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Excerpt from Field Guide to Interferometric Optical Testing
The Twyman-Green interferometer was invented and patented in 1916 and was originally intended for testing prisms and microscope objectives. The invention of the laser increased the utility of the Twyman-Green. Its applicability has grown ever since, and although the laser-based Fizeau is probably the most commonly used testing interferometer, the Twyman-Green is useful for introducing important concepts in interferometers.
The light source for a Twyman-Green is a quasi-monochromatic point source that is collimated by a collimating lens into a plane wave. This plane wave is split into a reference beam and a test beam by a beamsplitter. The basic setup is for the testing of flats, in which case the reference beam reflects off of a known reference flat and returns to the beamsplitter. The test beam is incident on the unknown test part and also returns to the beamsplitter. The beams are both split a second time, creating two complementary interferograms. One is projected towards the point source, while the more useful interferogram is relayed by an imaging lens to the observation plane.