Spotlight on automotive interior lighting

Peter Murphy
Automotive Engineering Manager
Chicago Miniature Lamps, an SLI company
Hackensack, N.J.

Edited by: Steven Mraz

Automakers are placing ever-greater emphasis on interior lighting. In fact, they've made interior lighting as much a marketing tool as a safety, convenience, and appearance.

Just look at the numbers. In 1995, the average car had just four interior lights, all incandescents. Today had just four interior lights, all incandescents. Today the number is closer to 15 and still climbing, with a mix of incandescents and LEDs. Not surprisingly, the more upscale the car, the greater the variety and scope of available lighting schemes.

Auto companies are emphasizing interiors for a simple reason: they offer designers more freedom and opportunity to differentiate and establish a competitive advantage than do exteriors. Exterior styling is heavily constrained by energy-efficiency considerations and regulations. Put a prototype car in a wind tunnel and optimize its aerodynamic efficiency, and the solution will be a minor variation on the familiar "jelly bean" theme. It doesn't leave much creative elbow room for exterior styling teams.

Move the battleground inside, though, and design opportunities abound. Few regulatory constraints apply and the challenge simply becomes how to make the passenger compartment more appealing, convenient, distinctive, and personalized for drivers and passengers.

The most direct, cost-effective solution is to upgrade the lighting. Dollar for dollar, nothing transforms the look of a space like lighting.

Several automakers are already putting lighting to work in innovative new ways:

• An automotive company is developing a temperature-indicating cupholder. A hot cup activates a red LED; a cold cup activates a blue LED.
• Several companies are developing mood lighting. Drivers can change interior lighting levels, and colors, to warm up the mood or cool it off.
• A new SUV features side-view mirrors with direction-indicating LEDs activated by the turn signals. Red LEDs form an arrow easily visible to drivers in cars behind the SUV when directionals are activated. Otherwise, the mirrored surface masks the LEDs from view.
• Chicago Miniature Lamps is working with a company to manufacture a flashlight and built-in charger as a convenience feature for consumer vehicles.
• We are also in the early stages of developing touchless automotive switches for overhead or map lights. Rather than groping for a small switch in the darkened car, you simply wave your hand near the light and it goes on.

On the bright side
There's good news and some cautions in these trends. On the plus side, significant developments in lighting have given interior designers more to work with. Here are some recent developments:

Mini-incandescents. Incandescent lamps have gotten smaller. In fact, new 12-V Neo Wedge incandescents are about the same size as through-hole LEDs and one-third the size of standard automotive incandescents. If you need to miniaturize, you need not rule out economical, proven incandescent lamps. This can be a cost saver, especially with white light.

Hardier incandescents. Recently we introduced surface-mounted incandescent lamps with reflector housings and hardier filaments. These improvements make incandescent lamps suited for smaller, more rigorous, limited-access applications requiring long life and high-reliability.

Light where you want it. Advanced designs, principally optical lightpipes, let engineers separate the light from its source. Light travels from the source to the point of display or illumination via flexible, optical fibers or molded, optical plastic. In one instance, the backlighting in an instrument panel was replaced with lightpipes, reducing the number of lamps required from four to one. Even with fewer lamps, lighting uniformity on the panel is slightly better than the original.

Better SMT LED availability. Surface-mount LEDs have become more plentiful and economical, making it possible to miniaturize interior display assemblies and put light where it couldn't be put before. In a joint venture with Stanley Electric, our company has just opened a plant that will use world-leading technologies to initially produce 120 million SMT LEDs /yr and eventually ramp up to 600 million/yr by mid-2002. Most of that output is earmarked for automotive interior lighting.

Brighter LEDs. An R&D development at our sister company in Europe has produced white and red/amber SMT LED prototypes five times brighter than conventional LEDs by using a different heat-sink design. This could lead to using LEDs for true illumination as well as for indicators and displays. Despite the new LEDs' greater brightness, they are the same size and geometry as conventional SMT LEDs and can be placed on circuit boards in the same way. They are expected to cost half as much as other high-brightness LEDs.

Ready for 42 V. Work is well underway on efficient lighting solutions for 42-V electrical systems. When the market moves to 42 V, the lighting will be ready.

Complete subsystems with lower part counts. Companies are partnering with more first-tier suppliers to provide turnkey optical engineering, support and production of larger, lighting subsystems. For example, one subsystem might include lighting clusters, lightpipes, and trim molding for HVAC and audio controls. Buying specialized optical engineering expertise gives first-tier vendors better solutions with lower parts counts.

On the dark side
Putting ever more lighting systems into cars, SUVs, and trucks will inevitably raise the bar for stylists, optical engineers, and manufacturers. Space and energy conservation will become major design considerations. Getting separate lighting systems to work with each other, and work uniformly, will also become issues as lighting density increases, especially for displays. For example, it is still a challenge to deliver absolutely uniform lighting for all five backlit characters on a gearshift display. Any nearby display lighting, say to outline a cupholder or seat-warmer control, must work with the gearshift lighting when both are on and off, and in all outside lighting conditions. Differences between the backlighting and display lighting in intensity or color will be painfully obvious. The eye is a severe critic and interior features are close at hand. The more interior lighting per vehicle, the better it must all be.

In short, as automotive interior lighting proliferates, its design and fabrication will inevitably become more sophisticated. Subsystem vendors must be prepared to supply more engineering and application support and to deliver more vertically integrated turnkey packages.

Updating the mainstays
The incandescent lamp, the principal source of automotive exterior and interior lighting for nearly a century, will probably lose market share to other sources, principally LEDs. But it will always have a place, especially in illumination. That's because incandescents still have the lowest costs. They've also been miniaturized and "ruggedized" as mentioned earlier. Limited space is no longer a reason not to use incandescents.

Recently, for instance, we helped a Tier-One supplier reengineer a backlit dashboard indicator light. Because of its small size, the assumption at the project's outset was that an LED would be the answer. Instead, the best solution turned out to be an incandescent neo-light. That's because the application required white light, and incandescents cost about 10 cents apiece installed, while a white LED would cost much more. That situation may change with time. But for the near term, incandescents remain considerably more economical than white LEDs.

Incandescent bulbs remain the source of choice for most general illumination and are widely used for backlighting, and panels with information-dense overlays. Incandescents are not well suited for direct-view, high-resolution displays because of their size and heat output.

LEDs, however, are definitely the comers, at least for indicators. Their small size, low-power requirements and solid-state longevity make them good choices for instrument clusters, lightpipes, and other high-resolution displays and indicators. Also, LEDs create their color directly — whites, reds, greens, blues and ambers, and blends of them. No need for color filters and their energy losses.

LEDs, like incandescents, have been miniaturized and "ruggedized." This should open up interior applications involving a lot of handling. And brighter LEDs will naturally capture some share of the illumination market now held by incandescents. The size of that share will depend largely on comparative costs once next-generation LEDs go into volume production. Over time, we expect LEDs to represent at least half of all automotive interior applications.

On the horizon
Halogen, electroluminescence, and cold-cathode fluorescent (CCFT) will each find its niche in interior lighting, but will likely remain in limited use. The biggest breakthroughs in interior lighting will lie in the optics, principally lightpipes and fiber optics.

Halogens have begun to appear, particularly in "light engines," in which a single lamp or lamp cluster delivers light to several locations through fiber-optic cables or lightpipes. Halogen lamps produce pure white light and have great lumen capacity and excellent energy efficiency compared to incandescents. Unfortunately, halogen lamps generate so much heat, they melt most plastics including plastic optical fibers. It takes a large, elaborately designed package and glass optical fibers to combat the heat, all of which increase costs and space requirements.

Electroluminescence creates uniform light from thin (0.04-in.) flexible panels. High-voltage ac power excites a thin layer of phosphors, makes them luminesce. However, light output is relatively low and the panel is vulnerable to moisture infiltration.

Cold-cathode fluorescent tube (CCFT) lamps come in linear and standard folded configuration, and can backlight virtually any area. They provide uniform brightness in a relatively slim package. The output lends itself well to filtering for required colors on panels.

Electroluminescent panels and cold-cathode fluorescent tubes have several common benefits and trade-offs. Both offer uniform area illumination, which is good for backlit panels such as instrument-panel clusters. Conversely, both require high voltages and alternating current, which generate cost, packaging, and safety issues.

The final and most important lighting technology coming down the pike are lightpipes. They can be made of be rigid optical-quality moldings of polycarbonate, or flexible assemblies of fiber-optic cable runs of any length. The rigid type is best suited for integration within electrical/electronic devices, such as the audio instrument panel or HVAC controls. Flexible fiber-optic cables, on the other hand, are often the better choice for longer runs between components or integrating the "light piping" into a structural or ornamental component such as a sun visor.

One downside to fiber-optic systems is the classic "all eggs in one basket" problem. If the system's light source fails, the entire light array goes dark. The remedy will likely be redundant light sources.

Another potential pitfall of lightpipes is that the optics can get complicated when it comes to balancing all lighting originating from a single source. For example, a single light engine might feed pipes that individually illuminate the six buttons for preset radio stations. The output from one or more of several display nodes may need filtering or other modification to make it all uniform.

When designing any interior lighting element, the most challenging technical issue will likely be design of the reflector, socket assembly, lightpipe and fiber-optic path. Designers must also consider the issues of optical clarity and lumen output, heat management, cost, part count, design for assembly, reliability, and maintainability. These tasks should be handled by experienced, full-line automotive lighting specialists. They can provide better answers, fewer false starts, and greater speed to market.