A multifaceted reflector (often abbreviated MR) light bulb is a reflector housing format for halogen as well as some LED and fluorescent lamps. MR lamps were originally designed for use in slide projectors, but see used in residential lighting and retail lighting as well. They are suited to applications that require directional lightings such as track lighting, recessed ceiling lights, desk lamps, pendant fixtures, landscape lighting, retail display lighting, and bicycle headlights. MR lamps are designated by symbols such as MR16 where the diameter is represented by numerals indicating units of eighths of an inch. Common sizes for general lighting are MR16 (16⁄8 inches, 51 mm) and MR11 (11⁄8 inches, 35 mm), with MR20 (20⁄8 inches, 64 mm) and MR8 (8⁄8 inch, 25 mm) used in specialty applications. Many run on low voltage rather than mains voltage alternating current so requiring a power supply.
Most MR lamps consist of a halogen capsule (or, bulb) integrated with a pressed glass reflector with a base conforming to the bi-pin GU5.3 standard. The compact size of the MR base allows for much smaller, more discreet fixtures than the incandescent reflector bulbs that pre-dated MRs.
The reflector controls the direction and spread of light cast from the lamp. MR lamps are available with different beam angles from narrow spotlights of as small as 7° to wide flood lamps of 60°.
Some lamps use an aluminum coating as a reflector. Others use a selective dichroic coating that reflects visible light and allows infrared radiation to pass through. This type reduces the heating of illuminated objects since less infrared radiation is present in the light beam. However, dichroic lamps must only be used in compatible fixtures that can dissipate the heat.
Dichroic lamps must not be fitted to recessed or enclosed luminaires with the IEC 60598 No Cool Beam symbol.
The brightness of MR lamps can be adjusted when used with appropriate light fixtures and dimmers. However, the color temperature changes significantly when the lamp is dimmed, shifting dramatically to the warmer end of the spectrum.
Like all halogen lamps, MR lamps produce significant heat and care must be taken to avoid contact with skin or proximity to flammable materials when the lamp is on or has been on recently.
Halogen lamps (18lm/watt typical) are more energy-efficient than regular incandescent lamps (15lm/w typical) but still fall far behind other more-recent types such as fluorescent lamps (80-100 lm/watt), gas discharge lamps (100-200 lm/watt depending on types), and LEDs (125-150lm/watt typical in bright white depending on style).
With both types of incandescent bulbs, useful life can be considerably shortened if their filaments experience mechanical shock or vibration. Using an electronic transformer with a soft start feature can considerably extend life, as it reduces the characteristically high inrush current that occurs initially when the lamp is cold. Dimming also extends life significantly.
MR lamps, like all quartz-halogen lamps, produce some undesirable ultraviolet light. Usually, this must be filtered out. Also, the quartz capsules of the lamps may rupture or explode upon the failure of the lamps. For these two reasons, some MR lamps include a cover glass that serves as an integrated ultraviolet filter and explosion shield. MR16 lamps lacking this cover require the use of a fixture that incorporates an external piece of glass specifically designed to provide both ultraviolet and physical protection.
MR lamps are available in 10–75-watt power ratings (150–800 lumens).
MR lamps most often operate at 12 volts, although they are also available in other voltages. These lamps use a bi-pin connector for power: 12-volt MR11 bulbs usually use a GU4 base, and 12-volt MR16 bulbs usually use a GU5.3 base. The common 12-volt MR16 lamps, therefore, require a ferromagnetic or electronic transformer—sometimes misnamed as a ballast—to convert the 120- or 230-volt mains voltage to the extra-low voltage required by the lamp.
Certain MR lamps can operate directly on the mains voltage. These lamps typically use a GU10 turn-and-lock base, so they cannot be accidentally interchanged with low-voltage lamps. GU10 is distinguished from the lower-voltage MR lamps by the U-shaped ceramic base mount with a 10 mm (pin center-to-center distance) 2-pin bayonet mount. MR16 lamps with an integrated transformer are also available. These lamps have screw bases to fit standard medium-base Edison sockets.
MR lamps are commonly available in a range of color temperatures, from about 2700 K to 7000 K, to satisfy various applications.