Interferometry is an optical technique used to measure sub-micron displacements by superimposing two coherent light beams. When two waves superpose, constructive and destructive interference produces a pattern of light and dark fringes. If the two wave fronts are in phase at their point of superposition, a bright fringe results. Antiphase results in a dark fringe. Points in between the brightest and darkest fringes represent different states of phase/antiphase.
Interferometry can be used to compare the form of two surfaces - a reference flat and a test surface. If the test surface is displaced or has defects, then the relative phase of the beams changes. As a result, the observed interference fringes change position as the test surface is scanned.
These fringes can be used to indicate relative differences or changes in the path length that the two beams have travelled. The exact value of the difference can be calculated from the wavelength of the light used. The separation between a dark and light fringe represents a path length difference that is ½ of the wavelength of the light present.
In practice, light from a single source is split into two beams that travel along different optical paths. One beam, the test beam, is reflected from a sample. It is then superposed with the second beam (the reference beam) to generate interference. The difference in phase between the reflected wave-fronts of the test beam and the reference beam is seen as a number of light and dark fringes, each one of which represents a difference of ½ the wavelength of the light.
Interferometry is typically applied in the manufacturing industry to measure surface defects, roughness, vibrations, or small displacements caused by external forces (stress, strain, weight).