Experimental verification of improvement of phase shifting moiré interferometry using wavelet-based image processing

Phase shifting interferometry is combined with wavelet-based image processing techniques to extract precise phase information for applications of moiré interferometry. Specifically, a diffraction grating identical to the specimen grating is used to introduce the additional phase shifts needed to implement phase shifting moiré interferometry. The phase map is calculated with the four-step phase shifting algorithm with 90-deg relative shifts between adjacent frames. Subsequently, continuous wavelet transform processing is applied to the derived phase map to remove the noise and extract precise phase information. This precise phase map is then used to calculate the in-plane strain of the specimen under loading. A curve-smoothing algorithm based on a discrete wavelet transform is applied to the resultant in-plane strain map to eliminate the noise generated through the derivative calculation. To demonstrate the usefulness of this technique, uniform vertical loading is applied to a three-layer ball grid array (BGA) package and the strain is experimentally obtained. The reconstructed phase and in-plane strains are compared to the simulation results obtained using Ansys®.