Fringe shift measurement system having an optical phase mask with phase-shifted first and second halves

A fringe shift measurement system for determining the shift of fringes of an interferometer, the system comprising:

• an optical phase mask configured to receive light fringes from one or more optical flats, the optical phase mask having first and second halves, each of the first and second halves having alternating patterns of opaque rings and transparent rings, with the first half pattern being phase-shifted with respect to the second half pattern, each of the first and second halves configured to alternately block the incoming light fringes or allow the incoming light fringes to pass through the optical phase mask;
• a splitter having a plurality of segments, each of the segments configured to direct incoming light fringes from the optical phase mask in a desired direction;
• a plurality of concentrators positioned to receive light fringes from the splitter segments, the concentrators further configured to focus the light fringes in a desired location;
• a plurality of detectors is configured to receive the focused light fringes from the concentrators and further configured to convert the focused light fringes into digital signals;
• and
• a control unit configured to compare and analyze the digital signals.