Electroluminescent foils could be used to create new spatial lighting effects in cars, says Christian Schreiner, Managing Director of Schreiner VarioLight, a German supplier specializing in this technology.
To demonstrate some options of using electroluminescent lamps, the company worked together with automotive textile supplier Strähle+Hess to add blue ambient electroluminescent (EL) lighting to the interior compartment of a Porsche Cayenne. The show car is currently being presented to automobile manufacturers and automotive suppliers until the summer of 2008. To date, EL lamps are a rarity in vehicles. One of the first automotive series applications of light-emitting foils was the 2002 Mercedes-Benz E Class speedometer dial lighting, which is part of the instrument cluster developed by Siemens VDO (now part of Continental). In 2006, license plates with EL foil illumination received the official type approval in Germany (following the first single road application approval in the Volkswagen Phaeton in 2005).
The current Schreiner EL lamp design consists of six layers. Between the front and back laminates there are (from top to bottom) the transparent front electrode, the luminous pigments, a dielectric/reflector, and the back electrode. The obvious benefit of EL foil lamps is their potential ease of integration, as they only require a few millimeters of depth for installation. Greater design freedom is also enabled by the foil’s moldability and suitability for in-mold decoration. The supplier is convinced that this makes it a lot easier to adapt EL foils to existing vehicle assembly conditions.
In principle, the glare-free and dimmable EL lamps can be installed in a very wide range of vehicle interior locations. As the installation does not require any fixtures or supports, as usually needed for LED lighting, ambient lighting could be done with minimal effort. Also, the EL lamp requires little electric power, which is a benefit in terms of fuel efficiency: a letter-sized EL sheet, for instance, consumes electric power in the single-digit watt magnitude. Depending on the application, the EL lamp can be very rugged. For door scuff plate illumination, the load-bearing capacity can be up to 250 kg/cm2 (3600 lb/in2).
Since the inverter needed to operate an EL lamp has to meet strict standards of electromagnetic compatibility (EMC) to avoid interference effects on other onboard systems, Schreiner has developed its own range of EMC-optimized inverters. To facilitate automotive use, the most compact version needs just 5 mm (0.2 in) of installation height.
While this ambient lighting application is mostly a styling option, Schreiner recommends users go one step further and employ EL foil to develop new types of switches that integrate foil-based sensors and switching elements. The supplier calls the resulting control element an intelligent surface. It consists of sensor, EL light, and front-printed pictogram. The integrated lighting makes the control visible and indicates its on/off state. Low- and high-intensity illumination makes clear whether the device is on and can also highlight which function or part thereof has been activated. This method of drawing the driver’s attention to relevant control elements can reduce driver workload at the human-machine interface. All components of the switch are printed on a film or sheet. The sensor mechanism can be based on capacitive, resistive, or piezoelectric principles.
Yet, there is a potential downside of the foil switch: currently, the driver gets no haptic feedback when activating a function. The person behind the wheel does not feel the click of a fully pushed button. This might lure him or her into a certain level of additional visual control to make sure that the fingertip actually hit the right spot. But then the use of backlit pictograms would probably make this visual confirmation quick, and finding a conventional switch requires some eye activity as well.