but uses the MR form-factor are available. For example, LED-based MR16 lamps appear similar to halogen lamps and can be used in most fixtures designed for MR lamps. The same is true of MR11-compatible LED lamps. Fixtures designed for halogen MR16 or MR11 lamps that use electronic transformers may need to be retrofitted with LED-compatible transformers. There is a wide variety of designs, varying significantly with regard to beam width, light colour, efficiency and luminous power. Unlike halogen MRs, LED-lamps often do not have the multifaceted reflectors that give MRs their precise beam width control. Some rely on the optics of the LED(s) to control the beam width. Some designs may have simple cut-off apertures that limit beam width, or even individual reflectors or lenses for each LED. As with other LED lamps available today, the quality and color temperature of the white light produced by such lamps varies. Many tend towards the blue end of the spectrum, being even "cooler"-coloured than fluorescent lighting. Because of this variability, some MR16 and MR11 compatible LED lamps will create significantly more natural looking light than others. The least efficient of these lamps produce about 26 lumens per watt (lm/W), which is similar to the efficiency of halogen MRs. The most efficient of these lamps available today produce about 160 lm/W, which exceeds the efficiency of compact fluorescent lamps. In terms of total luminous power, such lamps range from being significantly less powerful than their halogen counterparts, to being comparable to the lower power halogen MR16s. The brightest available halogen MR16s are still slightly brighter than the brightest available LED versions. MR-compatible compact fluorescent lamps are also available. Advantages MR lamps offer several advantages over other lamps with equivalent power ratings. They are typically smaller (transformers excepted), provide better beam control and offer a whiter light than ordinary incandescent lamps. The small size of the lamp allows designers more flexibility in placing the lamps and with the option of various beam widths, the light beam can be very specifically placed. White LED MR lamps can be manufactured with different colour temperatures, normally ranging from warm whites, attempting to match the colour of halogens, all the way through to cool whites at 5500 K or more. Another advantage is when used in 12-volt track lighting systems the bare wires/connectors can be touched without fear of electrocution. Disadvantages MR lamps have several disadvantages over other