Multi-reflector 16 (MR16) halogen bulbs are a standard size of light bulbs that contain halogen and have both residential and commercial applications. Originally developed for slide projectors, they project light forward in a flood or spotlight. They are manufactured by many companies in both low-voltage and line-voltage grades.
The designation MR16 refers to the size and shape of the halogen bulb. MR is an abbreviation for the multifaceted reflector that gives the bulb its characteristic cone shape and reflects light out of the front of the bulb as a spot or flood. The numeral 16 is the diameter of the bulb in eighth-inch (3.175 mm) units, indicating that the bulb is 2 inches (50.8 mm) across. In addition to MR16 halogen bulbs, smaller MR11 and MR8 bulbs exist for more specialized applications.
Like other halogen bulbs, MR16 halogen bulbs use a tungsten filament in a compartment that is filled with pressurized halogen gas. Tungsten is the same material used in the filament of standard incandescent bulbs because of the way it glows when an electric current passes through it. In a standard bulb, tungsten evaporates away from the filament until it becomes too thin to sustain a current. In a halogen bulb, the halogen gas enables the tungsten to burn at a higher, brighter temperature while slowing the evaporation so that the filament lasts longer than in an incandescent bulb. Low-voltage MR16 bulbs might be expected to last for three to six years of continuous operation.
MR16 halogen bulbs are not interchangeable with incandescent bulbs, because they require different current needs and use a socket that is different from those used by incandescent floodlights and spotlights. Low-voltage MR16s are generally 12 or 24 volt, 10 or 20 watt, and require a low-voltage transformer to step down standard household current. They generally use a two-pin GU5.3 base. Line-voltage MR16s can accept 120 volt current and generally use a twist-and-lock base.
One of the primary advantages of halogen bulbs is the purity and intensity of the light that they produce. They also have a longer lifespan than incandescent bulbs. Low-voltage MR16 halogen bulbs generally are slightly more energy efficient than corresponding incandescent spotlights.
The primary disadvantages of halogen bulbs are the safety risks when they are improperly used. Halogen bulbs can suffer what is euphemistically referred to as "non-passive failure" when the pressurized halogen compartment shatters. For this reason, every MR16 halogen bulb must either have a heavy glass cover over the filament compartment or must be used in an enclosed fixture. Secondly, as with incandescent bulbs, 90 percent of the energy used by halogen bulbs is given off as heat instead of light. MR16s are commonly used in recessed lighting, so using too hot of a bulb creates a fire risk.
Light-emitting diode (LED) replacements for MR16 halogen bulbs have begun being manufactured. Although they are more expensive than the bulbs they replace, LED bulbs do not generate as much heat. They are therefore more energy-efficient and safer. LED replacements for MR16 do carry a risk of being unable to meet the minimum energy load for their transformers and might not work or might flicker or strobe. Some testing has suggested that LED replacements might not meet manufacturer claims or generate as much light as their halogen counterparts.