Aug 4 2014
The Twyman-Green interferometer is an instrument used to test optical components such as lenses and prisms. It was invented and patented by the English electrical engineer Frank Twyman and the English chemist Arthur Green in 1916.
The Twyman-Green interferometer is a variation of the Michelson interferometer. The Michelson interferometer has a fixed mirror, whereas the Twyman-Green has a rotatable one. Likewise, the light source is mostly an extended source in the Michelson interferometer but the Twyman–Green‘s light source is always a point-like source.
The Twyman-Green’s light source is a quasi-monochromatic point source which is collimated by a collimating lens into a plane wave. The plane wave is then split by a beamsplitter into a reference beam and a test beam.
A lens is tested by placing it in the beam path, so that only one of the interfering beams passes via the test lens. Irregularities in the lens will be revealed in the resulting interference pattern. Irregularities such as coma, spherical aberration and astigmatism are clearly revealed as specific variations in the fringe pattern.
The Twyman-Green interferometer has thus enabled the creation of optical systems with an accuracy that is within a fraction of a wavelength.
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