A metric unit abolished in 1967 but in continued use, = 10−6 meter. Symbol, μ.  The name and symbol were adopted in 1879 by the CIPM1 and again in Resolution 7 of the 9th CGPM (1948). In 1967 the 13th CGPM abolished the micron (Resolution 7). The approved SI term for the same 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 ).

Certain authors in the mid-20th century use the plural “micra”.

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 Like the micron itself, the millimicron is obsolete.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).

Micron converter

The boxes below may be used to convert microns to thousandths of an inch, and vice versa.

1. BIPM.
Procès-Verbaux Comité International des Poids et Mesures. 1879

Page 41.



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.

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