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An optical interferometer is an interferometer for visible light, and/or nearby portions of the neighboring bands, infrared and ultraviolet. Use of them is called optical interferometry.
Some research-telescope optical instruments incorporate optical interferometry internally, e.g., a Michelson interferometer in an imaging Fourier transform spectrometer. Gravitational-wave detectors such as LIGO also incorporate them. A grating within a spectroscope works by interference, but generally is not termed an interferometer.
Optical interferometers analogous to astronomical radio and microwave interferometers have been created, for measurements and for images with a very fine angular resolution, functioning by analyzing interference of the light entering two or more optical telescopes. Unlike radio interferometers, which create the interference between electronic signals picked up by antennas, optical interferometers create the interference optically before sensing the light. The wave frequency is too high to handle with analogous electronics. There are significant challenges in designing, building, and operating optical interferometers, which require extremely precise, adjustable optics. Otherwise they would be in more common use. Examples:
Some large research telescopes consisting of more than one identical telescope at the same site offer optical interferometry as an option:
Past examples:
A particular type of optical interferometer, an intensity interferometer, operates by different principles, not using wave interference (as do those listed above). In discussions of both types, the term amplitude interferometer is sometimes used for those using wave interference.