A sealed beam is a type of lamp that includes a reflector and filament as a single assembly, over which a front cover (lens), usually of clear glass, is permanently attached. The previous design was like that of common flashlights (electric torches), which have a separate small bulb which is fitted in front of a parabolic reflector and covered with a transparent cover, which in the case of a headlamp is ribbed to avoid glare from the filament. This cover would be clamped on with a grommet in between to try to seal it; the method's deficiencies were what prompted the sealed beam system. The subsequent design has the reflector sealed to the cover and the small bulb inserted through a hole in the rear of the lamp.
Headlights for automobiles may be of the sealed beam type, meaning that the reflector, the lens array on the front and the bulb are all one unit that must be replaced together in case of burnout. They are clamped into a structure for aiming the beams to meet safety requirements. Every time the headlight is replaced, the aiming of the beams must be checked. Headlights using sealed beams became mandatory in the United States in 1940; cars prior to that date could have a variety of shapes of headlamps.
In stage lighting, sealed beam type fixtures are often used. A common size, also used in rock concerts, as well as outdoor architectural lighting, is the parabolic aluminumized reflector 64 (PAR64). PAR lamps are measured in non-SI units of measurement equal to one eighth of an inch, so a PAR64 light is a light that is 8 inches in diameter. The fixtures that such sealed beam lights go into are called "PAR cans", so a PAR64 fixture is an 8 inch diameter can.
Other popular sizes are PAR56, PAR38, and PAR36.
Sealed beams come in various voltages, most commonly 6, 12, 28, 120 and 230 V. Aircraft landing lights, which are commonly used in open air concerts and for stage lighting, are sealed beams that have a very narrow beam spread.
A PAR64 sealed beam typically comes in 250 W, 500 W, or 1000 watt.
Modern sealed beams have an additional envelope around the filament, whereas older types do not. The inner envelope contains halogen to improve the life of the filament and enable more light for the same power; for this to work, the halogen must be confined to the area around the filament by the second envelope, commonly made of fused quartz to withstand the filament's heat. These "halogen sealed beams" appeared on U.S. cars in 1978 to enable halogen technology under Federal Motor Vehicle Safety Standard 108, which at the time required sealed beam headlamps; they continued even after FMVSS 108 was amended to permit composite headlamps in 1983, and came to dominate the sealed beam lamp market.