Light-guide tech saving space in aircraft
Airlines are constantly being challenged to add services and provide improved passenger experience. One of the key ways in which they can improve the traveller experience is through seat comfort and entertainment controls as found in the passenger control unit (PCU), as well as real-time signage such as passenger service units (PSUs) to show the status of available services.
Status indicators such as those found in the PCU can help users understand the different positions of their seat and whether it needs to be restored to the upright position ready for takeoff or landing, as well as the status of different features on their infotainment devices. More extensive seat-back indicators for advanced PCUs can show audio volume and other status information about the entertainment functions. The indicators could be combined with capacitive touch controls to make it easier to change settings without calling for a separate remote control unit that takes up valuable space between seats. But the electronics needed to provide these upgraded user interfaces cannot come at the cost of seating and bulkhead space.
One way in which suppliers have reacted to the need for interactive displays and interfaces is through the combination of electronic controls and displays into single modules. However, even then, a key problem is the amount of space required to house the illumination elements that ensure the displays remain visible under all operating conditions. There is also the issue of information overload for passengers and staff. They do not want to be distracted by signs and PSU indicators that are not relevant to them at that time. That drives the requirement for a display technology that is effectively invisible until it is needed.
For the aircraft interior, there are a number of technology choices. But the constraints of the environment mean some potential choices are quickly ruled out. Although flat-panel displays are now routinely employed for crew information and seat-back entertainment, their rigid rectangular shape makes them difficult to integrate in an increasing number of locations. Galley insert equipment (GAIN), as well as cabin management systems (CMS) controls are more likely to continue to be based on conventional, rectangular structures because of the need to maximise storage and working space. But the demand in passenger-visible areas for more stylish interiors that rely on the extensive use of curved surfaces puts new demands on indicator design.
Flat-panel displays are difficult to hide when inactive – the only realistic solution is to have them retract out of sight, which means the introduction of additional space-consuming motors and hatches in areas often allocated for PSUs such as the spaces below overhead cabins. However, for many years, aircraft designers have been able to use for PCUs and PSUs display technologies that use secret-until-lit panel designs. They ensure crew and passengers are only aware of status lights and icons when they are illuminated.
The drawback of the conventional secret-until-lit technology is due to the illuminators they employ. The lamps and power supply circuits demand a comparatively deep amount of space directly behind each display element. This makes the display difficult to mount in seats and armrests – the ideal locations for displays tend to interfere with the structural elements of the seats.
If the illumination could be made smaller or moved to more convenient locations, the secret-until-lit panel becomes a much more viable option. Light-guide technology provides the ability to do both. Rather than demand a light source to be mounted behind each indicator, the light is delivered from a source mounted off to one side to where it is needed. Light-guide technology employs a similar approach to that of fibre optic cables, conveying light along shaped conduits according to Fresnel’s Law of Refraction.
The transparent light guide is shaped so if the photons hit the surface of the material at a shallow enough angle they will reflect back into the body of the material. However, if the light hits the surface at a large enough angle, it can escape. The size of the critical angle is determined primarily by the refractive index of the material. This is leveraged in advanced light-guide technologies through the use of different mixtures of materials, each with its own characteristic refractive index. The combination of materials and shape makes it possible to create light guides that are customised for each installation, optimising the use of space around the display panel. A key advantage of light-guide technology is that it readily supports the requirement for integrated with curved surfaces. In terms of light sources, high-brightness LEDs provide compact and efficient illumination, helping to bring overall size down.
Traditionally, the method of construction for the light-guide is centred on the use of an injection-moulded plate onto which are assembled light sources, reflectors and diffusers. The multi-indicator panel required for aircraft designs leads to the need for the manufacture and integration of many different moulded components into the final assembly. This leads to increased cost and difficulties in reducing overall size.
A different approach is to depart from the use of moulded light guides mounted onto a substrate and simply form them from an acrylic sheet. This integrated light-guide process cuts a cavity into an opaque polymer layer, acting as a receptacle for individually printed and laser-cut acrylic optics. The cavity material serves to isolate adjacent light-guides, with the benefit of preventing bleed from one indicator to another.
The light-guide architecture allows LEDs to be placed anywhere within the cavities. They could even be mounted externally with their light coupled into the guide at the edge. The integrated light-guide can be built with a thickness of 1.2mm or less.
As the electronics can be moved to the side of the actual display, it becomes much easier to integrate capacitive touch technology. There are no electric fields generated by the light sources and driver circuitry to interfere with those needed to support capacitive. This makes it possible to combine touch control with a secret-until-lit indicator panel. For example, the user can touch a dimly lit button that activates a set of indicators and their associated touch controls. Once the passenger has changed the settings, they can deactivate the display and remove it as a source of distraction.
The combination of touch with secret-until-lit displays that light-guide illumination offers creates many other opportunities to create novel and attractive user interfaces and make them part of the aircraft's overall styling, which can be incorporated easily in systems that range from PCUs to CMS controls.
