their data shows the percentage of a batch of LEDs that will reach a certain percentage of their peak light after a specific usage time. This information is provided for any combination of junction temperature and drive current. Data is expressed in what is known as a B/L lumen maintenance curve, and is available at www.philipslumileds.com.

An example maintenance curve is shown in Figure 1. Online tools such as the LED Reliability Tool (LRT) and Usable Light Tool (ULT) can be used to model the impact of this lumen maintenance data in a user’s intended implementation. These are available from www.futurelightingsolutions.com.

These tools will allow the system designer to make informed choices when determining the drive current to be applied, the number of LEDs to be used, the required cooling mechanisms, and when tuning the design for its intended environment.

 

 

	Future Lighting Solutions / Usable Light Tool
	Future Lighting Solutions / LED Reliability Tool