Micrometre

The micrometre (International spelling as used by the International Bureau of Weights and Measures;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). 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. 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. 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.

Micron (journal)

Micron is a monthly peer-reviewed scientific journal in the field of microscopy. It was established in 1969 and is published by Elsevier.

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Wool measurement

A micron (micrometre) is the measurement used to express the diameter of a wool fibre. The lower microns are the finer fibres. Fibre diameter is the most important characteristic of wool in determining its greasy value.

Every fleece comprises a very wide range of fibre diameters—for example a typical Merino fleece will contain fibres of as low as 10 microns in diameter, and there could be fibres with diameters exceeding 25 microns, depending on the age and health (or nutrition) of the sheep. What is usually referred to as wool's "micron" is the mean of the fibre diameters or average diameter. This may be measured in a number of different ways.

Small samples can be taken from the side or fleece of a sheep and measured using a portable instrument such as an OFDA2000 (Optical Fibre Diameter Analyser); or a mobile instrument system called a Fleecescan. Both these systems have been studied extensively and used correctly should give reasonably reliable results. Pre wool classing micron test results are a useful guide for classers in determining lines of wool to be made. Samples of fleece can also be shorn from the animal and sent to a laboratory for measurement ("midside sampling"). Most fleece-testing laboratories nowadays use related instruments to those mentioned—either the OFDA models or the Laserscan. Merino stud rams are mid-side sampled and the test results are displayed in the sale catalogues.