Automotive Headlamps
Sealed beam headlamps were introduced in the United States in 1939, and became mandatory from the following year until the 1984 model year. Cars prior and subsequent to that date could have a variety of shapes of headlamps, using any of a wide variety of replaceable bulbs. Halogen sealed beam headlamps appeared on U.S. cars in 1978 to enable halogen technology under Federal Motor Vehicle Safety Standard 108, which at the time required all headlamps to be of sealed beam construction; eventually halogen sealed beams came to dominate the sealed beam lamp market.
The limited range of standardized sealed beam headlamp sizes and shapes restricted styling options for automobiles. Replaceable-bulb headlamps provide the regulated light distributions while allowing greater design and engineering freedom.
Read more about this topic: Sealed Beam