Manufacturers of commercial aircraft interiors are faced with a challenge that can be hard to resolve. Each airline regards the interior design of their passenger-jet cabins as being a physical representation of their brand values and image. Styling and decorative flair now drive the design of cabins, from seating to gangway signage.

The manufacturers and integrators of these interiors have to take into account the practical reality of the designers’ wishes and make them feasible. This is a particular challenge for systems with an electronic human-machine interface (HMI), where not only manufacturing practicality needs to be considered but also factors such as safety and passenger usability.


Passengers want to have easy access to the controls for their seat’s position, ventilation and entertainment systems. But as the screens of the entertainment systems get bigger to provide a better visual experience, space around the seat back is becoming ever more precious.

Armrests and seat mouldings are now making use of curved fittings to improve comfort in highly space-constrained situations. Integrating controls and indicators into these locations is challenging for many electronic display technologies. For example, LCD touchscreens and panels could provide a solution but they continue to rely on flat surfaces and are difficult to form into the irregular shapes that would be needed to incorporate them into armrests and the recesses between screen and tray tables.

Overhead space is limited by the need to provide ample room for hand luggage, lighting and ventilation systems. There are further constraints in terms of how signage is deployed. Although such signs need to comply with strict safety regulations, interior designers want them to fit seamlessly into their overall styling paradigm. That calls for indicators that are secret until lit and which do not stand out when they are not required. But placing signs such that they are flush against bulkheads and internal partitions means space behind the signs themselves can be extremely limited.

Although secret-until-lit indicators are in common use in aircraft interiors, they are hampered in traditional implementations by the placement constraints of the lamps and backlighting systems, such as overhead signage and emergency signage.

One way to reconcile the needs of styling and manufacture is to employ light-guide technology. The light guide makes it possible to have the moulded indicator panel spatially separated from the electronics and bulbs required to control how the individual icons on the panel behave. A key advantage of light-guide technology is that it readily supports moulded surfaces with practically any sort of curvature and surface finish and, through the selection of appropriate translucent materials, supports secret-until-lit behaviour. The light each indicator requires can be directed into place from the side – and from locations that fit better around the structural elements of bulkheads and seats.

The key concept behind light-guide technology is the same as that used for the fibre-optic cabling that, increasingly, carries electronic signals at high speed through the aircraft. Optical fibres and light guides take advantage of Fresnel’s Law of Refraction. This law determines the critical angle at which light can escape from a transparent material.

When light is projected through the core of a transparent material it will eventually encounter a surface. But if the light hits this surface with a medium with a sufficiently different refractive index at a shallow enough angle, the beam of light will be reflected internally. A larger angle, however, will allow the light beam to escape into the external medium, typically air. With a suitable mixture of materials with the right refractive indices, structures can guide the light to predefined points. In the case of light-guide displays, these are the outer surfaces of the indicator icons within a moulded panel.

Light-guide technology has been in use for some time but, in traditional implementations, presented issues for reliable and cost-effective manufacture. The conventional technique used for light-guide integration is to build light sources, reflectors and diffusers onto an injection-moulded plate. For the kinds of aircraft designs now being prepared, this requires the manufacture and assembly of multiple moulded components. The resulting module can be costly to implement.

An alternative, and the approach adopted by TT Electronics is to make the light-guide a fundamental part of the overall module. The technology allows multiple light guides to be integrated on a single substrate by forming them using an optically transparent acrylic material that is deposited into cavities cut into an opaque polymer layer. The optically opaque cavity serves to isolate adjacent light guides from each other and prevent any light bleed between them.

LEDs can be placed anywhere across the light-guide substrate depending on the needs of the application or optically coupled to the guide at the edge. The result is a light-guide with a thickness of 1.2mm or less: a significant reduction compared with traditional approaches.

The process has significant advantages in terms of the flexibility of component placement and design-for-manufacture (DFM) principles. LED drivers and other electronic devices can be integrated readily into the assembly with ICs positioned either on the upper light-guide surface or the opposite surface.

The technology supports both colour changing and static-colour LED lighting. Where there is a static colour requirement, the light-guide provides the ability to use white LEDs. Colour filters can easily be laminated into the assembly or a translucent print applied to the graphic material. This provides the potential to use white LEDs for their higher luminance and so deliver higher-brightness safety indicators, for example, than would be possible using coloured LEDs.

A further advantage of TT’s support for integrated electronics is the ability to use capacitive sensing, making it possible to save space by combining indicators with touch controls. With conventional display and backlighting technology, one of the major issues with integrating capacitive switches is illuminating the switch graphics – such as on/off indicators – without interfering with the electric fields needed for sensing. Because the light-guide technology provides illumination from comparatively remote LEDs there will be little to no interference with the capacitive electrodes mounted around the indicator windows themselves.

Through TT, manufacturers can satisfy the often-conflicting requirements of modern styling concepts and manufacturability. And integrated electronics go further than traditional secret-until-lit panels by providing support for additional features such as interactive touch-based controls.

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