Interferometer

An instrument that employs the interference of lightwaves to measure the accuracy of optical surfaces.
A beam of light or another type of electromagnetic radiation is splitted into two equal halves using what’s called a beam-splitter, a piece of glass whose surface is very thinly coated with silver, half the light passes straight through and half of it reflects back—so the beam-splitter is like a cross between an ordinary piece of glass and a mirror. One of the beams known as the reference beam shines onto a mirror and from there to a screen or camera. The other beam shines at or through something you want to measure, onto a second mirror, back through the beam splitter, and onto the same screen or camera. This second beam travels an extra distance or in some other different way to the first beam, so it gets slightly out of phase.
When the two light beams meet up at the screen or camera, they overlap and interfere, and the phase difference between them creates a pattern of light and dark areas (a set of interference fringes). The light areas are places where the two beams have added together (constructively) and become brighter; the dark areas are places where the beams have subtracted from one another (destructively). The exact pattern of interference depends on the different way or the extra distance that one of the beams has traveled.
Interferometers are used extensively for testing optical elements during manufacture. Typical designs include the Michelson, Twyman-Green and Fizeau interferometers.