What is the unit called a micron?

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micron

A metric unit of length = 10⁻⁶ meter, abolished by the CGPM in 1967 but in continued use. Symbol, μ. The name and symbol were adopted in 1879 by the CIPM¹ and again in Resolution 7 of the 9th CGPM (1948).² In 1967 the 13th CGPM abolished the micron (Resolution 7)³, partly because its symbol had been transferred to the new “micro-” prefix. The approved SI term for the micron's length is micrometer (symbol, μm).

Nonetheless, “micron” is still the term most commonly used in certain fields, including semiconductor fabrication. It is often used in describing the sizes of particles retained by air and water filters, the range of wavelengths of light to which an optical instrument responds, and in machining. One application in which its use is currently specified by national standards is in describing the fineness of wool and other textile staples (see www.ymccoll.com/micron_reports.html).

The micron was often encountered in the millimicron = 10⁻⁹ meter, or ¹⁄₁₀₀₀th of a micron. Symbol, mμ. Like the micron itself, the millimicron is obsolete. The current SI unit having the same value is the nanometer (symbol, nm).

1. Bureau international des poids et mesures.
Procès-Verbaux comité International des Poids et Mesures. 1879

2. Comptes rendus des séances de la 9e Conférence générale des poids et mesures, réunie à Paris en 1948.
Paris: Gauthier-Villars, 1949.

3. Conférence générale des poids et mesures (13e. 1967-1968. Paris).
Comptes rendus des séances de la 13e Conférence générale des poids et mesures, Paris, 10-16 octobre 1967, 15 octobre 1968.
Sèvres: Bureau international des poids et mesures, 1969. 105.

sources

The micron is most commonly used for infra-red wavelengths greater than 1 μ but for shorter wavelengths either of the other units is commonly used.

A. B. Calder.
Photometric Methods of Analysis.
New York: American Elsevier Publishing Co., Inc., 1969.
Pages 1-2.

From the official report of M. Tresca it appears that this meter is 118.9 mikrons longer than the Metre des Archives at 13°.7 Centigrade, a mikron being equal to 1/25400 inch, or ordinarily, with sufficient exactness, 1/25000 inch.

William A. Rogers, in
George M. Bond, editor.
Standards of Length and Their Practical Application.
Hartford, CT: Pratt and Whitney, 1887.

Page 4. Rogers was an Assistant Professor at Harvard University, an expert in metrology whom Pratt and Whitney hired to trace the length standards they used in making gauges to the American, British and metric prototypes. The above volume, which has been digitized by Google, is a record of that work.