A Brief History of Light

As far back as 1886, cars such as the Benz already had headlights.  These cars already had gas-discharge lamps, but not as we know them today.  Rather, they were lanterns that burned acetylene or kerosene.

A major advance was when cars developed a complete electrical system.  Originally, the electricity was limited to the ignition equipment.  With the advent of the electric starter on a 1912 model, electric lights began to replace the gas lanterns.

The earliest electric light bulbs used filaments that actually became hot enough to emit light when an electric current was passed through the filament.  Filaments of impregnated paper gave way to carbon and then tungsten, which burned brighter and lasted much longer.  These were known as incandescent bulbs.

At Present: Halogens and Multi-reflectors

Lighting technology in most new cars is still limited to incandescent.  Halogen bulbs are still classified as incandescent, because they still rely on a filament to provide the light.  They are a step up from conventional incandescent because inside each glass bulb is not a vacuum but a concoction of halogen gases such as krypton.  The wire glows from heat, and this excites the gas molecules enough that they also emit powerful light energy.  Halogen bulbs can be up to 50%  brighter than conventional incandescent, and can last up to twice as long.

Making sure all that light output is "thrown" out efficiently onto the road ahead is the job of the reflector-essentially, a paraboloid mirror.   The bulb is positioned at the mirror's         focus to reflect all rays of light parallel to the mirror's axis-that is, reflected out the front of the car.  In a low beam/high beam bulb, the high beam is at the focus while the low beam is positioned a few millimeters in front of the focus, to aim the light downward.  A shield incorporated in the bulb prevents reflections from travelling upward into the night sky.

Aiming is accomplished with the use of ribs and grooves of different thicknesses in the lens in front of the reflector.  These act as prisms and make sure the light is aimed into the appropriate distance in front of the car, and with enough lighting at the sides too, to illuminate curbs and other objects that the driver would want to avoid.

They also make sure that oncoming drivers are not blinded by the headlamps.  Due to legal requirements, American-market cars tend to emphasize this non-offensiveness, and thus have a fuzzier cutoff compared to European-market cars.  Cars meant for Europe have a more aggressive headlamp approach, with a longer beam throw and a sharp horizontal cutoff, perhaps necessary due to all that London fog and Autobahn speed.