Car headlights typically employ filament-based light sources such as halogen lamps, or high-intensity discharge lamps that produce electric arc illumination by electrical discharge between electrodes in a high-pressure gas ambient. A halogen lamp is widely used in a conventional headlight. The halogen lamp is provided with two filaments, that is, a filament for irradiating a high-beam and a filament for irradiating a low-beam, so a high-beam mode or a low-beam mode may be selectively operated as desired. High voltage discharge lamps are also in use as a light source for an automobile headlight device. The discharge lamp emits light from a light source generated by arc-discharge powered by a square alternating voltage or a direct current voltage. The discharge lamp is an arc-discharge type lamp in which xenon gas is enclosed in a discharge lamp bulb (arc tube), and light is emitted from the discharge lamp bulb using discharge generated between a pair of electrodes therein. The light emitted from the discharge lamp bulb is white light similar to sun light. A gas discharge which emits a very intensive light is generated in a closed discharge vessel between two electrodes in known discharge lamps. A discharge lamp usually comprises a lamp base, a burner comprising a discharge vessel, and an outer bulb. The lamp base serves to retain the burner and to position the lamp and contact it electrically in a headlight. The actual light generation takes place in the discharge vessel. Usually, an outer bulb of glass is arranged around the discharge vessel. The high-pressure gas discharge lamp is inserted directly into the ballast through an opening in the reflector. An elastic sealing compound is inserted between the reflector and the molded housing to protect against harmful effects of moisture and corrosion. The ballast is supplied with voltage from the vehicle's on-board electric system over a simple pair of cables. This bulb serves to filter out UV radiation from the emitted light. A quantity of light from the discharge lamp can be equal to or more than twice as much as that from a conventional halogen lamp while using about 30% of the electric power compared to the halogen lamp. Accordingly, when the discharge lamp is used as a vehicle head lamp, visibility in front of the vehicle is improved. A dedicated lighting-control circuit is required to light the discharge lamp. A lighting control circuit is mainly constructed by a converter, an inverter and an igniter. The converter increases the direct-current (DC) voltage from a battery, and the inverter converts a DC voltage to an alternating-current (AC) voltage.

Automobile headlights are designed to emit a low or high beam by causing light from a light source bulb to be reflected forward by a reflector. The low-beam headlights are a compromise between providing forward illumination for the driver and avoiding glare and possible blinding of oncoming traffic by the vehicle headlights. High beam headlights are used most often in country night-time driving, where there are usually no oncoming vehicles. When an oncoming vehicle is encountered, the driver switches to the low beam headlights. Vehicle head lamps most typically contain two separate light bulbs that can be independently activated. When one of the bulbs is activated, a low beam lamp output is produced and when a second bulb of the two bulbs is activated a high beam lamp output is provided. All motor vehicles include a control for switching between high beam and low beam operation of the motor vehicle headlamps. This control may be implemented with a stalk on the vehicle steering column that is easily actuated by a motor vehicle operator. When the high beam option is selected, the vehicle headlamps are focused in a direction that illuminates further in advance of the vehicle to improve a motorist's ability to see details in his or her line of sight. When an oncoming vehicle approaches, the driver can switch to low beam operation to avoid temporarily blinding the oncoming driver. Fog lamps are occasionally installed on vehicles to direct a lamp output downward in a direction closer to the vehicle to enable the motorist to more clearly see during rain and foggy conditions. Though different light sources are generally used for low and high beams respectively because the direction of emitting the low beam differs from that of emitting the high beam, there are also known vehicle headlights using a single light source. Car headlights using a single light source are adapted to switch between a low beam and a high beam by mechanically displacing optical elements such as a light source bulb and a reflector and to this end they have a beam switching device.

There are various constructions of car headlight assemblies switchable between their low-beam and high-beam operating modes. The low-beam headlights are designed and mounted on the vehicle in a manner which concentrates the low-beams below the horizontal, i.e. onto the road rather than toward oncoming traffic. The low-beams are also preferably used in snowy, rainy, or foggy driving conditions to reduce back-scattered headlight illumination which can blind the driver. In general, a low beam headlight has a reflector, a light source and a light-permeable disk arranged in the path of the light rays reflected from the reflector. The light-permeable disk has at least one localized optical element, by which the light beam reflected from the reflector is deflected or guided during its passage through it. The light beam issuing from the low-beam headlight has an asymmetrical upper light-dark boundary with an approximately horizontal section directed toward the on-coming traffic side of the vehicle. Low beam headlights are typically designed such that light which is emitted at 10 degree to 90 degree up from the axis of the lamp capsule is blocked from exiting the headlight, because this light forms what is known as veiling glare. A high beam headlight assembly has a light source and a reflector. The light source has a light emitting body and the reflector has two different parts, by which light issuing from the light emitting body of the light source is reflected differently. The light is reflected by one reflector part as a horizontally scattered beam and by the other reflector part as a concentrated light beam with lesser scatter and longer range. The light beam emitted by the high beam headlight has the characteristics of both sufficient horizontal scattering and also sufficient range. High beam and low beam headlights either in separate light bulbs or implemented by dual filaments and lens in a single bulb project different light patterns.

The typical vehicle headlight has a partially spherical back surface from which extends a pair or more of male terminals which are in turn connected to the vehicle electrical system by a suitable electrical connector. A typical electrical connector consists of a molded plastic socket containing female terminals that grip the male headlight terminals as the socket is plugged to the headlight. While the discharge lamp bulb is lit, its temperature becomes high. Generally, ventilation holes are provided in the lamp housing to eliminate any pressure differences between the inside and the outside of the lamp housing. A headlight of the elliptical type consists generally of a reflector having a first focal zone, with a light source placed in the vicinity of the first focal zone, together with a second focal zone. The light from the light source, after being reflected by the reflector, is concentrated in the vicinity of the second focal zone. Such a headlight also includes a convergent lens, which is typically of the planar-convex type, which is focused in the vicinity of the second focal region of the reflector and which is capable of projecting on the road the above mentioned light patch. Dipped headlights for motor vehicles consist of a reflecting mirror sending light rays emitted by a light source towards the front of a vehicle and a glass possibly altering the path of these rays, in order to obtain a light beam having a predetermined photometry. A vehicle may also have a daytime running light circuit that automatically operates the headlights at reduced intensity for daytime running whenever the vehicle is operating and the headlight switch is off. Headlight covers are well known in the automotive industry, both as original equipment and as aftermarket add-ons.