A car headlamp consists of a housing (usually recessed into the body of the vehicle), the reflector, the light-diffusing glass, and the bulbs with their sockets.

The purpose of the reflector is to concentrate the light that the bulb emits in all directions and directing it ahead of the vehicle. A reflector is a parabolic mirror which is so shaped that all the rays of light emitted from a light source at its focus are reflected in a direction parallel to the axis of the reflector. If the light source is not located at the focus, the light output of the headlamp in the desired direction will be greatly reduced by scatter of the rays.

The reflectors are made of steel, the reflecting surface being ‘silvered’, usually by a thin coating of aluminium applied as vapour in vacuum. They reflect about 89% of the light that strikes them. The headlamp glass functions as a light diffuser, i.e., it must so distribute the light that the zone on each side of the vehicle and directly in front of it must also be illuminated.

This effect is achieved by providing the glass with grooves or ribs which function as prisms and deflect the light. This diffusion does, of course, reduce the brightness of the beam in the main direction.

The principal requirements that the electric bulbs have to fulfil are that the filament must be accurately located at the focus of the reflector and that the bulb should be as small as possible and yet be very bright. The first requirement is fulfilled by accurate location of the fixing tabs on the base of the bulb in relation to the filament. Thus, if the bulb is correctly fitted, the filament will be exactly at the focus of the reflector. A high light efficiency is obtained with coiled or ‘coiled coil’ tungsten filaments enclosed in bulbs filled with an inert gas.

At the present time most headlight bulbs are of the twin-filament type, with a built-in screen. The filament for high-beam (‘far beam’) is located at the focus of the reflector.

The filament for low beam (‘dimmed light’) is placed a few millimetres in front of the focus and a little higher, so that the light emitted by this filament is reflected as a downward-directed spreading beam. Rays from this filament striking the lower part of the reflector would be reflected upwards, but these rays are intercepted by the screen installed (inside the bulb) under the filament.

Most cars in the United States are equipped with sealed-beam headlights. This type of light contains the filament, reflector, and lens, all assembled into one unit. The entire sealed-beam fits into the car's headlight housing and, if replacement becomes necessary, the entire unit is removed and a new one is inserted.