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Fringe Spacing
The fringes on an interferogram form a contour map of the wavefront. This wavefront typically has a few fringes of tip and/or tilt.
For a single-pass interferometer, the fringe spacing is one wave on the wavefront, which corresponds to ½ wave surface error on a mirror.
For a double-pass auto collimation interferometer, the fringe spacing is ½ wave on the wavefront. The corresponding surface height error is ¼ wave per fringe.
Notes: