In-plane motion measurements with Fourier lensless holography
A holographic and moire technique for measuring inplane motions of a holographic plate attached to an object is described. This one seems easier to implement than a similar approach given by Roychoudhuri and Machorro. It is well known that when spherical and plane coherent wave fronts interfere, they give rise to a Fresnel-zonelike interference pattern. This is so when a lensless Fourier hologram of an empty pupil is recorded.If between two such exposures the holographic plate is rotated or translated inplane, the two overlapping interference patterns will produce low-frequency moire patterns. If the displacement involved is small (less than the innermost Fresnel ring), the moire pattern consists of equally spaced straight lines perpendicular to the direction of the displacement.a and the interfringe spacing is inversely proportional to the magnitude of the displacement.