Editing Michelson interferometer (section)

===Twyman–Green interferometer===
[[File:Twyman-Green interferometer.png|thumb|300px|right|Figure 6. Twyman–Green interferometer.]]
The [[Twyman–Green interferometer]] is a variation of the Michelson interferometer used to test small optical components, invented and patented by Twyman and Green in 1916. The basic characteristics distinguishing it from the Michelson configuration are the use of a monochromatic point light source and a collimator. Michelson (1918) criticized the Twyman–Green configuration as being unsuitable for the testing of large optical components, since the available light sources had limited [[coherence length]]. Michelson pointed out that constraints on geometry forced by the limited coherence length required the use of a reference mirror of equal size to the test mirror, making the Twyman–Green impractical for many purposes.<ref name=Michelson1918>{{Cite journal
| doi = 10.1073/pnas.4.7.210
| last1 = Michelson | first1 = A. A.
| title = On the Correction of Optical Surfaces
| journal = Proceedings of the National Academy of Sciences of the United States of America
| volume = 4
| issue = 7
| pages = 210–212
| year = 1918
| pmid = 16576300
| pmc = 1091444
|bibcode = 1918PNAS....4..210M | doi-access = free }}</ref> Decades later, the advent of laser light sources answered Michelson's objections.

The use of a figured reference mirror in one arm allows the Twyman–Green interferometer to be used for testing various forms of optical component, such as lenses or telescope mirrors.<ref name=Malacara2006_2>{{Cite book | last1 = Malacara | first1 = D. | chapter = Twyman–Green Interferometer | doi = 10.1002/9780470135976.ch2 | title = Optical Shop Testing | pages = 46–96 | year = 2007 | isbn = 9780470135976 }}</ref> Fig.&nbsp;6 illustrates a Twyman–Green interferometer set up to test a lens. A point source of monochromatic light is expanded by a diverging lens (not shown), then is collimated into a parallel beam. A convex spherical mirror is positioned so that its center of curvature coincides with the focus of the lens being tested. The emergent beam is recorded by an imaging system for analysis.<ref name=OPITwyman>{{cite web|title=Interferential Devices – Twyman–Green Interferometer|url=http://www.optique-ingenieur.org/en/courses/OPI_ang_M02_C05/co/Contenu_31.html|publisher=OPI – Optique pour l'Ingénieur|access-date=4 April 2012}}</ref>