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For the micrometer as a measuring instrument, see Micrometer (device) A micrometre (American spelling: micrometer, symbol µm) is an SI unit of length equal to one millionth of a meter, or about a tenth of the size of a droplet of mist or fog. It is also commonly known as a micron, although that term is officially outdated. It can be written in the expanded mathmatical notation (1×10−6 m) The symbol µ (Unicode character U+00B5; HTML µ) is the "micro sign", which should look identical to the Greek letter mu (μ) (the two may or may not look the same, depending on the font). The symbol "um" is sometimes used, when the µ and μ are not available, for example when using a typewriter. The micrometer is a common unit of measurement for wavelengths of infrared radiation. Some people (especially in astronomy and the semiconductor business) use the old name micron and/or the solitary symbol µ (both of which were official between 1879 and 1967) to denote a micrometer. This practice persists in the face of official discouragement, perhaps to help disambiguate between the unit of measurement and the micrometer, a measuring device. nanometer << micrometer << millimeter
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