Precise method of determining systematic errors in phase-shifting interferometry on Fizeau interferences.

Fizeau phase-stepping interferometry permits multiple-beam interferences to be evaluated exactly with a resolution in the subnanometer range when a special four-bucket algorithm is applied. The step width, which is set at 2π/4 for the four-bucket algorithm, has to be maintained precisely to avoid erroneousphase determination. A method is described by which deviations from the exact value of the step width can be quantitatively determined. In numerical calculations the phase dependence of the step-width error is determined and confirmed by accompanying experiments. Step-width errors recognized by the phase dependence of a difference measurement can be corrected by means of a general four-bucket algorithm. The topography of Fabry-Perot plates can therefore be measured with a high degree of accuracy, of λ/200.