Moire contouring method by parallel light is realized using holography. In Fourier-transform holography, the object placed at the focus of a collimating lens is equivalent to the one placed at infinity. Regular fringes are projected onto the object by tilting the illuminating parallel light slightly between the exposures of double exposure holography. These fringes are deformed by the relief of the object. Imaginary hyperboloids of interference fringes are formed by fine lateral displacement between the two exposures. As the plate is located at the focus of the lens, it is considered that the hyperboloids can be approximated by a set of parallel planes after passing through the lens. These parallel fringes are observed at infinity as a regular grid. The combination of this regular grid with the deformed fringes produces contouring fringes on the object at infinity.

A method is proposed for contouring of diffused objects using digital holographic moire interferometry in lensless Fourier transform configuration. Fringe projection moire technique combined with digital double-exposure holography produces the contours in this method. Two digital holograms of a 10 mm aluminum alloy cube are recorded by tilting the illumination angle slightly between exposures, and a third one is recorded by translating the detector a little laterally with the final illumination angle unchanged. Upon numerical processing of the first two holograms, a plane parallel fringe system seems to be projected onto the object. This fringe system can be referred to as the modified grid. Processing of the second and the third hologram results in another grid, the reference grid. In effect, processing of the first and the third hologram combines the modified and the reference grids to produce the moire contour fringes. The range of contour intervals obtained remains between 2.73 and 0.38 mm with seven different contours in between. The present method can measure details of a great variety of sizes on objects of large dimensional range. Deviations in the measured contour intervals from the theoretically calculated values are found to be within 12%–18%. This seems to be because of the deviation in the present experimental geometry from the ideal theoretical configuration, the hologram digitization, and the particular reconstruction algorithm used in the present experimental arrangement.

Contouring Method by Parallel Light Moire Holography

Citations

Contouring of diffused objects using lensless Fourier transform digital moiré holography

3. Sheeding light on the subject

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