Fig. 2 CAD model of the OH cage system. C is the collimator, Ȝ/2 is the Ȝ/2 wave plate, SFA is the spatial filter assembly which contains OBJ, the objective, PH, the pinhole and C, PBS is the polarizing beamsplitter cube, Ȝ/4 is the Ȝ/4 wave plate, M is the mirror, PZT is the piezoelectric transducer, P is the polarizer, and CCD is the CCD camera. 2.2 Shape measurement with the two-wavelength contouring technique In order to determine the shape of the object, a dual-wavelength holographic contouring technique [5] is applied by generating depth contours related with geometry of the object. This requires acquisitions of a set of amplitude and phase information at wavelength 1O, considered as the reference or undeformed state, as well as acquisitions of a second set of amplitude and phase information at wavelength 2O , considered as the modified or deformed state. The phase difference [6] of the two sets of data, I ' , is given by 2 1 2 OPL S I I I ' / (1) where 1I is the phase of the optical path length recorded at the first wavelength 1O, 2I is the phase of the optical path length recorded at the second wavelength 2O , and OPL is the optical path difference defined as the distance from an illumination point to an object point and to an observation point. / is the synthetic wavelength given by 1 2 1 2 OO O O / (2) The phase difference [7] related to depth contours I ' can also be represented by the fringe-locus function:, 202
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