Automotive News Europe
Headlamps hae come a long way from the carbide/methane gas-burning carriage lamps of the very early cars, but the fundamentals haven't changed at all. Cars need lighting to see, and to be seen.
The difference between then and now is quality. Advances in lenses and reflectors have put more light on the road where it is helpful. And new light sources make more light available from less energy.
An early system
Lighting provided one of the first systems delivered by a supplier. In 1913, Robert Bosch supplied what it claims to be the first complete electric lighting system for cars. It included headlamps, side lamps, number plate and dashboard lamps plus generator, switch-box and battery.
The introduction of powerful generators was the key to the switch from gas lights to a tungsten filament bulb. For nearly 80 years the filament bulb has ruled the industry, though it is under threat today.
Light for illuminating a dark road needs to be concentrated forwards, and waste needs to be limited.
The parabolic headlamp reflector was the answer.
Reflectors don't magnify light, they merely collect the light escaping from the back of the bulb and reflect it, adding to the forward-directed light. Then came the idea of making a lens of the glass that protects the light source from road filth, further concentrating the light.
An early challenge was to keep headlamps from dazzling approaching drivers. The early answer was a mechanical system to dip the headlights downward.
Refinements of the idea, involving a tilt to the reflector, were still in use in the mid-1930s, although the double-filament headlamp with low beam and high beam was invented in 1924.
Oddly enough, one of the latest inventions is an echo of the first mechanical systems.
Bosch is working on variable light distribution, where a number of small reflectors in the headlamp are adjusted to give optimum light as needed according to different conditions, such as a long range light for motorways and a wide, low beam for city use.
If successfully developed, a single lamp could also light the road appropriately for fog, robbing some drivers of their excuse for hanging extra lamps to the front of their cars. Added lamps have a tendency to increase drag, contributing to fuel consumption.
In the mid-1950s the asymmetric low beam became commonplace, where a combination of a shield over the bulb and a specially shaped headlamp lens gave a long finger of light on the edge of the road, and a well-cut-off low beam in the center facing oncoming drivers.
Generators continued to have problems supplying enough electricity for lamps. In the 1960s, the industry switched to the mechanically simpler alternator, which generates ample power even at idle.
The alternator revolutionized the entire electrical system of cars, allowing higher electrical loads for all sorts of devices, including the halogen bulb.
In these bulbs, the filament is enclosed in a quartz bulb containing gas enriched by one of the halogen gases: fluorine, chlorine, iodine and bromine.
Such bulbs operate at higher temperatures and give twice the light of plain tungsten bulbs.
Smaller and smaller
In the last decade, demands for style and aerodynamic shape drove the evolution of more compact headlamps, which could deliver the same or better illumination from a smaller opening in the bodywork.
First an ellipsoid reflector, together with a bulb shield and a light-collecting lens, almost doubled the light power of the traditional parabolic lamps, and in a smaller space. Then glass was abandoned for polycarbonate plastic lenses. Plastics allowed optical designers to deliver excellent performance from astonishingly distorted headlamps. Polycarbonate lenses resist shattering better than glass, and they are lighter. Plastic reflectors coated with metal also weigh less than their predecessors.
Freedom of design in lenses and plastic reflectors has allowed so many improvements that a plain cover glass is all that is required for a very small yet powerful lamp, exemplified in the Hella 'free-form' headlamp family used in the Alfa Romeo GTV.
Manufacturers are beginning to adopt a new light source. Gas-discharge lamps are replacing the halogen bulbs, starting at the luxury end. Such lamps fill a quartz tube with xenon gas and an arc is struck between two electrodes. As in a fluorescent lamp, an electronic ballast is needed, adding to the cost.
The resulting light is nearer daylight in wavelength, and it more than doubles the light energy of a halogen bulb while requiring only 35 watts of power instead of halogen's typical 55 watts. Bosch claims to be first in the field, with its Litronic system developed in 1991.
Despite all the advances, headlamps remain inefficient.
Of the actual light flux delivered by the source, whether tungsten, halogen or gas-discharge, traditional parabolic headlamps delivered only 27 percent of the low beam illumination to the road ahead. With elliptical reflectors, Hella says this went up to 36 percent. Free-form reflectors lift the figure to 52 percent. The remaining 48 percent goes elsewhere, and gives developers a reason to continue research.