Digital Speckle Pattern Interferometry(DSPI)[
Acta Photonica Sinica, Volume. 51, Issue 4, 0412003(2022)
Phase Stitching Based Multi-CCDs Deformation Measurement in Digital Speckle Pattern Interferometry
Digital Speckle Pattern Interferometry (DSPI) provides an effective means of full-field and non-contact 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 sub-images. 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 μm 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 cm×4 cm to 10 cm×4 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 μm , the expanded measurement uncertainty is 0.2 μm 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.
0 Introduction
Digital Speckle Pattern Interferometry(DSPI)[
In the current DH aperture synthesis methods,sub-holograms are obtained by a multi-step image acquisition operation as the object or camera is scanned along the x and y-axis[
The use of multiple CCDs to cover the full field of view can overcome the disadvantages of multi-step image acquisition schemes. However,the relative positions among the CCDs need to be addressed. Using multi-CCDs to cover the full field of view can overcome the drawback of the multi-step image acquisition scheme. However,the relative positions among CCDs need to be addressed. This is usually estimated by calculating the similarity of intensity images in overlapping regions between adjacent images[
To evaluate the effectiveness of image registration of multi-CCDs DSPI system,a dual-CCDs DSPI system was constructed,the relative positions between CCDs were estimated based on the unwrapped phase diagram,the effect of the size of the overlapping area on the image registration accuracy was analyzed,and a compensation method of the phase deviation between CCDs was proposed. Finally,the registration accuracy of the stitching method is evaluated with a calibrated artifact.
1 Method
The optical setup of the two CCD DSPI system is shown in
Figure 1.Multi-CCD DSPI experimental setup
The speckle interferograms before
Figure 2.The flow chart of proposed method of phase stitching
After the unwrapped phase maps are obtained,shown as
where the overlapping area of the two unwrapped phases is
The ZNCC criterion defined in
Then compensating the phase deviation between CCDs by
where
where
2 Experiment and results
The experiment setup is shown in
Figure 3.Experiment setup
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Figure 4.The relationship between the standard deviation of the difference of before and after stitching and size of overlapping area
In order to prove the effective of the proposed method,the relative error which is defined as
where
Figure 5.Unwrapped phase map and the relative errors of the overlapping area before and after stitching
To further evaluate the proposed method,a total of 9 displacement loading points are included,and three groups of values are measured by CCD#1,CCD#2,and the phase stitching method. The fitted curves and residual errors are shown in
Figure 6.Least square fitting and fitting residuals
Based on the results obtained above,it can be concluded that phase stitching is effective and allows for FOV expansion.
3 Conclusion
A DSPI system with two CCDs is used to record multiple sub-aperture images to avoid scanning errors in the micro-positioning stage when moving the CCD or object. The registration positions were calibrated to eliminate phase errors between SASPIs. In order to obtain the maximum FOV with fewer CCDs,the relationship between the standard deviation and the size of the overlapping area was investigated. The size of the overlap zone is approximately 10%,which may be appropriate in terms of the trade-off between FOV and accuracy. To demonstrate the effectiveness of the phase stitching method,a calibrated loading device driven by a piezoelectric actuator was used. The measurement accuracy of the phase stitching method is approximate to that of the single-camera method when comparing the measurements of the calibration points by RMSE metric. Since the true value of the full field is unknown,the difference in overlap area between the single CCD method and the phase stitching method was calculated to evaluate the confidence of the full field values obtained by the proposed method,and these values are less than 1%. Furthermore,with more CCDs,the measurement range and axial resolution are increased to a certain FOV.
[2] Sijin WU, Jing YANG, Siyang PAN et al. Dynamic deformation measurement of discontinuous surfaces using digital speckle pattern interferometry and spatiotemporal three-dimensional phase unwrapping. Acta Photonica Sinica, 47, 0212002(2018).
[4] Hao YAN, Jun LONG, Chiyue LIU et al. Review of the development and application of deformation measurement based on digital holography and digital speckle interferometry. Infrared and Laser Engineering, 48, 603010(2019).
[21] Ming ZHAO, Kemao QIAN. Quality-guided phase unwrapping implementation: an improved indexed interwoven linked list. Applied Optics, 53, 3492-3500(2014).
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Jun LONG, Ping CAI, Shuyuan PAN, Chiyue LIU, Hao YAN. Phase Stitching Based Multi-CCDs Deformation Measurement in Digital Speckle Pattern Interferometry[J]. Acta Photonica Sinica, 2022, 51(4): 0412003
Category: Instrumentation, Measurement and Metrology
Received: Sep. 18, 2021
Accepted: Nov. 11, 2021
Published Online: May. 18, 2022
The Author Email: CAI Ping (pcai@sjtu.edu.cn)