Publisher Summary Accurate measurement of length is a very critical application of interferometry. This essentially involves the determination of the mean interference order over a specified area of the interference pattern. Such length measurements are usually carried out on material standards of two types: line standards—consisting of a set of fine lines engraved on a polished surface—and end standards—consisting of a bar with polished, flat, parallel ends. Measurements on line standards, such as scales, are carried out with an interference comparator. The scale to be calibrated is mounted on a movable carriage that also carries one of the mirrors of a Michelson interferometer. Measurements of end standards are commonly made with a Kösters interferometer. This is basically a Michelson interferometer that uses collimated light and incorporates a dispersing prism to select a single spectral line from the source. Another method involves the use of zero-order white-light fringe to judge the equality of two optical paths in the interferometer; this criterion can be used when comparing two end standards of nearly the same length. Another method is optical multiplication. If two Fabry-Perot interferometers are placed in series, the spacings of the plates in the two interferometers can be adjusted using fringes of superposition, so that they are in an integral ratio. This technique makes it possible to compare a long optical path with a shorter known optical path.