This area only for techno-geeks like me. Proceed with caution!
Headlight & Halogen History
One characteristic of tungsten filaments is that the tungsten evaporates while the light source ison. This evaporation process will concentrate at some weak spot, scratch, or mark on the tungsten surface. After many hours the tungsten becomes thinner and finally the filament will burn out. The concept of the halogen cycle, and the benefits regarding redeposition of evaporated tungsten on the filament were discovered in 1912 (Ehrhardt, 1979). Certain halogen gases, when inserted in a bulb, combine with the evaporating tungsten molecules to form a compound that will circulate back to the filament, where the tungsten will dissociate and redeposit on the hot filament. This allows the filament to burn brighter and hotter without shortening its life. However, it is not perpetual motion, and eventually halogen bulbs will also burn out. Automotive applications of halogen headlamp bulbs did not occur until the middle sixties. Europe was quicker to take advantage of this improved light source than the U.S. Single filament H1, H2, and H3 light sources were made and used in headlamps and other lighting applications on vehicles in Europe. The double-filament H4 bulb was used in the late sixties in European headlamps. Halogen bulbs were not installed in U.S. sealed beam headlamps until the 1970s. Some refinements in the low-beam headlamp specifications occurred in 1965 when SAE J579a was written (see Figure 5).
The U.S. government passed the Motor Vehicle Safety Act in 1966. The U.S. National Highway Traffic Safety Administration (NHTSA) was created in 1968 to improve the safety of road transportation by focusing on the design of vehicles. Previously, lighting was regulated solely by states. Within two years, many existing SAE lighting standards were adopted, in whole or in part, in the Federal Motor Vehicle Safety Standards (FMVSS).
At the beginning of the 1970s two headlamp sizes were legal in the U.S.: 7 inch (178 mm) round (two headlamps per vehicle) and 5 3/4 inch (146 mm) round (four headlamps per vehicle). In 1974 NHTSA approved a petition to allow a new four-headlamp rectangular sealed beam system, 4 inches by 6 1/2 inches (100 mm by 165 mm). Two years later, a 142 mm by 200 mm rectangular two-headlamp system was allowed. Then in 1978, NHTSA increased the maximum allowed headlamp intensity per vehicle from 75,000 cd to 150,000 cd (NHTSA, 1978). This made the installation of halogen bulbs in sealed beam headlamps advantageous, because seeing distance could be improved
with the additional lighting intensity.
Most headlamps in the world had used glass lenses up to this time. With the use of halogen light sources in the U.S., the hermetic seal provided by the all-glass sealed beam was not required. In halogen lamps the tungsten filament was contained in a gas-filled inner glass bulb. Adhesive-bonded, all-glass sealed beams were manufactured along with sealed beams with plastic lenses and reflectors. Materials and coatings were developed to provide good abrasion resistance, and the plastic materials increased the impact resistance to stone breakage.
During this time period, Europe had a higher percentage of headlamps with halogen bulbs than the U.S. Most European headlamps used replaceable bulbs. Some European countries allowed sealed beams (a very small percentage of the total number of headlamps) meeting the ECE beam patter requirements.
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