Abstract
A fine reference grating and coarse specimen grating in series are illuminated so that two dominant diffraction sequences take symmetrical paths in the intergrating space. In this optimum configuration, observation is on or near the axis of symmetry. Moiré sensitivity is that of the reference grating and independent of the specimen grating. The contour map described by moiré fringes depicts in-plane displacements uniquely, being insensitive to thickness and taper of gap between gratings. Moiré fringes are nearly perfect two-beam interference fringes of superb contrast. Existing gratings blazed for single-order dominance can be used under conditions that produce the required double-order dominance. In experimental evaluations, coarse gratings of 200 rulings/cm were mated with various reference gratings up to 12,000 rulings/cm for fringe multiplications by factors up to 60.
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