Lighting 101(Lea en español)
Unfortunately humans do not always see well enough. Yet our need to see doesn't end when the sun goes down! The risk of having an accident at night is about twice as high as during the day despite the fact that there is less traffic on the road. While the lights of modern vehicles are typically more than sufficient to fulfill the legal requirements, they can often be added to, or improved upon in order to see even more details in low light, or no light situations.
A car's headlight consists of bulb(s), reflector, lens, and housing. While many newer cars, vans and light trucks in the United States are equipped with headlights that have replaceable bulbs, most earlier cars feature sealed-beam headlights that combined the filament, reflector and lens into one unit that fits into the car's headlight housing (the entire unit is removed and a new one is inserted if replacement becomes necessary).
Most replaceable headlight bulbs are the twin-filament type, with a built-in shield preventing light from escaping in the wrong directions. The filament for the low-beam ("city beam") is placed a few millimeters in front of, and higher than the reflector's focal point so that the low-beam light will be reflected forward and down. The filament for the high-beam ("highway beam") is located at the exact focal point of the reflector, so that the high-beam light will be reflected straight ahead.
The bulbs should be as small as possible, emit a very bright light, and must be accurately located relative to the reflector. High light efficiency is obtained with coiled tungsten filaments enclosed in relatively small bulbs filled with an inert gas. The accurate location of the bulb is made possible by locating tabs on the base of the bulb in relation to the filament. Thus, if the bulb is correctly fitted, the filament will be exactly where it is desired to be relative to the reflector. If the light source is not located correctly, the headlamp's output will be greatly reduced due to a random scattering of the available light.
The purpose of the reflector is to concentrate the light that the bulb emits by directing it through the lens. The reflectors are typically made of metal, with the reflecting surface being "silvered," usually by a thin coating of aluminum applied as vapor in vacuum.
The headlamp lens allows the light to pass directly through it or is used to distribute the light in front of the vehicle. Light distribution is achieved by manufacturing grooves or ribs in the glass lens that function as prisms and deflect the light. This diffusion does, of course, reduce the brightness of the beam in the main direction.