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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.
A burnt-out sealed beam, broken open to show internals. When the lamp burns out the whole assembly (reflector, lamp, lens array) must be replaced. The advantage is very good alignment, and being completely impervious to dirt, moisture, etc.
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.
In theatrical 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 Aluminum Reflector 64 (PAR64) . The PAR is a non-SI unit of measure equal to one eighth of an inch, so a PAR64 light is a light that's 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, and 120 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.
Beam spreads are designated as FL (flood) , SP (spot) , NSP (narrow spot) , and VNSP (very narrow spot) , as stamped on the back of the lamp's reflector.
A PAR64 sealed beam typically comes in 250 W, 500 W, or 1000 Watt.