A micrometer, or one millionth of a meter, is a unit of length commonly used to measure objects like cells or the wavelength of infrared radiation. The abbreviation for it is µm, and the unit is also often referred to as a micron or micrometre. The limits of visual acuity for the human eye is often cited as objects 50 micrometers in diameter, about the size of a dust speck. The average width of a human hair is 80 micrometers.
The building blocks that make up every living thing, the cell, have sizes in the micrometer range. A typical bacterial cell has a width of 1-10 micrometers, while a red blood cell has a diameter between 6-8 micrometers. A strand of spiderweb has an average width of 4-5 micrometers. Typical eukaryotic (non-bacterial) cells have a diameter of 7 micrometers.
Light with wavelengths in the micrometer range falls into the infrared portion of the spectrum. Infrared light carries heat energy, and is released by the Sun in all directions in large quantities. This frequency is invisible to most animals, the pit viper, rattlesnakes, vampire bat, jewel beetles, and certain darkly pigmented butterflies being important exceptions. The infrared portion of the spectrum contains wavelengths between 750 nanometers and 1 millimeter, spanning five orders of magnitude. Human beings radiate their own warmth at a wavelength of about 10 micrometers, which can be seen using night vision goggles.
As of 2008, the smallest electric engine ever built was just 1/5th of a micrometer on a side. In the field of microfluidics, channels are regularly built with dimensions in this range.