Editing Interferometry (section)

===Double path versus common path{{anchor|Double path}}===
{{See also|Common-path interferometer}}
[[File:Four common path interferometers.png|750px|thumb|Figure 4. Four examples of common-path interferometers]]

A '''double-path interferometer''' is one in which the reference beam and sample beam travel along divergent paths.  Examples include the [[Michelson interferometer]], the [[Twyman–Green interferometer]], and the [[Mach–Zehnder interferometer]]. After being perturbed by interaction with the sample under test, the sample beam is recombined with the reference beam to create an interference pattern which can then be interpreted.<ref name=HariharanBasics2007/>{{rp|13–22}}

A ''[[common-path interferometer]]'' is a class of interferometer in which the reference beam and sample beam travel along the same path. Fig.&nbsp;4 illustrates the [[Sagnac interferometer]], the [[fibre optic gyroscope]], the [[point diffraction interferometer]], and the [[Shearing interferometer|lateral shearing interferometer]]. Other examples of common path interferometer include the [[Zernike phase-contrast microscope]], [[Common-path interferometer#Selected examples|Fresnel's biprism]], the [[zero-area Sagnac interferometer|zero-area Sagnac]], and the [[scatterplate interferometer]].<ref name=Malacara2006>{{Cite book | last1 = Mallick | first1 = S. | last2 = Malacara | first2 = D. | doi = 10.1002/9780470135976.ch3 | chapter = Common-Path Interferometers | title = Optical Shop Testing | pages = 97 | year = 2007 |isbn=978-0-470-13597-6}}</ref>