The Comparison of Complex Object Geometries Using a Combination of Electronic Speckle Pattern Interferometric Difference Contouring and Holographic Illumination Elements

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Abstract
This paper demonstrates that two-wavelength speckle-pattern interferometry may be used to compare complex objects of nominally identical geometries. The principle of the technique is that the inspection object (usually of non-specular finish) is illuminated by a smooth 'master wavefront' at wavelength u 1. The u 1 interferogram formed between the speckle image of this object and an in-line reference beam is then subtracted from the corresponding pattern observed at a second wavelength u2. (u2 m u1 is typically 10 nm.) Speckle pattern correlation fringes observed in the subtracted pattern define differences between the inspection object and master wavefront of magnitude Gu1u2/(u2 m u1), where G is a function of the object geometry. Master wavefronts of complex geometry are reconstructed by holographic elements which initially record the light field reflected from a specular master component. This field is reconstructed to form the required illumination wavefront by re-illuminating the hologram with a wavefront of identical geometry to the recording reference wavefront but propagating in the opposite direction. Theoretical and practical aspects of the work are discussed and a summary of experimental results is presented.