Absolute flatness measurement of plane and self-calibration of an interferometer: 2 Surfaces and 4 positions by lateral shifts

The paper describes a new absolute flatness measurement method by an interferometer. The method uses two surfaces and four interferograms obtained at different positions of a specimen. The principle of the proposed method is based on the autonomous calculation of inclination angles which occur with the lateral displacements of the measured specimen. The total systematic error of each CCD-pixel which includes wavefront error due to the reference mirror and optical path distortion in the interferometer is also calibrated by this method. The influence of the accidental error of the sampled data with CCD-pixels which remains as the only cause of error of the proposed absolute measurement method is estimated by the error propagation rule. The uncertainty amount of the obtained absolute flatness is reduced to about a double of that of the sampled data of each pixel due to the accidental error. The method is available in the aperture expansion by connecting stepwise shifts and measurement of a specimen with an area larger than the aperture of the interferometer.