Micrometre

A micrometre or micrometer (pron.: /ˈmaɪkroʊmiːtər/, symbol µm) is an SI derived unit of length equaling 1�-10⁻⁶ of a metre (SI standard prefix "micro-" = 10⁻⁶); that is, one-millionth of a metre (or one-thousandth of a millimetre, 0.001 mm, or about 0.000039 inches). The symbol µm is sometimes rendered as um if the symbol µ cannot be used.

The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of cells and bacteria.^[1]

The symbol for the SI prefix micro-, µ, is a Greek lowercase mu, μ. In Unicode, it has a codepoint distinct from that of the Greek letter lowercase mu so that it can be recognized as a symbol and not a letter. Many fonts use the same glyph for the two characters.

ISI Standardization

The term micron and the symbol µ, representing the micrometre, were officially accepted between 1879 and 1967, but officially revoked by the ISI in 1967.^[2]

Nevertheless, in practice, "micron" remains a widely used term in preference to "micrometre" in many English-speaking countries, both in academic science (including geology, biology, physics, and astronomy) and in applied science and industry (including machining, the semiconductor industry, and plastics manufacturing).

Additionally in American English the use of "micron" helps differentiate the unit from the micrometer, a measuring device, because the unit's name in mainstream American spelling is a homograph of the device's name. The term "micron" is still extensively used in most English-speaking countries.

In engineering it is common to use the micron as a replacement for the traditional thou or mil, each of which represent a thousandth of an inch. 1 thou = 1 mil = 25.4 microns. So a trash bag (in British English: a rubbish bag) may be originally specified as 0.35 mil thick, but stated as 8.89 microns.

See also

• Metric system
• Si prefix
• Orders of magnitude
• Wool measurement

References