Optical Shop Testing

Chapter 2.8 - Variations From The Twyman-Green Configuration

2.8.   VARIATIONS FROM THE TWYMAN–GREEN CONFIGURATION

Many variations of the Twyman–Green interferometer have been invented; interesting
among them are some small, compact interferometers designed by Van Heel and
Simons (1967) and by Basile (1979).

A carbon dioxide laser has been used as a light source of long wavelengths
(10.67m 1m) in a Twyman–Green interferometer (Munnerlyn et al., 1969, Kwon
et al., 1979, and Lewandowski et al., 1986) in order to measure unpolished or rough
surfaces.

Another interesting development is the invention of a cheap interferometer whose
defects are corrected by means of a hologram (Rogers, 1970). First, a photograph is
taken in a very imperfect and inexpensive interferometer, introducing a large tilt.
This gives rise to an exceedingly fine set of fringes invisible to the naked eye but
capable of being photographed. Then a second exposure is taken on the same
photographic plate, after introducing into the interferometer the plate to be tested.
A moire´ pattern appears on the developed plate, giving the contours of the surface
quality of the plate under test.

Another interesting holographic Twyman–Green interferometer has been
described by Chen and Breckinridge (1982). In this design, a single holographic
optical element combines the functions of a beam splitter, beam diverger, and null
compensating lens. McDonell and DeYoung (1979) designed a large aperture interferometer
using a holographic compensator.

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