Twyman-Green setup
The most flexible interferometer setup

A Twyman-Green interferometer is a modified Michelson interferometer. Here the beam splitter is separated from the reference surface. The advantage of this configuration is greater flexibility, because both interferometer arms can be modified independently of each other. So the intensity of the reference and test arm can be easily adapted to each other depending on different sample reflectivities in order to get maximum fringe contrast. This increases the range of applications enormously. Only a maximum fringe contrast enables a maximum resolution in depth. The reference surface can be a surface that is inexpensive and accurately producible regardless of the sample size. The adaption to the sample size is done by conventional beam shaping optics introduced to the test arm. Contrary to the beam shaping optics for Fizeau interferometers these optics do not require an expensive Fizeau surface as a final surface.

As a consequence of this flexibility, the interference patterns are not caused by the sample errors only but also by the aberrations of the additional optics in the individual interferometer arms. However, nowadays samples are no longer evaluated by visual inspection of the fringe pattern but by computer controlled analysis of the phase map causing the fringe pattern. During this analysis the aberrations of the additional optics can easily be taken into account. Finally, the software provides an objective digital measurement result.

Fizeau interferometer for wavefront and surface measurement
The most commonly used interferometer setup

The last surface of the beam shaping optics is the so called Fizeau surface. It has to have the same shape as the sample to be tested (commonly spherical or flat) and is placed concentrically into the optical path, so the individual rays intersect perpendicular to the Fizeau surface. The majority of the light passes the Fizeau surface and is reflected at the test surface. The returning light interferes with the part of the light reflected at the Fizeau surface. So the Fizeau surface acts as beam splitter as well as reference surface. So the cavity is formed by the gap between the Fizeau and the test surface containing no additional optics. That is the reason why a Fizeau interferogram commonly directly shows the deviations of the test sample from the reference surface, i.e. Fizeau surface. The quality of the Fizeau surface determines the accuracy of the Fizeau interferometer. Fizeau surfaces are commonly available with a quality of λ/10 – λ/20 PV, better on request.