White-light devices are improving, but challenges remain.
Mitch Sayers, Visteon Corp.
Given the styling flexibility, unique brand-image possibilities and the increased performance levels that they offer, LEDs may represent the most significant advancement in automotive lighting sources since the advent of the high-intensity discharge arc source.
Today, LEDs can be seen on luxury vehicles under nearly every brand name as well as on several high-volume vehicles. For example, LED-based daytime running lights are used on some Audi models; the Mercury Montego has LED stoplights and the Cadillac DTS uses LED backup lights. Other applications include center high-mount stoplights, rear combination lights, front turn signals, side markers and specialized interior lighting. Now the automotive lighting community eagerly awaits the availability of LEDs that produce enough output at a low enough cost to be used in such forward-lighting applications as head-and fog lights (Figure 1).
Figure 1. Visteon has developed and tested numerous LED forward-lighting applications, as shown here on a concept vehicle. Images by Doug Nork.
Recent advancements
Important strides are being made in the development of high-luminance white LEDs that will enable automotive forward-lighting applications. Advancements in LED source output, improvements to thermal management and headlight design, and increased understanding and alignment among component suppliers and automakers are all contributing factors.
New materials, optics and electronics emerging from related industries also are contributing to the advancement of these uses. For example, silicone has superseded epoxy as an encapsulant, enabling high-temperature operation of LED chips. Similarly, ceramics have replaced high-temperature plastics for base and housing materials, and solder and gold have replaced adhesives for joining layers within LED packages. In addition, optics of smaller scale and higher precision, such as miniature lenses and index-coupled lightguides and lenses, are replacing optics previously adapted from reflector and lens applications.
Improvements in luminous flux (total light output), luminance (flux per emitting area per solid angle), efficacy (luminous flux per unit of input power) and color control, as well as enhancements to thermal resistance and operating temperature, are other factors. Luminous flux, measured in lumens, and luminance, measured in nits, are particularly significant to producing a well-designed, high-output headlight because most other design parameters relate to those requirements.
Newer LEDs can achieve higher luminance levels through higher efficiency in thermal resistance and drive current or through lower optical magnification. Less efficient LED sources require more power and lower thermal resistance but can still achieve similar luminance levels. However, practical reductions achievable in a lamp’s thermal resistance are limited, and improvements to all elements are needed to ensure optimum performance of LED sources in a headlight application.
Luminance also is important to overall headlight design because the optics drive its package, style and structure. The luminance and flux values give an LED the light output required to produce the beam content for a headlight application. Luminance also drives the scale of the optics through the principle of etendue — the preservation of intensity and area. For example, a requirement for the minimum and target maximum beam intensity, or hot spot, as well as the relationships between the sizes of the optical components and the luminance, dictate a certain luminance minimum at the LED’s emitting surface. To exceed halogen sources in headlight performance, the luminance values of white LEDs must exceed the luminance of tungsten filaments that can produce >20 Mnt.
The development of more efficient LEDs in terms of efficacy — lumens/watt — also contributes to headlight design. Improvements in efficacy have partially resulted from chip epitaxy and from phosphor enhancements. However, new high-luminance LEDs — producing >20 Mnt — often have significantly lower encapsulation-related efficacy, partially offsetting the greater chip and phosphor efficacy.
From the perspective of an LED integrator, there is little to leverage among these parameters for a headlight system. However, one efficacy-related lever can be used: temperature. The efficacy of all LEDs drops with increased operating temperatures. Therefore, the better the thermal management and the cooler the environment, the more efficient the devices will be. Because drops in efficacy can be greater than 10 percent under high operating temperatures, it is not in anyone’s interest to drive LEDs to their performance limits. Depending on the vehicle platform, however, efficacy may be traded off with cost and package requirements.
Another area of improvement — perhaps the most recent topic of discussion in LED source technology — is color control. Because automotive exterior LED applications are not as prominent as general LED lighting applications (streetlights and billboard signage, for example), most people do not notice the significant color differences that the devices produce. Although the current color range of white LEDs may not be acceptable for a high-end vehicle headlight, strides are being made toward finding an acceptable nominal color space and toward limiting color variations, to improve the color output. In addition, industry experts are working to resolve conflicting views on the best range in terms of color (for visibility) and glare, as well as signature and appearance for LED headlights.
To provide a wider range of operating conditions and to improve LED cost-effectiveness (measured in lumens per dollar), both thermal conductivity — often expressed as the converse, “thermal resistance” — and maximum operating performance (usually junction temperature and drive current) have dramatically advanced in the past two years. These improvements allow higher power dissipation per LED package (power density) and, therefore, a tendency toward higher source luminance. The result is the ability to develop smaller, higher-temperature systems with increased overall thermal capability. However, higher lamp temperatures also bring elevated requirements for materials and construction of the rest of the system.
Industry alignment
Cooperative developmental work among automotive suppliers and manufacturers is improving system integration and mounting capabilities for LED headlight applications.
The starting point for greater industry collaboration has been the need to fulfill US and European motor vehicle safety standards. These standards raise light system requirements to include outage detection, temperature compensation, elevated environmental temperatures and other features not demanded of current lamps. Greater alignment among various technology suppliers is proving critical in other areas as well. From suppliers of electronic substrates to thermoplastic resin suppliers to heat sink suppliers, each is developing a better sense of not only what works, but also how to improve their own components.
Part of the challenge to suppliers is that the internal architecture of LED lights bears little resemblance to today’s high-intensity-discharge or halogen lights. They are distant relatives of the same species, and it may take some time before technical solutions begin to settle. In addition, design shifts are tectonic, and discussions about active or passive thermal management still pose complex questions. For example, is a plausible and compelling vehicle styling enhancement worth the large investment it will require for validation? Such questions affect not only departments and companies, but also entire industries.
Current challenges
Although commercial white LED lights are suitable for forward-lighting applications in terms of their thermal resistance and mechanical packaging, the luminous flux and luminance that they generate remain insufficient to compete with high-intensity discharge lamp systems.
Another challenge is the thermal management of the overall LED-based headlight system. The extent to which heat is dissipated from the devices into the surrounding environment strongly determines how effective the headlight application will be in terms of cost, performance and packaging. Lamp systems using LEDs must have sufficiently low thermal resistance to obtain the required light output at the required drive current. This necessitates a cooling system. However, passively cooled systems need greater heat-sink mass, which complicates packaging and the mechanical structure of lamps, and actively cooled systems are costly.
Although these factors, along with electronic control and optical alignment, represent significant challenges, the effort appears well compensated for by the benefit of the enhanced exterior styling achieved by using LEDs.
Establishing a market
Lamp manufacturers are responsible for demonstrating the unique styling, performance features and benefits that LEDs can offer, but the vehicle manufacturers’ technology and platform teams must better understand the related consumer value.
Research has been conducted in the past several years to elicit consumer views about LED forward-lighting as well as about the vehicle manufacturers’ desires for brand differentiation. Concepts and prototypes have been developed, and tests have been run. Now it is time for the market to speak.
Supply chain providers constantly are coming up with ideas to further enhance LED technology and the related consumer experience. So the question remaining is not whether automotive companies are going down the right path in pursuing LED forward-lighting applications, but how soon we can achieve an alignment of the needs and desires of consumers and vehicle manufacturers using LED technology in a way that maximizes value to each group.
Meet the author
Mitch Sayers is manager of advanced lighting applications at Visteon Corp. in Van Buren Township, Mich.; e-mail: [email protected].