Sealed beams utilize a parabolic aluminized reflector, and are thus known as "PAR" lamps. Round PAR lamp diameter is expressed in eighths of an inch, so a PAR56 lamp, for example, is 56/8" (i.e., 7 inches) in diameter. Other popular sizes are PAR30, PAR36, PAR38, PAR46, and PAR64. Rectangular PAR lamp size is expressed in millimeters, so a PAR200×142 is 200 mm wide and 142 mm tall.
Sealed beams are available in a variety of nominal voltage ratings, most commonly 6, 12, 24, 28, 120, and 230 V. The actual operating voltage may differ from the nominal rated voltage; for example, "12 volt" sealed beam headlamps are meant for use in automobiles with typical line voltage of 13.5 to 14.2 with the engine running.
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. Between 1940 and 1956, all U.S. cars had to have two 7-inch round headlights. . Each headlight had dual filaments, so that one bulb provided both high and low beam. In 1957, regulations changed to allowed four sealed beam headlights (two on each side, each measuring 5 ¾ in.). Two served as low beam and two were high beams. This influenced the overall design of the American-made vehicles. 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.