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/W 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 (80–100lm/W), gas discharge lamps (100–200lm/W depending on types), and LEDs (125–150lm/W 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 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). Variations 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 range of color temperatures, from about 2700 K to 7000 K, to satisfy various applications. Alternatives Retrofit lamps that generate light by a different principle