Micrometre

The micrometre (International spelling as used by the International Bureau of Weights and Measures;^[1] SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, 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 inch).^[1]

The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria,^[1] and for grading wool by the diameter of the fibres.^[2] The width of a single human hair ranges from approximately 10 to 200 μm. The first and longest human chromosome is approximately 10μm in length.

Examples

Between 1 μm and 10 μm:

• 1–10 μm – length of a typical bacterium
• 10 μm- Size of fungal hyphae
• 5 μm – length of a typical human spermatozoon's head ^[3]
• 3–8 μm – width of strand of spider web silk^[4]
• about 10 μm – size of a fog, mist or cloud water droplet

Between 10 μm and 100 μm

• 10 to 55 μm – width of wool fibre^[5]
• 17 to 181 μm – diameter of human hair ^[6]
• 70 to 180 μm – thickness of paper

SI standardization

The term micron and the symbol μ were officially accepted for use in isolation to denote the micrometre in 1879, but officially revoked by the International System of Units (SI) in 1967.^[7] This became necessary because the older usage was incompatible with the official adoption of the unit prefix micro-, denoted μ, during the creation of the SI in 1960. In the SI, the systematic name micrometre became the official name of the unit, and μm became the official unit symbol.

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. In spoken English, they may be distinguished by pronunciation, as the name of the measuring device is invariably stressed on the second syllable, whereas the systematic pronunciation of the unit name, in accordance with the convention for pronouncing SI units in English, places the stress on the first syllable.

The plural of micron is normally "microns", though "micra" was occasionally used before 1950.^[8][9][10]

Symbol

The official symbol for the SI prefix micro- is a Greek lowercase mu (μ).^[11] In Unicode, there is also a micro sign with the codepoint U+00B5 (µ), distinct from the codepoint U+03BC (μ) of the Greek letter lowercase mu, so that machines can recognize it as the SI prefix symbol rather than as a letter. According to The Unicode Consortium, the Greek letter character is preferred,^[12] but implementations must recognize the micro sign as well. Most fonts use the same glyph for the two characters.

See also

Look up micrometre in Wiktionary, the free dictionary.

• Metric prefix
• Metric system
• Orders of magnitude (length)
• Wool measurement

Notes and references