Many customers in the commercial sector are moving from traditional lighting technology over to LEDs - YEG Opto Marketing Services Manager demystifies the technology and weighs up the benefits for the aviation industry.

There are many reasons for switching to LED technology, some are obviously clear, such as greater energy efficiency and longer life, but additional benefits are less obvious and particular benefits to aerospace applications include: the LED package is a more robust solution (less affected by vibration and stress factors), a high light output can be achieved from a far smaller device, far less weight, more control over light output, more choice in the Colour Rendering Index or CRI (the ability to perceive colours more naturally under the light source), instant start-up, easy dimming control, no or low UV/Infra Red emissions and more flexible colour options.  Add to this the fact that there are no harmful chemicals in the product (unlike fluorescent lamps for example which have a cost to the environment in their manufacture and for disposal) and  the case for moving over to LEDs becomes a pretty convincing argument. 

Light Output
In the Aerospace industry perhaps more than any other, LEDs offer benefits in several areas that are crucial and integral to the industry such as lower weight and power consumption and secondary benefits such as CRI and colour control which can improve the passenger experience.  For example, LED manufacturers use binning to identify the colour and wavelengths of their devices.  This makes designing a white or colour changing system easier and can ensure consistency of colour throughout the aircraft.  Lens technology is also an important aspect of LED luminaire design.  Inherently LEDs are non directional in their light out put – LED manufacturers use lenses on board the package to create a degree of light dispersion control, however, in some areas additional lenses will be required to diffuse and disperse light to the required area – when using lenses there will be a reduction in output and possible CRI dependant on the lens quality.

LED brightness (measured in Lumens) is an area where design engineers have strived to improve their products and there has been rapid progress in this area in recent years.  Just two years ago the output on a pure white LED (4500 Kelvin+) was under 100 lumens.  Now manufacturers are achieving output of 118+ lumens.

Compact Lighting Source
Smaller packages are now being produced in high power devices; previously the package size was 8mm, now there are devices as small as 3.5mm but still capable of producing the same lumen output.  This has clear advantages in aerospace applications where size and weight are critical factors and empowers designers with more flexibility in light source positioning within the aircraft.

Colour Perception  - the CRI factor
A high CRI can be beneficial in an aircraft environment creating a more natural light source particularly important with the various mediums and surfaces the passengers and crew would be subjected to during a flight – from entertainment via flat panel screens, controls and instrumentation to books, walkways and dining etc. the CRI of a light source will affect the way a passenger or crew member perceives these images. 

Ken Poole, Marketing Services Manager for the Young Electronics Group comments: “ We are now seeing significant progress with an increase in CRI (Colour Rendering Index), this is very important as the higher the CRI the truer the colour will appear when using artificial lighting.  At present this is mainly being developed on warm and natural white colours, some manufacturers are now at 93 CRI, although over the last year we have seen and increase on the pure white as well, previously 70 CRI now at 73.  The only downside on high CRI LEDs would be the reduced lumen output.  A warm white high power LED running at 350mA with a CRI of 80 would produce an average of 80 lumens, where as the same LED with a CRI of 93, would only produce 60 lumens.

There is a fine balancing act with regard to any lighting environment where LEDs are used – in critical environments such as medical and control environments a high CRI would be beneficial, but in non critical areas there is a trade-off -  true colour or more light?  More light can be achieved by adding more LEDs, but CRI cannot.”

Thermal and other issues
The technology is not without it's problems and one specific area which is an important factor in the Aerospace industry is in thermal management.  LEDs loose over 80% of their energy consumption through heat and this poses several potential problems, especially for devices in enclosed areas. The efficiency, life and stability of an LED depends a great deal on the optimum operating temperature – too high temperature and all of these factors can be compromised.
 
OEMs and designers are constantly developing new solutions for the efficient transfer of heat ranging from the actual LED package design to special PCB substrates such as Ceramics and other heat sinks and highly efficient thermal transfer materials or forced air cooling.  YEG Opto has considerable experience in helping OEMs overcome the problems of thermal management and work with a range of materials and products which offer solutions depending on the application. Which ever option is chosen, it is clear that the design of the LED and light fitting is an important factor and consideration has to be given to thermal management.

Scott Lacey, Divisional Manager for Young Electronics Groups Powerline division explains, “Although LEDs for this type of application consume considerably less power than conventional lighting; LEDs still create significant heat which must be managed.

Taking heat away efficiently will lower the LED PN junction temperature to an acceptable level. Failure to do this will result in reduced light output and unwanted colour shift; both will result in the finished module not meeting the original LED or customer specification and further to this the module will have reduced operational life.

Nearly all heat generated by an LED chip is conducted through the bottom surface of the package. Conventionally the heat is then conducted to the heat sink via the solder pad, composite PCB  and thermal interface. The heat sink will have radiating fins to disperse the heat by convection. As LED light intensity increases further; this type of heat sink may become a limiting factor. To overcome this; incorporation of a heat module design which uses heat pipe in conjunction with active fan cooling will become more common. Young Electronics Group have extensive experience in this kind of active thermal management.

Fans originally developed for note books have been developed further to minimise acoustic noise. These specialist fans are now tuned to have imperceptible noise in everyday environments. However, the user will realise that as a mechanical device a fan can be an unacceptable life limiting factor.

Regardless of whether passive or active thermal management is used; the key to optimal thermal conduction is the minimisation of thermal resistance between the LED chips(s) and the heat sink / module. This can be controlled by selection of LED and PCB design. The PCB is composite type; e.g. IMS (insulated metal substrate). These composite PCBs ensure excellent thermal conduction through the board. The bottom side is a metal substrate and is mechanically fixed to the heat sink and a thermal interface material is used between the PCB and the heat sink.

The purpose of the thermal interface material is two-fold; it provides electrical isolation and improves thermal conduction by minimising micro air gaps between PCB and heatsink. Presence of any air gaps no matter how small will affect thermal transfer efficiency. An interface material with a smooth surface finish at the start will require less material compression, less assembly force and therefore easier installation.

Further factors in selecting the right thermal interface material will be thermal conductivity electrical isolation, mechanical strength and compressibility. “

Electrical Supply Considerations
LEDs are also voltage sensitive so the lighting systems must be designed to include a stable supply with a voltage above the threshold of manufacturers recommendation and a current below the rating of the LED.  LEDs are also electrical polarity dependant and unlike incandescent bulbs, will not work on reversed electrical polarity.

Electrical systems in aircraft can vary from 3 phase in heavy transport aircraft to AC power in light commercial aircraft.  Therefore careful consideration needs to be given to the design of the LED driver since a more complex driver circuit will be required to adapt from a 3 phase system and a stable supply is critical for efficient LED operation.

Cost of ownership
Although LED based lighting solutions cost more than traditional lighting products at present, they are considered “Green” as they use a lot less energy than current technologies and LEDs have a longer lifespan, especially High Power which have an estimated lifetime of 50,000 hours, this adds another benefit by reducing the cost of ownership, maintenance and replacement of the lighting systems.

Economy of Scale
As manufacturing output increases, driven by demand, the economy of scale will eventually mean the cost of LEDs will come down.  The demand for more energy efficient devices accelerates as energy costs rise but Legislation and environmental issues  can be influencing factors on demand, adding momentum to the growth in LED manufacturing output.  Although on the increase, the production volumes of high power LEDs for lighting applications fall far short of the current production of  lower power LEDs used in products like TVs, video screens and hand held devices.

The Future’s Bright
The development of high power LED lighting solutions means applications in aviation have been extended and recent practical applications include airport and heliport lighting with solutions in the areas of runway lights, runway centerline lights, taxiway centerline and edge lights, signage and obstruction lighting as well as maintenance areas, emergency lighting, airport vehicles and fixtures in terminals.

YEG Opto provide a complete range of LED solutions for developers, designers and manufacturers of LED products, including LEDs, Drivers, Thermal Solutions and Secondary Optics.