May 2008 Articles & Features
By Mike Dale | May 2008
The science behind using diodes as a light source was mapped out more than 80 years ago. After a slow start, LEDs are finding widespread use in today’s vehicles, including headlight systems.
Automotive lighting is all about the driver’s need to see and be seen, usually in difficult conditions. Darkness, twilight and varying degrees of rain, mist and fog are all obstacles to driver vision. Vehicle lighting right now is in the midst of technological changes that will affect all vehicle systems. Intelligent lights are coming that will bring new vision capabilities to the driver. With them will come new service opportunities.
Vehicle lighting is important for reasons beyond driver vision. The headlights are the “eyes” of the vehicle and play a key role in styling. Interior lighting can be used to set the mood and ambiance in high-end luxury vehicles. The power consumption of the lighting system affects what power is available to other electrical systems in the vehicle. Turn signals, CHMSLs and emergency flashers compete for the attention of motorists already distracted by cell phones, stereos and rowdy kids.
There are three main types of lights currently being used in automotive applications—incandescent bulbs, high-intensity discharge (HID) lamps and light-emitting diodes (LEDs). Incandescent bulbs produce light by heating a filament made of tungsten by an electric current to the point of incandescence. The filament is housed in a glass bulb that has been purged of oxygen. For headlamp application, gases such as halogen are added as a means of improving the efficiency and the quality of the light output.
HID lamps produce light by striking an arc across the terminals inside the bulb. No filament is needed, as the light comes from the arc. The inside of the bulb is filled with xenon gas with mercury or bismuth added. These gases give the light its characteristic bluish color. An HID lamp requires a special electronic ballast to create the arc and then to keep it lit while the lamp is in operation.
LEDs are highly optimized devices that produce light when current flows through the P-N junction of the diode. Although this phenomenon has been known since the 1920s, it’s only recently that indium gallium arsenide diode technology has evolved to the point where enough light can be produced to make LEDs a candidate for use in headlamps. Other, lower output LEDs are being used for interior lighting, parking lights, brake lights and other lighting applications.
LED lighting also requires special control electronics. To survive in automotive applications, LEDs must be fed from a constant-current power supply. A Lamp Driver Module (LDM) prevents variations in battery voltage from overdriving the LED and possibly causing it to fail.
The LDM also deals with a problem unique to LEDs—a normal, gradual reduction in light output over the life of the device. This change, called lumen maintenance, can amount to about a 10% loss in light output for headlamp LEDs and even greater losses for small-signal LEDs. It’s possible to compensate for this in the drive electronics, so the loss is not noticeable.
All three possible sources of light for forward vehicle lighting have advantages and disadvantages. Halogen bulbs have been with us for more than 30 years and are tried, proven and cheap. They require no special electronics. But they’re energy hogs. A typical halogen high beam will draw about 55 to 65 watts. At typical vehicle battery voltages, this means about 4 amps per bulb, or 8 amps for the pair.
One advantage with HID systems is that they produce about three times the amount of light as the halogen bulbs for a little more than half the energy input. A typical bulb, drawing 35 watts of power, will last considerably longer than a halogen bulb since there’s no filament to wear out or break. Another major plus is that the light output has nearly the same color or frequency content as natural daylight. Since this is what the human eye responds to best, it means that drivers can simply see better when the road ahead is lit up by HID lamps.
While HID lighting has been available since the early ’90s, it never really has become inexpensive enough to make it into mainstream U.S. production. Currently, HID lighting has about half the market in Europe. The main cost driver is not the bulb but the electronics needed to power it. There are two sections inside an HID ballast. One produces about 20kV to start the arc between the bulb’s electrodes. The second part feeds and maintains the arc with about 600 volts and a regulated amount of current.
LEDs are even more efficient than HID lamps in terms of energy consumption, although it’s difficult to say how much more efficient, as new developments and improvements keep coming at a rapid pace. The expectation is that LEDs will match the performance of HID systems while using perhaps half the energy of HIDs and a quarter of the energy of incandescents.
LEDs have been shown through testing to have over three times the life and reliability of incandescent bulbs. Yet another advantage is that the actual LED chips are smaller and lighter than the components of either HID or incandescent systems. This weight saving is made even greater by the fact that the supporting structure in the headlamp system can also be made lighter.
The real impact of LED lighting becomes apparent when you add up the number of lights in use on a typical vehicle. LEDs used in the instrument panel are much smaller than those used for the headlights. But then there are lots of them. Add to this all of the lights in the front, rear and side of the vehicle and the total current draw becomes impressive. A vehicle driving at night with the high beams on may need 20 amps or more just for lighting.
The net power saving with LEDs is important because it frees up electrical energy that can then be used for other accessories, such as electric power steering or an electric-powered water pump or a/c pump. Converting the Front End Accessory Drive (FEAD) loads from mechanical to electrical loads is one way to save fuel. Since these accessories need not run all the time, it’s more efficient to do them as electrical devices that can be switched on and off as needed.
Generating the needed light is only part of the automotive lighting story. That light must be focused and directed in accordance with very detailed federal regulations written with the primary idea of protecting oncoming drivers. They specify how the forward headlights must be aimed and how bright they can be.
To meet these requirements, it’s necessary to gather, focus, project and direct the light from the light source. Light leaving a filament structure or HID arc does so in all directions. Very little is truly aimed where it needs to go—that is, forward of the vehicle. The traditional solution to this has been to place the source of the light near the focal point of a paraboloid reflector. This reflector gathers light not headed in the car’s forward direction and reflects it onto the right path. It was also possible to include optical elements in the front lens of the lamp assembly to further form and direct the output of the lamp in the pattern of a forward-directed beam.
There are other ways to do this in current use. The Free Form Reflector (FFR) concept directly assigns an area of road to be illuminated to a specific area in the reflector. Hella Lighting says that the surface of the reflector is calculated in computer software with what it calls deflection strategies. The software, called Helios, creates the design for the Free Form Reflectors that do a better job of putting the light where it’s needed.
Another light-directing method seeing a lot of use on late-model high-end vehicles is called the projector type system. This, too, is a free-form system. The light generated by the source is gathered near a first focal plane, then projected onto a second focal plane. The second focal plane has a shield on it whose contour is mapped to the road by a lens.
The lens itself is the distinguishing feature of this headlamp system. It’s a convex lens about 2 in. in diameter that looks kind of like an “eye” from which the shaped light beam emerges.
Now this wouldn’t be the modern day and age if someone didn’t figure out how to attach a computer and a motor(s) to the projection lens. The Hella Varilus and Variox systems are in production, with both computers and motors attached to the forward vehicle lighting system.
With a single free-form drum-shaped reflector system, the headlamp can be varied through a half dozen different light projection patterns. The light pattern designed for use in urban areas has a wide projection designed for short-range vision on roads that are lit by street lamps. The rural light pattern has a bright, sharp projection of light along the right shoulder with reduced projection into the oncoming lane of traffic. For Interstate or highway driving, more of the light is put in the center of the road for long-range vision. (Interstates are, of course, divided, so the concern of blinding the oncoming lane of traffic is reduced.)
As with everything that’s technology-driven, headlamp and vehicle lighting systems are only going to become much more sophisticated. The opportunity for the service industry will come from the higher mechanical complexity of the motors and sensors that adjust and aim these systems. As usual, the technicians who understand how they work and how to adjust and/or repair them will earn the service dollars that knowledge deserves.