Digital Speckle Pattern Interferometry(DSPI)provides an effective means of full-field and noncontact measurement of deformation or displacement. With the advancement of the aerospace and automotive industry,deformation measurements with a large Field Of View(FOV),high resolution,and wide measurement range are becoming more and more urgent. However,it is difficult to increase the FOV for a given size of CCD without compromising the lateral resolution of the deformation measurement. To solve this problem,a technique for stitching the phases of multiple sub-images to enlarge the FOV without impairing the lateral resolution was investigated. The existing aperture synthesis methods usually obtained multi-images by moving CCD or object. They are only applicable to the measurement and observation of stationary objects. For deformation measurements,at least two surface states of the object are involved, corresponding to before and after deformation. Thus,the positioning errors and axial misalignment between corresponding hologram pairs are difficult to estimate. To overcome the disadvantages of multi-step image acquisition schemes. An experimental setup with multiple CCDs was constructed to obtain multiple subimages. The phase of each CCD was extracted by the Fourier-transform method,and then the unwrapped phase maps of the overlapping areas were used to estimate the relative positions. Subsequently,the phase deviations between adjacent sub-image pairs were estimated and compensated for correct phase stitching. In order to obtain the largest possible FOV using as few CCDs as possible,the effect of the size of the overlap area on the stitching results was analyzed. The relationship between the standard deviation and the size of the overlapping area was investigated. The standard deviation is less than 0.015 mu m when the size of the overlap area is between 141 and 461 pixels,corresponding to a percentage of the overlap area between 8.8% and 28.8%. Therefore,the size of the overlap area is approximately 10%,which may be appropriate in terms of the trade-off between FOV and accuracy. With the proposed method,the FOV was expanded from 5.5 cmx4 cm to 10 cmx4 cm and only two CCDs were used. The maximum relative error before and after stitching of the overlapping area was less than 1%,which illustrates the effectiveness of the proposed method. In addition,to further demonstrate the effectiveness of the phase stitching method,a calibrated loading device(the loading range is 0-9 mu m,the expanded measurement uncertainty is 0.2 mu m with the coverage factor k= 2) is driven by a piezoelectric actuator was used. A total of 9 displacement loading points were included,and three groups of values were measured by CCD# 1,CCD#2,and the phase stitching method. The Least-Square(LS) method was used to fit the measured deformation of the three groups and the fitting residuals were evaluated. Additionally,the coefficient of determination R and the Root Mean Square Error (RMSE) of the quality of the fitting were compared. The measurement accuracy of the phase stitching method was equivalent to that of the single-camera method when comparing the measurements of the calibration points by the Root Mean Square Error(RMSE)metric. In summary, the proposed phase stitching method based on multi-CCDs deformation measurement is an effective means to increase the FOV without impairing the lateral resolution. At the same time,with a certain FOV,the measurement range and axial resolution can increase. Theoretically,for the deformation distribution similar to the cantilever beam,the measurement range can increase with the increment of FOV.
