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The light at the end of the tunnel

As automotive designers try to balance limited cabin space with additional display elements, many are turning to light guides as a solution.

Today, vehicle makers see autonomous driving as key to their future market strategies. It will transform their businesses and require some radically different approaches in terms of automotive design.

Internal styling choices will become more important, having a knock-on effect on the designers of electronic subsystems. They will need to integrate their products into surfaces that will make use of complex curves to make the most of confined space. Electronic controls and their associated displays will be built into the arms of seats and ceiling door panels so they can remain active and accessible no matter the configuration of the internal space.

There’s a long road ahead before the autonomous vehicle becomes a mainstream, but the safety considerations that make self-driving cars practically inevitable will inform the design of the next wave of vehicles. Cars are not just becoming more automated but are more complex in terms of features that are intended to supplement the travelling experience rather than help with driving. As a result, some refer to the modern car as a “smartphone on wheels”. But it cannot be designed to have a user interface that relies on a single large touchscreen.

Driver distraction is a key issue for any control surface in a moving vehicle. The touchscreen exacerbates this issue in two ways. One is the need for the driver to look away from the road ahead to look for virtual buttons and dials. The second is the sheer volume of information – much of it non-essential – that a high-resolution display can present.

Manufacturers are now looking to gesture-based touchless interfaces for controlling in-car systems. They also need to find ways to put critical driving-related displays in the eyeline of the driver. That means making the most of regions such as the steering wheel and the upper parts of the dashboard as well as mirrors and the doorframe.

Simple icon-based displays mounted on the steering wheel and these other areas can present vital, immediate information that aid driving instead of distracting from it. LED-driven icons on the mirror or close to it on the inner doorframe can warn of vehicles in the driver’s blind spot that have been detected by external sensors. Icons on the steering wheel can provide visual confirmation of guidance from the navigation system and warn of conditions such as excess speed. Even when the journey is over, strategically mounted indicators on the door can ensure continued safety - blind-spot sensors can pick up approaching cyclists and flash a warning on the doorframe to the occupant as they reach for the door handle.

The problem facing the automotive designer is fitting the necessary displays into the car’s structure. When it comes to installing the touchscreen elements, the dashboard designer has the advantage of working with a large volume able to accommodate what is a relatively inflexible structure. For rear

passengers, seat-back panels can fit into the headrest section with reasonable ease. Some automotive designers have even made the touchscreen a structural feature of the dashboard and central-pillar region.

“Light-guide technology will ensure carmakers can deploy advanced user interfaces for systems inside the vehicle.”
Terry Moss, Sales Director, Stadium IGT

Display elements that need to be mounted on door panels or on the steering wheel do not have that luxury of space. Many of the best surfaces for visibility will have structural elements running through them close to the surface. As a result, the depth behind the surface panel will be extremely limited. However, there will be usable voids nearby that can be used to hold control electronics and other support components.

Similarly, as designs mature and adapt to changes in consumer preference, it seems likely automotive designers will look to alternative display technologies to complement flat touchscreens so they can make more use of the structural elements that hold the screens in place. But this will often make placement of the display elements harder to achieve. Whereas the touchscreens often sit in front of what may be a large void, the smaller displays built into the curved framework will have very little depth with which to work. They will need to be arranged around structural elements that cannot be moved. A technology that can take advantage of these factors is based on light guides.

Light guides

Light guides make it possible to place LED-based backlit indicators into extremely confined spaces. The only requirement is to have enough depth behind the surface to implement colour filters and cutouts. In these structures, light is steered according to Fresnel’s Law of Refraction such that, at low incidence angles, the photons are confined to remain within a translucent material. At a large enough angle, the photons escape through the surface of the material. Careful selection of materials and constructions makes it possible to guide photons to a pre-defined point, such as an icon cutout, some way from the driver and control electronics.

Important both for styling and for reducing driver distraction, light-guide technology makes it possible to build invisible-when-inactive indicator panels. If the icon is not lit up, the surface looks seamless. When the icon appears, it is immediately apparent, providing clear visual feedback.

One problem that has faced designers wanting to use light-guide technology is that of construction. Conventionally, the display elements are assembled onto injection-moulded plates, which can be expensive and restrictive in terms of design. An alternative is to use a technology that makes greater use of moulding allowing multiple light guides to be integrated on a single moulded substrate that can be just 1.2mm thick. At Stadium IGT we use a process that cuts a cavity into a polymer layer into which laser-cut acrylic optics are placed. The approach makes it possible to place driver LEDs anywhere on the substrate. Alternatively, they can be optically coupled to the light guide at the edge. The cavity material is optically opaque so that light does not bleed from one adjacent light guides to another.

The result is a structure that allows visible indicators to be placed on almost any surface with the control electronics and driver LEDs located in available voids that are easily accessible to production and maintenance personnel.

The light-guide display technology can be combined with touch controls so that the driver need not look elsewhere to make a change to a setting. For example, they can indicate a roadblock ahead to the navigation system or suspend it by pressing a virtual button on one of the indicators. A problem with conventional backlighting technologies is electromagnetic interference between the capacitive sensors and the control electronics. Our technology makes it possible to separate these elements spatially, so the interference between them is reduced practically to zero.

No matter where the industry is on its path towards full autonomy, light-guide technology makes it possible to tune the human-machine interface for the needs of the vehicle user and ensure manufacturers can support the trend towards increasingly safe driving. Even when full autonomy is achieved, light-guide technology will ensure carmakers can deploy advanced user interfaces for systems inside the vehicle that fit the many possible use-cases and styling possibilities they envisage.

Author
Terry Moss

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