Interferometers measure distance in terms of wavelength and determine the wavelengths of light sources. They use two or more flat mirrors to split off or pick off beams from a light source such as a laser, lamp, or star. The beams are then combined to interfere with one another. The result is the discovery of alternating bands or fringes of light and dark. These fringes are bright where the beams are added together and dark where they cancel each other out. The interferometer optical configuration helps to determine the location of the fringes. Fizeau interferometers or image plane interferometers use a reference and test light-path of unequal distance. Fabrey-Perot interferometers are frequency-tuning devices that use the properties of interference between two adjacent, flat and parallel surfaces. Michelson interferometers have a half-silvered mirror at a 45° angle to the incoming beam. Half the light is reflected perpendicularly and bounces off a beamsplitter. The other half passes through and is reflected from a second beamsplitter. Twyman-Green interferometers are similar to Michelson interferometers in terms of the beam splitter and mirror arrangement, but different in terms of illumination. A Michelson interferometer uses an extended light source. A Twyman-Green interferometer uses a monochromatic point source. The light passing through the mirror must also pass through an inclined compensator plate to compensate for the fact that the other ray passes through the mirror glass three times instead of one.
Specifications for interferometers include resolution, zoom, aperture size, light source, and laser wavelength. Resolution measures minimum detectable dimensional variations, flatness variations, position, and surface roughness. The zoom range of the interferometer is usually specified as 2x, 4x, or 7x. Aperture size is the opening in the interferometer through which light passes. Aperture size can be fixed or adjustable and can alter the focal point or working distance or collimation. There are three choices for laser type: carbon dioxide (CO2), helium neon (HeNe), and helium xenon (HeXe). CO2 lasers use a mixture of carbon dioxide, nitrogen (N2) and helium (He) to produce a continuous output of laser light at a wavelength of 10.6 micrometers. With HeNe lasers, the active medium is a mixture of helium and neon. Typically, interferometers that use HeNe lasers are used in alignment, recording, printing, and measurement applications. HeXe lasers use a mixture of helium and xenon. Interferometers also differ in terms of number of axes (single or multiple), orientation (vertical or horizontal), and special features. Some interferometers can be operated by remote control or have continuous zoom capabilities. Others have a high-resolution camera for image capture applications. Interferometers that can be controlled or monitored with a computer interface are also available.