# Micrometre

1 micrometre =
SI units
1.0000×10−6 m 1.00000 μm
US customary units (Imperial units)
3.2808×10−6 ft 39.370×10−6 in

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 symbol µm is sometimes rendered as um if the symbol µ cannot be used, or if the writer is not aware of the distinction.

The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria and is also commonly used in plastics manufacturing.[1] Micrometres are the standard for grading wool (referring to the diameter of wool fibres). Any wool finer than 25 µm can be used for garments, whilst coarser grades are used for outerwear, rugs and carpets.[2] A human hair ranges from 17 to 181 µm.

The symbol for the SI prefix micro- is a Greek lowercase mu. In Unicode, it has 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. Most fonts use the same glyph for the two characters.

## Examples

A 6 μm diameter carbon filament, compared to 50 μm diameter human hair.

Between 1 μm and 10 μm:

Between 10 μm and 100 μm

• 10 to 55 µm – width of wool fibre[5]
• 17 to 171 μm – diameter of human hair [6]
• 70 to 180 µm – thickness of paper

## SI standardisation

The term micron and the symbol µ were officially accepted for use in isolation to denote the micrometre, between 1879 and 1967, but officially revoked by the International System of Units (SI) in 1967.[7] This was necessary because that older usage became 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).[citation needed] 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 despite being homographs they are not homophones; 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.)

This older usage was pluralized either as "microns" or "micra",[8] the latter being arguably more correct,[9] although that debate became less relevant after 1967, as the micron and its plural forms have been deprecated for almost half a century.

## Notes and references

1. "micrometre". Encyclopedia Britannica Online. Retrieved 18 May 2014.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
2. "Wool Fibre". "NSW Department of Education and Communities". Retrieved 18 May 2014.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
3. Smith, D.J. (2009). "Human sperm accumulation near surfaces: a simulation study" (PDF). Journal of Fluid Mechanics. 621: 295. doi:10.1017/S0022112008004953. Retrieved 20 May 2012.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
4. Gordon Ramel. "Spider Silk". Archived from the original on 4 December 2008. Retrieved 4 December 2008. garden spider silk has a diameter of about 0.003 mm ... Dragline silk (about .00032 inch (.008 mm) in Nephila) Unknown parameter `|deadurl=` ignored (help)<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
5. IST - Innovative Sintering Technologies Ltd. "Fibreshape applications". Retrieved 4 December 2008. Histogram of cotton thickness<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
6. According to The Physics Factbook, the diameter of human hair ranges from 17 to 181 µm. Ley, Brian (1999). "Width of a Human Hair". The Physics Factbook.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
7. BIPM - Resolution 7 of the 13th CGPM (1967/68), "Abrogation of earlier decisions (micron, new candle.)"
8. "Proceedings of the Royal Society of Queensland". google.com.br.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>