Relative Size Of Micronics Particles
A formally abolished metric unit, = 10-6 meter. Symbol, µ. The name and symbol were adopted in 1879 by the CIPM (Comité International des Poids et Mesures / International Committee for Weight & Measures) and again in Resolution 7 of the 9th CGPM – 1948 (Conférence Générale des Poids et Mesures / General Conference on Weights and Measures). In 1967 the 13th CGPM abolished the micron (Resolution 7). The approved SI (Système International d’Unités / International Syatems of Units) 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. 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 Like the micron itself, the millimicron is obsolete.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 — Obsolete terms for micrometer, µm (10-6 m).
micron (µ) 
a metric unit of distance equal to one millionth of a meter. “Micron” is simply a shorter name for the micrometer. In 1968 the CGPM (Conférence Générale des Poids et Mesures) decided to drop the micron as an approved unit and recommend that micrometers be used instead. Microns, however, are still in common use.
micron (µ) 
an informal unit of pressure widely used in vacuum technology. In this use, a micron is a micron of mercury, that is, 0.001 mm Hg or approximately 1.333 microbars (µbar or µb) or 133.3 millipascals (mPa). For all practical purposes, 1 micron is identical to 1 millitorr (mTorr).