A formally abolished metric unit, = 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 Like the micron itself, the millimicron is obsolete.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).
1.
BIPM
Process-Verbeaux Comité International des Poids et Mesures.
Page 41.
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