Ed Kinnally
Marlene Schade
Durel Corp.
Chandler, Ariz.

Electroluminescent (EL) backlighting has an allure that continues to attract designers in the automotive industry. Its thin, uniform lighting properties let EL keep instrument cluster design time to a minimum. The resulting dashes are also comparably simple. EL lamps create uniform lighting for both silhouette and traditional instrument clusters though they are less than 0.25-mm thick (about half the diameter of the lead in a typical mechanical pencil). They are flexible enough to fit into a medley of configurations and need no light pipes or compensated graphics. They also don't generate much heat and won't burn out.

Ease of assembly

The retrofitting of EL into an instrument cluster requires no major changes to other components. But there are important benefits, such as size and weight reduction if the cluster is designed around EL technology.

EL lamps frequently mount between a subdial (backplate) and a graphic overlay (appliqué). The subdial can be inexpensive black ABS. The subdial, EL lamp, graphics appliqué, and EL driver electronics (EL inverter) can be a single unit that snaps into the instrument cluster. This provides one plug-in subassembly that completely illuminates the dial. Stepper motors that move the pointers can also mount on the subdial. If there's no requirement for a complete lighting subassembly, then EL and graphics-appliqué components can mount on the instrument cluster via alignment holes or slots.

The EL lamp and appliqué stay in place with a bezel and lens which snap or screw on top. Typical mounting options include affixing the lamp and appliqué at the center hole of each gage via ultrasonic welding or orbital forming. The lamp and appliqué edges are held flat against the instrument cluster by either snap fitting or adhesively bonding the faceplate to the assembly.

EL lamps need an ac drive signal, which comes from discrete inverter electronics or integrated circuit (IC) EL drivers. Lamp size and desired luminance determines the best approach. Mounting methods for EL inverter electronics include placement on the printed-circuit board with other cluster circuits or clipping them to the cluster housing.
Graphics screen-print directly onto the front of the EL panel when there's no need for a separate graphic overlay. This approach, however, must consider possible yield losses. Silhouette lighting or positive graphic designs (large backlit areas with opaque indicia) don't need lit pointers because the EL lighting silhouettes the pointers. Different appliqués also provide different looks for the same EL backlight. As part of an appearance package, designers can select standard graphic lighting over cluster designs that use silhouetting. The two cost about the same but there's a dramatic difference in appearance between them.

EL lighting systems can also get to market quickly. Prototypes need no hard tooling so they can be built fast and cheaply. Designers use CAD to model the design, generate artwork, and finally cut parts to shape with a laser.

Production hard tooling consists typically of one or two stamping dies (depending upon volume) that blank the periphery of the part. They are also inexpensive compared to injection-molding dies often required to produce light guides and reflectors for alternative lighting systems. Generally, design modifications consist of inexpensive artwork executed quickly.

EL manufacturers can virtually eliminate tooling costs during the development stage using rapid-prototyping lamps such as ProtoLight. These stock lamps can be either cut by hand with scissors or with a laser to the desired shape. This is a quick way to compare the EL approach with other lighting systems. Prototypes also serve in product mockups to evaluate design concepts. When evaluating EL lighting, however, it's important to use uncompensated graphics. All in all, use of design prototyping can greatly reduce development and costs.

Additionally, EL technology eliminates expensive optical-grade polycarbonate light pipes and/or reflectors necessary for LEDs, CCFL (cold-cathode fluorescent lamps), and incandescent bulbs. These localized sources of light force light to distribute itself across the instrument cluster through extra optical materials and components. The design of such systems can be complicated, often entailing several prototypes and tooling sets to get the right look and/or software modeling to predict lighting patterns. The EL approach also eliminates compensated graphics, an effect achieved by printing extra layers of ink on the graphic appliqué to reduce lighting "hot spots." The effort associated with compensated graphics can be lengthy and expensive.

Lighting themes

Both negative (traditional or standard) and positive (silhouette) lighting design themes serve in analog instrument clusters. With negative lighting designs, backgrounds are dark and the numbers and other symbols illuminate only at night. For example, white numbers on black backgrounds turn blue-green or white at night. And in a third configuration (pseudo-silhouette) the numbers and backgrounds are both in semitransparent inks. Here, although the traditional lighting theme looks like a standard cluster the entire gage face illuminates at night.

With standard (negative) lighting, alternate technologies can cost more. The reason is hidden costs such as sorting (binning) certain LED colors and the end-of-line failures associated with incandescent bulbs.

In positive designs, the numbers and other symbols are visible as dark silhouettes against illuminated backgrounds. They typically have black numbers on white (or gray) dial faces. Various combinations of design themes are also available, such as dark graphics on white backgrounds during the day, and illuminated graphics on dark backgrounds at night. Silhouette designs requiring large lit areas are generally only feasible with EL or more costly CCFLs. Other competitive lighting systems need substantially more design effort and carry a bigger price tag.

To keep costs down, only selective areas of the instrument cluster glow. And phosphor and other raw materials go only in areas they're needed. Selective lighting also keeps down power requirements, lowering the cost of the transformer inverter or EL driver IC. Only with EL lighting can designers choose exactly where the light should go. This eliminates the need to camouflage unwanted light leaks, again simplifying design efforts.

Typically, each EL lamp product design is unique. The EL maker needs to know the location of the lit areas, the required luminance and color, the product geometry, and the particulars of the connection scheme. The customer supplies a print of these basic parameters. The EL manufacturer uses this information to design the phosphor placement, color-shifting overprints, rear insulators, bus bars, and so forth. A print of the finished product then goes to the customer. Graphics can change luminance and color so EL manufacturers often lend assistance in characterizing customer samples and providing light transmission and color-shift data. They also assist with design constraints, luminance considerations, calculating useful life, colors, electrical connections, adhesives, inverter drive electronics, and other pertinent issues.

Colors

Instrument clusters use a variety of colors based on OEM specifications. EL lamps easily accommodate any of the various color requirements. Base colors of EL lamps are green, blue-green, blue, and orange. Changes in the drive frequency enable small shifts in color. A wider range of color comes through phosphor blending or cascading (overprinting) techniques. For example, Mercedes
E-Class and CLK instrument clusters use a lighting theme called white lighting which is created using a three-color phosphor blend. The same results are possible with cascading which "shifts" the color wavelength of the emitted light. This takes place through the use of dyes or pigments and possibly filters printed on the lamp's top surface (overprints). This results in a significantly wider color palette.

Underprinting can also shift color. Here, a dye mixes with phosphor ink (printed under the polyester surface). The dye works like an overprint, cascading the emitted light wavelength to a different color. Selectively screen-printing different areas of the lamps let designers specify multicolor segments.

Coordinating the EL lamp and graphics early in the development stage gives designers the specified color for the instrument cluster. While blue-green still remains the most popular color, white instrument clusters are becoming increasingly more popular.

Luminance and color are evenly distributed across the surface of the backlit cluster gage. This provides consistent, uniform two-dimensional light regardless of configuration or dimensions.

Standard (negative) incandescent bulb cluster graphics. White graphics turn blue-green at night.

EL silhouette (positive) cluster graphics shows the white background transformation to a blue-green hue at night.

The EL lamp cross section is about 0.25 mm (layer thickness not drawn to scale). EL lamps need high voltage to excite the light-emitting phosphor layer sandwiched between two conductive layers.


Constructing an electroluminescent lamp

Polyester (polyethylene terephthalate or PET) is the base material generally used for EL lamps. A conductive indium tin oxide (ITO) layer is sputtered onto one side of the polyester to create a front electrode. Then the ITO side of the polyester is screen-printed with a phosphor layer as well as dielectric and rear electrode layers. This creates a thin (0.25-mm), flat, light source which comes in a variety of shapes, sizes, and colors. The EL lamps can bend to fit various simple curves or offset to different heights to get a multidimensional appearance.

Luminance and color evenly distribute across the surface of the lamp. This provides consistent, uniform two-dimensional light regardless of configuration or dimensions. A single lamp can provide multiple colors through the use of multiple phosphors or the addition of fluorescent dyes or pigments.

Instrument clusters designed with EL lighting for dial illumination use less power than incandescent bulbs or LEDs. A typical EL instrument cluster lamp has a lit area of 200 cm2 and may draw up to 200 mA for lighting. A similar cluster using incandescent bulbs uses 1,600 mA. Most of the additional power for bulbs is wasted in heat.