Micrometre
The
micrometre or
micrometer, also commonly known as a
micron, is an SI derived unit of length equaling 1×10−6 of a metre; that is, one millionth of a metre. 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 cells and bacteria and is also commonly used in plastics manufacturing. Micrometres are the standard for grading wool. Any wool finer than 25 µm can be used for garments, while coarser grades are used for outerwear, rugs and carpets. The symbol for the SI prefix
micro-, µ, is a Greek lowercase mu, μ. In Unicode, it has the codepoint U+00B5, distinct from that of the Greek letter lowercase mu so that machines can recognize it as the SI prefix symbol rather than as a letter. Many fonts use the same glyph for the two characters.