Method and system for determining interferometric optical path length difference

An optical path length difference detecting interferometer system comprising:

• an interferometer including a radiation source of predetermined wavelength for producing an interferogram from radiation reflected from a first surface and radiation reflected from a second surface;
• means for varying the optical path length difference between said first and second surfaces in three steps at one quarter wavelength intervals;
• at least one detector for sensing the intensity of the incident interferogram radiation;
• means for scanning each said detector at each step to obtain from each detector a signal representative of the level of intensity at each detector;
• means for storing each intensity level derived from each detector to retain the interferogram image presented at each of said steps;
• means responsive to said storing means for determining the d.c. spatial frequency amplitude from the sum of the intensity levels derived from the first and third steps;
• for determining the cosinusoidal spatial frequency amplitude from the difference between the intensity levels derived from the first and third steps and for determining the sinusoidal spatial frequency amplitude from the difference between said d.c. spatial frequency amplitude and intensity level derived from the second step;
• means for combining said sinusoidal and cosinusoidal spatial frequency amplitudes to generate an amplitude representative of a trigonometric function of the phase angle of the radiation reflected from the first and second surfaces;
• and
• means, responsive to said amplitude representative of a trigonometric function of the phase angle, for generating an output representative of the optical path length difference between said surfaces at each position monitored by a said detector.