Headlights are built into the front of a vehicle to illuminate the road ahead of the vehicle when driving at night or in other conditions of reduced visibility. In headlights, most vehicles require two beams to provide for a high beam and low beam operation. The beams are created by separate filaments, included either in one light bulb or in separate bulbs. These filaments must be positioned correctly in relation to the highly polished reflector. This is called focusing and is carried out when designing the light assembly and lens for the vehicle. The high beam filament is positioned at the focal point of the reflector to project the maximum amount of light forward and parallel to the reflector axis Figure 39-18. This light is then shaped by the lens, which is made up of many small glass or plastic prisms fused together. These prisms bend the light horizontally and vertically to achieve the desired light pattern for road illumination.

FIGURE 39-18
High and low beam filament position in relation to the reflector.
FIGURE 39-19
Low beam filament position in relation to the reflector.

The low beam filament is often placed above and slightly to one side of the high beam filament. Mounting the low beam filament in this position produces a beam of light that is projected downward and toward the curb side Figure 39-19. With this arrangement, the high beam filament produces the most concentrated light output, while the low beam filament gives a downward and dispersed beam that is less likely to blind oncoming drivers.

Headlight Design

Generally speaking, the headlight circuit consists of the battery, the fusible link or maxi-fuse, headlight fuses, the headlight switch, the headlight relay(s), the beam selector switch, headlights, the high beam indicator light, wiring of a suitable size to carry the electrical current through the circuit, and the ground circuit. Modern vehicles use the BCM to manage headlight functions.

On older, noncomputer-controlled systems, when the headlights are switched on, current is supplied from the battery and proceeds through the fusible link or maxi-fuse, headlight fuse(s), headlight switch, and dimmer switch, reaching either the low or the high beams. In more recent, noncomputer-controlled systems, some vehicles use relays to control the current through the low or high beam circuits. In this configuration, the beam selector switch either powers or grounds the relay windings in each of the relays, depending on the position of the beam selector switch. Activating the beam selector switch in the low beam position creates a magnetic field inside the low beam relay that closes the relay contacts, which allows electrical current to flow to the low beams. Activating the beam selector switch in the high beam position creates a magnetic field inside the high beam relay that closes the relay contacts, turning on the high beams.

The beam selector switch is a single-pole, double-throw switch, meaning it has one movable pole but makes contact in two positions. In the high beam position, the current is switched to the high beam circuit. In the low beam position, the current is switched to the low beam circuit. The beam selector switch can be set to switch a common power input to two outputs, or it can switch a common ground input to two outputs.

As with other advanced-technology on-board systems, modern vehicles use a network/bus system to manage these and other headlight and dimmer functions. On many vehicles, the headlight switch and dimmer switch are just inputs to a control module that sends a headlight “on” or “off” request over the network. The appropriate light controller processes the request and actually commands the lights on or off, sometimes with the help of relays—especially for high beams.

FIGURE 39-20
Replaceable halogen bulb.

A sealed-beam headlight has a highly polished aluminized glass reflector that is fused to the optically designed lens. It is a completely sealed unit that has the filaments accurately positioned in relation to the reflector. Most older vehicles used two dual-filament 7" ( 178-mm) round lamps; for a time, four single-filament 5.25" (133-mm) round lamps were used. Other older vehicles used two large dual-filament 7.875" (200-mm) rectangular lamps, or four smaller rectangular single-filament lamps. Regardless of size, when a filament fails in a sealed-beam light, the whole sealed unit must be replaced.

A semi-sealed beam headlight uses a replaceable bulb with a prefocus collar. The collar locates the bulb in the headlight and also controls the correct positioning of the filaments to the reflector and lens Figure 39-20. Some replaceable headlight bulbs have a partial shield below the low beam filament. This shield prevents light from the filament from striking the lower part of the reflector, which would be reflected higher than the midpoint of the lamp. The shield provides the primary shape of the low beam. The final shaping of the beam is carried out by small cylindrical prisms in the headlight lens. This provides a low beam that is asymmetrical. The asymmetrical lens pattern causes light to be thrown upward at a 15-degree angle on the curb side to better illuminate objects, persons, or animals close to the road.

Types of Headlights

Headlight systems have traditionally used reflector-type lighting systems. In a reflector headlight, the light from the bulb is reflected forward by a specially shaped reflector. An alternative is a projection-type headlight system. This type of headlight often has a smaller front lens; however, it produces a high-intensity forward beam. It uses a lens system to project the light forward, rather than the traditional reflector system Figure 39-21. A projector-style light can use a standard incandescent bulb, or more commonly, an HID light.

FIGURE 39-21
Projector bulb assembly.
FIGURE 39-22
HID headlight assembly.

HID lights use light from an electric arc rather than heating up a filament until it glows Figure 39-22. High voltage is applied to tungsten electrodes. Xenon gas inside the bulb is then ionized and creates an electrical path between the electrodes, which lowers the resistance of the gap. As the temperature rises, metallic salts are vaporized and provide a stable arc, emitting much light. To initially jump the gap, a ballast and igniter raise the vehicle’s low-voltage direct current (DC) to as much as 25,000 volts alternating current (AC). Once the light reaches full operation, voltage is maintained between approximately 40 and 85 volts AC, depending on the system. HID lights give off a brighter and bluer light than halogen bulbs with less electrical energy, so they help fuel efficiency slightly. They last two to four times as long as a halogen bulb but are quite a bit more expensive. Also, they take up more room in the engine compartment.

Some vehicles use LED lights as headlights. They are not currently as bright as HID lights but are at least as bright as halogen lights. LED lights are known for low power consumption, but for LEDs to create enough light to function as headlights, they must consume almost as much energy as HID lights. Still, LED technology continues to improve. One drawback of current LED headlights is that high temperatures degrade or damage them. Thus, heat sinks and cooling measures are needed, which add complexity, cost, and space.

Night vision is another relatively new technology that enhances a driver’s visual perception in dark or poor weather conditions. There are two types of night vision systems: active and passive. Active systems use an infrared light generator that projects infrared light in front of and to the side of the roadway ahead. A special camera picks up the reflected infrared radiation. The image is then displayed either on the windshield using a heads-up display or on an LCD screen on the dash or navigation system.

Passive night vision systems use a heat-sensing camera (thermal imaging) to pick up thermal radiation emitted by objects. This system does not have an infrared light source on the vehicle. The captured thermal image is then displayed either on a heads-up display or on an LCD screen. One benefit of the thermal system is that it can be programmed to recognize pedestrians and animals and can then either place an outline around them on the display or flash a warning symbol.