Light emitting diodes for airport applications – not just ‘plug and play’

When people think of a new technology to produce light, it is widely thought of as ‘just another light bulb’. This has been true, in a general sense, since the invention of the incandescent light over 100 years ago. This incandescent technology, while improved over the years, has not changed significantly from the basic design. A filament is placed in an oxygen-free ‘bulb’ and excited by electricity causing the filament to glow, thus producing light. Incandescent lights are not very efficient. Most of the energy needed to illuminate the bulb is lost in heat, with very little available to generate its main function of producing light.

There have been developments in recent years with other technologies that can produce light more efficiently and without the need for the ‘bulb’. One of the technologies being considered by the Federal Aviation Administration (FAA) is the Light Emitting Diode (LED). This technology produces less heat and more light per energy consumed.

The FAA, like many other organisations around the world, wanted to take advantage of this new more efficient light source, while making no changes to the infrastructure already in place. It was the attitude, ‘it’s just another light bulb’. Just design it to look like the incandescent lamp, removing the old and plug in the new (plug and play). It was quickly realised whilst this technology is very efficient, the current infrastructure is designed around the incandescent source, which does not allow for effective and efficient ‘plug and play’ capability for new technologies.

Airfield lighting circuits in the United States are designed as a 6.6 ampere series circuit, using alternating current (AC) and constant current regulators (CCRs). After implementation of replacement LED versions of taxiway edge lights, it was found that when changing the intensity of the lighting circuits, the incandescent fixtures would dim to the proper level, while the LED versions on the same circuit would not appear to dim at all. Also, the LED version appeared brighter than the incandescent when set at the same intensity.

What we learned from this is that we could not just ‘plug and play’ and specifications would have to be readdressed before LEDs would work in the current infrastructure. Also, the plug and play method may not be most the efficient way to utilise this new light source.

As a result of this, the FAA initiated a research program to determine the best way to implement LEDs into the existing infrastructure. The two major goals are:

1. To determine what specifications need changing, to be able to use this technology in the existing system.
2. To develop the criteria for a new infrastructure based on non-incandescent technology, to obtain the most efficiency possible.

Guidance material in the form of an engineering brief (67b) was developed by the FAA, to help during the transition to new technology and before research results could be obtained.

The FAA’s Airport Safety Research and Development Sub-Team’s Visual Guidance Program, located at the William J. Hughes Technical Center in Atlantic City, NJ, has initiated research to develop these specifications. This research is in support of the office of airports safety and standards and the lighting systems office who are responsible for airport and approach lighting specifications.

A team was formed, in conjunction with the International Civil Aviation Organisation’s (ICAO), Visual Aids Working Group (VAWG), to identify and clarify what the issues were before meaningful research could be performed. There were eight issues identified and following is a brief discussion on each.

Issue One

How will this technology interact if interspersed with standard incandescent lights? Is there a problem in the ratios between lighting systems?

The FAA realised this problem early, when LED taxiway edge lights were first installed at airports and interspersed with standard lights. Pilots complained that all the edge lights did not look the same, and neither were they consistent. It was then decided that intermixing of these technologies was not desired. Wording was included in engineering brief 67b to prevent that situation.

After noticing in the field that the LED fixtures appeared ‘brighter’ than the equivalent incandescent version, a study was conducted to determine if indeed the LEDs were brighter. The results indicated that there was indeed a perceived ‘brightness’ difference, because LEDs are a more saturated light source than the incandescent. This however, only occurs when the atmosphere is clear. So, as the atmosphere becomes less clear and weather deteriorates, LED light signal colours become desaturated by scatter in fog, reducing this ‘brightness’ perception component. This, in effect, makes the LED a desaturated source equal to the incandescent in these conditions.

Issue Two

How will this technology interact with present airport systems?

Studies were conducted on airport circuits and concluded, that specifications need to be updated in order for this technology to interact in a positive way.

Issue Three

What are the impacts of intensity changes with LEDs?

The existing infrastructure changes intensity by altering the current/voltage supplied to a light fixture. LEDs do not dim the same when current/voltage is changed. One fix for this was, to include in the electronics of the fixture circuitry called Pulse Width Modulation (PWM), which enables the LED fixtures tops dim correctly using this voltage change. While not necessarily the most efficient way of dimming LEDs, it was the only way to accomplish the task within the present infrastructure. Work is ongoing to establish the operational criteria for a new infrastructure, to include ways of intensity control, and to take advantage of the more efficient non-incandescent sources.

Issue Four

Does the narrow spectral band of LED impact pilots with certain types of colour deficient vision?

Some issues have arisen, purely on speculation or lack of adequate knowledge of the issue of the LED technology. Before considering the LEDs, a brief discussion concerning colour is needed. Colour or chromaticity is defined and illustrated using a system of coordinates, depicted on a diagram, which shows the geometrical representation of colours and their relationship to one another. There exists a condition where a portion of the population has problems identifying colours, especially, between red and greens. This situation has been termed and quantified as chromaticity confusion lines. These lines refer to lines plotted on the internationally recognised Commission Internationale de l’Eclairage (CIE) chromaticity diagram. This condition is not related to a technology, rather, it is directly related to where a given light source’s chromaticity coordinates falls on the CIE diagram. In the case of the FAA’s aviation red boundary, which is the standard for most red lights at an airport, the region is very narrow compared to other red boundaries. This would make it less likely for a light in the region to fall on one of those lines, which means, it would be less likely that a person would mistake aviation red for some other colour.

Today’s incandescent lights produce a yellowish white light, which, when colour is desired, is passed through a coloured filter to produce the desired colour. Where the colour falls is restricted to be this yellowish white source and the capability and quality of the filter. This inability to precisely guarantee a particular colour makes it more likely that the chromaticity coordinates could fall on one of the chromaticity confusion lines. LEDs do not require filters, so, they can be specified to meet a more exact chromaticity coordinate range, which could reduce the likelihood of them falling on one of these confusion lines.

The FAA’s Airport Safety Research and Development Sub-Team’s Visual Guidance Program has been tasked to conduct research (given a technology that is more flexible concerning colour) into what the aviation colours chromaticity coordinates should be and if the aviation colour boundaries need to be revised. This is an exciting challenge which could not be addressed in the past due to the limited flexibility of the incandescent source.

An LED is not a ‘magic’ light. Another misconception is the concern that acquisition distance of a light signal from a LED source may not be as good as an incandescent source in lower visibility conditions. Aviation lighting is specified in terms of a quantity known as a candela. This measurement is independent of the source producing the light. Thus, a candle that produces one candela, an incandescent lamp that produces one candela or a LED that produces one candela, would all be seen at equal distances in a given visibility.

Issue Five

What is the impact of the reduced heat signature on the lens of LED fixtures, with respect to lens contamination due to environmental conditions?

Since LEDs do not produce nearly as much heat as an incandescent, the concern arose that they would not be able to clear themselves of condensation, produced as a result of ice or snow. To remedy this, the FAA in engineering brief 67b states that non-incandescent sources ‘must have an optional arctic kit or/and appropriate way of addressing of potential icing conditions to no less extent than present fixtures’.

Issue Six

Can LEDs be seen on an enhanced vision display?

Enhanced vision systems (EVS) use different types of sensors to detect the runway environment and surrounding area in visibility conditions in which the human eye may not be able to detect them. This has the potential for increasing capacity at airports in lower visibilities conditions. One type of sensor utilises the infrared (heat) from the surrounding area and also, from the incandescent lights that, as mentioned above produce mostly heat. LEDs, however, produce very little infrared and can not be detected by these enhanced vision systems.

The FAA conducted two independent studies which concluded that, at this time, it is not feasible to require the amount of infrared (heat) needed by these systems. In addition, a growing concern is that the trend in the industry is going away from incandescent lighting and towards the more efficient non-incandescent sources. The increase in energy use required the increase in cost. Plus, the vast amount of lights affected and the questionable availability of incandescents in the near future, suggests that alternative sensors such as millimeter wave technologies that do not require infrared, be considered for EVS. This therefore eliminates the dependency on the older incandescent source technology.

Issue Seven

Are current photometric tests for incandescent lights valid for LEDs?

There have been extensive discussions in the industry on whether the current photometric testing criteria are applicable to non-incandescent light sources. The consensus is that, some different items have to be considered for these sources that do not need to be considered for incandescent sources, such as, the test methods for chromaticity of these saturated sources.
A group from industry, academia and government was formed in 2002, to develop recommendations on solid state (non-incandescent) lighting standards. This group, the Alliance for Solid-State Illumination Systems and Technologies (ASSIST), as part of its charter, is developing the testing criteria for LEDs.

Issue Eight

How is the operational failure of LED fixtures identified?

As part of the ASSIST program, a recommendation was published for the determination of useful life. This recommendation defines LED life as the time it takes for an LED component or system to reach 70% lumen maintenance for general lighting applications.

The FAA is working towards a definition of useful life and is looking closely at this recommendation. ICAO has similar requirements for standard airport lighting.

Conclusion

The advent of new technologies for airport lighting applications present challenges that have not been of concern since the first airport was illuminated. Until now there was primarily one technology that would perform the needed applications.

The FAA is aggressively addressing these challenges and is encouraged by the ability to consider other technologies that promise better visual cues, life cycles cost reduction and energy savings.