Sealed beam light bulbs are a special kind of PAR (parabolic aluminized reflector) bulb specifically intended for outdoor use. The "sealed" part of the name refers to the fact that the bulb, reflector, backing, and glass are sealed together in a single package to keep weather out of the more delicate electronic parts of the bulb. "Beam" means the reflector focuses the bulb's light in one particular direction, producing powerful but narrowly focused illumination. The one kind of sealed beam light bulb everyone is familiar with is automobile headlights and fog lights. Larger sealed beam bulbs are often used to light theater stages, sports arenas, and concerts. Aircraft landing lights and lighthouse lamps also use sealed beam bulbs. On buildings, sealed beam bulbs can be used to provide spot illumination, either to highlight architectural details or for security and safety purposes.
The size of a sealed beam bulb is usually given as a PAR number. For a round bulb, the PAR number refers to how wide the lamp is in eighths of an inch, so for example a PAR36 lamp is 36 eighths, or 4.5", wide. Some sealed beam lamps also come in rectangular shapes. These are described by giving their length and width in millimeters. Sealed beam bulbs are also rated according to their wattage, ranging from 2-10 (for flashlights, hand lanterns, and building emergency lights) to 300 or more (for airport lights, floodlights, and spotlights). While sealed beam bulbs are intended to stand up to heavy use and outdoor conditions, if they do fail they can be more expensive to replace. Since the various components are sealed into a single unit, it is usually necessary to replace the entire unit -- lens, reflector, and all -- instead of just the bulb itself.