Light guides the way to vehicle ergonomics

April 16, 2018 // By Terry Moss, Stadium IGT
Light guides the way to vehicle ergonomics
Vehicle designers have eagerly embraced the potential of the high-resolution touchscreen to improve the attractiveness of the user interfaces of infotainment and driver information systems. But the touchscreen is far from being suitable for all user-interface requirements in vehicle design.

High-resolution touchscreens make it possible to bring together multiple indicators into one place and represent complex information, such as 3D maps of the route ahead, as well as providing a home for traditional indicators of speed, engine and fuel performance. But the concentration of information in large, discrete displays has drawbacks from the user-experience and ergonomics perspectives. The desire to pack too much information into the touchscreen leads to distractions for the driver as they try to hunt for icons and dials that are important to them at that time among less important graphics.

The steering wheel is one natural home for indicators that are important to the driver while the car is moving. It is where the driver can expect to find cruise control settings and the icons that show whether direction indicators are active as well as warnings generated by the advanced driver assistance system (ADAS) as a supplement to audio and vibrational alerts. For example, icons on the wheel can confirm to the driver which warning has been triggered, such as lane departure or slippery road conditions.


When the driver and other occupants are getting out of the vehicle, they should be aware of the outside environment. Cyclists and pedestrians may be about to pass by and risk being hit by a door that is opened too quickly. This is where ADAS can once again help, showing on an indicator whether there is someone passing by or another obstruction. When someone opens a door, they will most likely be looking in its general direction and not at one of the larger touchscreens in the dashboard or central pillar.  Indicators in the door control help ensure safety for those around the car and minimise the risk of damage to the vehicle from a bollard that is hard to see from the window. The indicator may show a green icon when there are no obstructions, moving to a red cross or other similar warning symbol if a potential hazard arises.

The problem facing the designer of automotive systems that need these controls is how to fit them into the limited spaces available within the steering wheel or doorframe and without compromising styling objectives. There can be extreme constraints on the panel depth, making it difficult to fit LEDs or other types of backlighting illumination behind the surface. What is required is a thin backlighting technology such that the depth required is little more than that needed to implement colour filters and cutouts.

Fig. 1: A light source at position A, exhibiting Total Internal Reflection at position B for is a material of
refractive index n₂, where n₁ is greater than the refractive index of the surrounding medium n₂.
At position C the light is scattered by a light extraction feature, and exits the light-guide

An effective way of achieving thin backlighting for indicator panels is to employ light-guiding technology. The light guide takes advantage of the same effects as those employed to send photons over long distances through translucent fibres. Transparent materials convey light according to Fresnel’s Law of Refraction. When light moving within a material hits one of its outer surfaces, one part normally reflects and another refracts out into the adjacent material. However, if the incident angle is higher than a critical angle, which is governed by the difference in refractive indices of the material and its neighbour, the light will be reflected back entirely into the material. This condition of total internal reflection allows light to be confined within the translucent material (Fig. 1). Using a mixture of materials with different refractive indices, it is possible to guide light precisely to defined points, ones at which the critical angle is no longer satisfied and the light can exit.


With a light guide, the sources of illumination can be mounted away from the indicators themselves. This makes it possible to mount the indicators where the depth below the surface is extremely limited. The control electronics and LEDs used to light the indicators can be mounted in cavities some distance away (Fig. 2). This can also be leveraged to provide easier access for networking and power connections during manufacture and maintenance. A further advantage of light-guide technology for indicator panels is the ability to hide elements that are not in use without an obvious gap in the panel. Only when the system is active do the LEDs illuminate the various graphical elements. When powered down or switched into an alternate mode, the unused icons effectively disappear into the substrate, making it easy for designers to preserve a clean flat look for styling purposes.

Fig. 2: Schematic illumination of an Active light-guide construction with the LEDs embedded
within the optical acrylic light-guide, bound by the light fast cavity.

Light-guide technology has existed for a number of years. The conventional technology for the light-guide is an injection-moulded plate designed using optical modelling software tools. The resulting moulded parts are mechanically assembled with light sources, reflectors and diffusers to obtain uniform light coverage. The problem with the conventional approach is that it takes multiple moulded components to build the complex multi-indicator panels required for many automotive projects. The construction would require multiple mould tools and a second process to integrate them to an additional structure to provide the mechanical placement and optical isolation between indicators.


Instead it is possible to apply a moulded light-guide design. This is an approach that demonstrates significant cost and performance benefits. The use of a moulded structure allows multiple light guides to be integrated on a single substrate. The process cuts a cavity into an opaque polymer layer into which the individually printed and laser cut acrylic optics are dispensed. This optically opaque cavity material serves to isolate adjacent light-guides and prevents any light bleeding from one guide to another. Using this technique, 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.

A light-guide thickness of 1.2mm or less is a significant reduction compared with the traditional approach of deploying top-emitting LEDs under a transparent electrode. The total thickness of the light-guide assembly is driven by available LED package and can be as low as 0.4mm. This makes it possible to place indicators in positions where they are needed rather than just where there is sufficient depth to place the active components. (Fig. 3) It also makes it possible to have indicators on the top surface of the door handle where visibility is best, but with the active LEDs driven from a circuit board placed close to the window and other comfort controls. Similarly, on a steering wheel, the indicators can be placed where they are least likely to be obscured even if that position has restricted depth caused by the presence of structural elements immediately below the fascia.

Figure 3: a) The light extraction graduation print is applied to the substrate,
with gradient indicated by the arrow.
b) Shows the position of the LED aperture within the acrylic light tile.
c) The assembly is bonded to the circuit to which the LEDs and capacitive
sensor pads are mounted.

The indicators made possible by a thin backlighting solution based on light guides need not be solely passive. The technology is highly compatible with capacitive sensing – allowing the integration of touch-based user interface 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, with light-guide technology, the LEDs and control electronics are mounted away from the capacitive sensor area the interference between the two types of electromagnetic source is minimised.


In this way, it becomes possible to integrate controls and indicator panels into space-limited but ergonomically good positions such as the doorframe, steering wheel or gearshift areas. This removes the need to design and build separate switches, which may be more difficult for the driver or passenger to find when they need to adjust the control related to the indicator.

Fig. 4: Example construction for 2 icons, with the optically opaque cavity
supporting a 6mm separation. Dimensionally a minimum overlap of 0.25mm
is recommended between the illuminated icon position, the light extraction
print and the inner surface of the cavity.

Thanks to light-guide technology, stylish indicator and control panels can be integrated easily into space constrained areas, providing more opportunities for adding features to dashboards and other parts of the car interior that have severely limited depth behind the bulkhead. Not only that, the surfaces can be curved to support the more adventurous styling concepts that designers now want to employ.

About the author: Terry Moss is Sales Director, Stadium IGT (www.stadiumgroupplc.com)

 

 

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