Shadow Moire method for the measurement of the source position in three-dimensional shearography

Shearography is a full field optical technique for measurement of surface strain. In multi-component shearography an accurate knowledge of the location of the source positions is required to resolve the displacement derivative components in three dimensions. Shadow Moire using a linear grating is an established technique for measurement of angle of illumination. The grating pitch can be varied to change the measurement range and sensitivity, and sub-fringe processing is used to obtain the angle of illumination, with ambiguity if multiple fringes are present. Shadow Moire using a circular grating may also be used to measure angle of illumination Multiple fringe processing is used to obtain the angle of illumination, with the sensitivity and measurement range again adjustable by varying the grating pitch. As the circular and linear gratings have different measurement ranges for the same grating pitch, they can be combined to extend the measurement range. In this paper circular, vertical linear and horizontal linear gratings have been combined to provide measurement of the position of the optical source in two directions with extended range and enhanced accuracy.