micrometre

The micrometre, also spelled micrometre in British English and abbreviated as μm, is a unit of length in the metric system equal to one millionth of a metre (10^-6 m). The micro- prefix comes from the Greek mikros, meaning small. In practice, the micrometre is used to express sizes that are too small for millimetres but easily measured in lab and manufacturing contexts. The official SI unit is the micrometre, while the term micron is commonly used informally as a synonym, though it is not an SI unit.

One micrometre equals 0.001 millimetres or 1000 nanometres. It is a convenient scale for describing microscopic structures and optical wavelengths. For example, bacteria typically range from about 0.5 to 5 μm in length, red blood cells are around 6–8 μm in diameter, and human hair can be tens to hundreds of micrometres thick. Optical wavelengths in the visible spectrum are roughly 0.38 to 0.75 μm, while infrared wavelengths extend from about 1 μm to tens of micrometres.

Measurement and instrumentation commonly involve the micrometre. Precision tools such as micrometers (micropitch screw gauges) are used to measure small mechanical parts with resolutions often in the micrometre range (and sometimes fractions thereof). In scientific imaging and microscopy, features and structures are frequently specified in micrometres.

The micrometre is a standard unit in physics, chemistry, biology, and engineering for describing small scales. Its ubiquity reflects the typical sizes of cells, fibers, and microfabricated components encountered in research and industry.