There are four basic types of headlights used on automobiles today:
- standard sealed beam
- halogen sealed beam
- high-intensity discharge (HID)
FIGURE. Sealed-beam headlight construction.
From 1939 to about 1975, the headlights used on vehicles remained virtually unchanged. During this time, the headlight was a round lamp. The introduction of the rectangular headlight in 1975 enabled the vehicle manufacturers to lower the hood line of their vehicles. Both the round and rectangular headlights were sealed-beam construction. The sealed-beam headlight is a self-contained glass unit made up of a filament, an inner reflector, and an outer glass lens. The standard sealed-beam headlight does not surround the filament with its own glass envelope (bulb). The glass lens is fused to the parabolic reflector, which is sprayed with vaporized aluminum that gives a reflecting surface that is comparable to silver. The inside of the lamp is filled with argon gas. All oxygen must be removed from the standard sealed-beam headlight to prevent the filament from becoming oxidized. The reflector intensifies the light that the filament produces, and the lens directs the light to form the required light beam pattern.
FIGURE. The lens uses prisms to redirect the light.
The lens is designed to produce a broad, flat beam. The light from the reflector is passed through concave prisms in the glass lens. Lens prisms redirect the light beam and create a broad, flat beam. The illustration shows the horizontal spreading and the vertical control of the light beam to prevent upward glaring.
FIGURE. The prism directs the beam into (A) a flat horizontal pattern and (B) downward.
By placing the filament in different locations on the reflector, the direction of the light beam is controlled. In a dual-filament lamp, the lower filament is used for the high beam and the upper filament is used for the low beam.
FIGURE. Filament placement controls the projection of the light beam.
FIGURE. A halogen sealed-beam headlight with iodine vapor bulb.
The halogen lamp most commonly used in automotive applications consists of a small bulb filled with iodine vapor. The bulb is made of a high-temperature-resistant quartz that surrounds a tungsten filament. This inner bulb is installed in a sealed glass housing. With the halogen added to the bulb, the tungsten filament is capable of withstanding higher temperatures than that of conventional sealed-beam lamps. The halogen lamp can withstand higher temperatures and thus is able to burn brighter.
In a conventional sealed-beam headlight, the heating of the filament causes atoms of tungsten to be released from the surface of the filament. These released atoms deposit on the glass envelope and create black spots that affect the light output of the lamp. In a halogen lamp, the iodine vapor causes the released tungsten atoms to be redeposited onto the filament. This virtually eliminates any black spots. It also allows for increased high-beam output of 25% over conventional lamps and for longer bulb life.
Note: Because the filament is contained in its own bulb, cracking or breaking of the lens does not prevent halogen headlight operation. As long as the filament envelope has not been broken, the filament will continue to operate. However, a broken lens will result in poor light quality and should be replaced.
FIGURE. A composite headlight system with a replaceable halogen bulb.
By using the composite headlight system, vehicle manufacturers are able to produce any style of headlight lens they desire. This improves the aerodynamics, fuel economy, and styling of the vehicle. Composite headlight systems use a replaceable halogen bulb.
Many manufacturers vent the composite headlight housing because of the increased amount of heat these bulbs develop. Because the housings are vented, condensation may develop inside the lens assembly. This condensation is not harmful to the bulb and does not affect headlight operation. When the headlights are turned on, the heat generated from the halogen bulbs will dissipate the condensation quickly. Ford uses integrated nonvented composite headlights. On these vehicles, condensation is not considered normal. The assembly should be replaced.
FIGURE. HID headlamps; note the reduced size of the headlamp assemblies.
High-intensity discharge (HID) headlamps are the latest headlight development. HID headlights use an inert gas to amplify the light produced by arcing across two electrodes. These headlamps put out three times more light and twice the light spread on the road than conventional halogen headlamps. They also use about two-thirds less power to operate and will last two to three times longer. HID lamps produce light in both ultraviolet and visible wavelengths. This advantage allows highway signs and other reflective materials to glow. This type of lamp first appeared on select models from BMW in 1993, Ford in 1995, and Porsche in 1996.
FIGURE. Comparison between light intensity and patter. Halogen lamp is shown on the left and the xenon (HID) lamp is on the right.
FIGURE. HID bulb element.
The HID lamp consists of an outer bulb made of cerium-doped quartz that houses the inner bulb (arc tube). The inner bulb is made of fused quartz and contains two tungsten electrodes. It also is filled with xenon gas, mercury, and metal halides (salts). The HID lamp does not rely on a glowing filament for light. Instead it uses a high-voltage arcing bridge across the air gap between the electrodes. The xenon gas amplifies the light intensity given off by the arcing. Hie HID system requires the use of an ignitor and ballast to provide the electrical energy required to arc the electrodes. The ignitor is usually built into the base of the HID bulb and will provide the initial 15,000 to 25,000 volts to jump the gap. Once the gap has been jumped and the gas warms, then the ballast will provide the required voltage to maintain current flow across the gap. The ballast must deliver 35 watts to the lamp when the voltage across the lamp is between 70 and 110 volts.
FIGURE. HID headlight schematic showing the use of a ballast and ignitor.
The greater light output of these lamps allows the headlamp assembly to be smaller and lighter. These advantages allow designers more flexibility in body designs as they attempt to make their vehicles more aerodynamic and efficient. For example, the Infiniti Q45 models are equipped with a seven-lens HID system to provide stylish looks and high lamp output.
FIGURE. Seven-lens HID headlamp.
Due to the increased amount of light that the HID headlamps produces, they are not used in a quad headlight system as a high beam lamp. Instead they are used as the low beam lamp only and a halogen bulb for the high beam. Since the quad headlamp system uses all four bulbs for high beam operation, using a quad lamp system with HID lamps would blind any oncoming drivers due to the excessive amount of light output. Also, it is not possible to reduce the light intensity of an HID by PWM of the current to the element.
To overcome these issues and to still be able use the HID, manufacturers have started to use bi-xenon headlamps in their dual headlamp systems. Bi-xenon refers to the use of a single xenon lamp to provide both the high beam and the low beam operation. Hie full light output is used to produce the high beam. Low beam is formed by either moving the xenon bulb within the lens or by moving a shutter between the bulb and the lens.
Systems that use the shutter will have a motor within the headlamp assembly that raises and lowers the shutter. The position of the shutter dictates the amount of the projected light that will be allowed to escape from the lens and its pattern.
Note: The actual direction that the shutter travels, to block or unblock varies based on application.
In some systems a motor is used to change the position of the bulb. The bulb is physically raised in the reflector housing to produce the high beam output. In low beam mode, the bulb is lowered in the reflector housing. The amount of reflection dictates the light intensity and pattern.
Using the same lamp for both low and high beam operation permits both modes to have the same light color. This produces less visual contrast when switching between modes and is less stressful to the eyes of the driver.