Mood swings with EL lighting: the future looks bright for electroluminescent interior lighting

Automotive Industries, Nov, 2003 by Barrett Kalellis

Recent advances in polymer chemistry have given new life to thick-film electroluminescent (EL) lighting applications. A technology first discovered in the 1930s, EL now finds the availability of more efficient and economical materials, coupled with modern manufacturing techniques, enabling it to provide a practical, lightweight, thin, cool, and versatile means of illuminating controls and displays.

In current usage, electroluminescent lamps are used as backlighting for LCDs in pagers, cell phones, watches, control panels and as lighting sources for safety strips and graphics highlighting.

An EL lamp is essentially a capacitor with an inorganic zinc sulphide phosphor sandwiched between the electrodes (see chart on opposite page). When a small alternating current voltage is applied, the changing electric field within the phosphor particles causes them to emit light. With a d-c power source, an inverter is used to generate 60-115 volt A/C and between 50-1000 Hz frequencies.

Rather than shining on an area to be lit from a distance, electroluminescent illumination evenly distributes and sources light only where it is needed. Therefore, it is not wasted or diffused by distance. And because the power levels are relatively low and the dissipation is spread over the surface area of the lamp, the heat that the electroluminescent lamp generates is not usually a problem, unlike the point source of an LED or incandescent lamp.

In recent years, automakers have become increasingly interested in using EL in vehicle interior situations. In a move to replace incandescent bulbs, designers are turning to solid-state alternatives, primarily to EL and LEDs. Although LEDs appear to have the edge in many of the newer applications, EL has attractions of its own.

At present, current applications are limited to instrument cluster gauge backlighting, gear shift ("PRNDL") illumination, window and lock control buttons in doors, and most recently, as "mood lighting" on the ultra-luxury Maybach 62 and 2003 Mercedes E-Class--semi-transparent roof EL liner membranes whose diffused light intensity can be controlled by rear seat passengers.

According to some industry estimates, EL penetration in vehicles now has reached three to four percent, with the expectation that it might exceed ten percent in the next few years.

The primary advantage of EL is its ability to illuminate over a wide area instead of simply a point or line source like other forms of illumination. This allows areas to be lit without having to build mechanical and thermal structures to accommodate numerous lamps.

In addition to the uniform lighting of extended areas, EL has the added quality of generating a cool light with virtually no heat to dissipate. It can be manufactured into thin structures, and it offers a highly efficient use of power. All this in a solid state, reliable package.

It would take about four T-3 incandescent lamps, each at one-half amp current draw, to provide the same candela illumination of a 40 sq. in. EL panel rated at .038 amps. And while individual LEDs also offer low current draw, six of them in series would be needed for equivalent illumination.

Some companies are discovering ways to expand this technology to make it fit applications that are both visually appealing as well as functional. Key Plastics, an automotive trim supplier based in Farmington Hills, Michigan, has developed a unique way to create three-dimensional forms of EL material.

Starting with the thin sheet that has been screen printed with the EL phosphors and other layers, Key Plastics uses a patented process that allows the sheet to be injection molded into a variety of three-dimensional shapes and sizes, depending on the application configuration that is desired.

In the process, the lamp, which is about the thickness of a standard applique, is placed in an injection mold where plastic is shot in front or in back of it. The result is a one-piece plastic part that emits light when a voltage is applied, usually through the use of a D/C to A/C voltage inverter. Different light effects can be achieved by varying the position of the lamp within the plastic part, along with the translucence and color of the plastic itself.

Phosphor particle deterioration from moisture and humidity, formerly a serious problem with electroluminescent panels, has been solved by encapsulating the lamps with a protective, optically clear coating.

"At present, there's very little functional ambient light being used in vehicle interior environments," says Richard Pudney, director of advanced technology for Key Plastics. "We think that there are a lot of possibilities just around the corner for automotive EL lighting."

Pudney listed some immediate applications that show promise: Door handles that are illuminated whenever the doors open, or when the key is removed from the ignition; lighted kick panels at the bottom of doors, to aid when stepping out of the vehicle; illuminated bezels around air ducts, to make them visible at night. In an exterior application, Pudney suggests an EL-illuminated hood release latch, making it easier to find when light is low, without dirtying your hands.