Optical path length difference interferometer method - uses intensity levels sensed at three different optical path length differences, and trigonometric function

摘要

The method for determining interferometric optical path length difference includes varying the interferometric optical path length difference between a first and a second surface in three steps at one-quarter wavelength intervals. The intensity of the interferogram radiation at at least one position of the interferogram is sensed at each of the steps. The intensity sensed a each position is stored at each step. For each of the positions the intensity of the first and third steps is added to produce a d.c. spatial frequency amplitude. The intensity of the third step is subtracted from that of the first to obtain the cosinusoidal spatial frequency amplitude, and the intensity of the second step is subtracted frequency amplitude. The sinusoidal and cosinusoidal amplitudes are combined to produce a trigonometric function of the phase angle of the radiation reflected from each position of the first and second surfaces. From the trigonometric function of the phase angle an output representative of the optical path length difference at each position is generated.