ASPM-SI is a type of advanced laser speckle interferometric technique which is highly immune to environmental noise.
Unlike in conventional electronic speckle pattern interferometry (ESPI), in which the reference speckle pattern is taken at time zero and this reference is subtracted from all deformed ones obtained after time zero, ASPM-SI is a three-step process.
Step 1), an additive speckle pattern is taken while the object deforms or the optical path difference (OPD) in an interferometer changes repetitively at a frequency much higher than the video frame rate. This additive speckle pattern contains the information about the object deformation or the OPD change. The intensity distribution can be described by Bessel function. Due to the high level of self-interference term, fringe contrast of the additive speckle pattern is very low and the fringes are barely visible.
Step 2), a similar additive speckle pattern is taken but with the phase of the reference beam in the interferometer shifted by Pi.
Step 3), the above two additive speckle patterns are subtracted to reveal a high contrast additive-subtractive fringe pattern. By repeating the process from the first through the third step, a real-time ASPM-SI fringe pattern depicting the repetitive deformation or OPD change is obtained.
For quantitative measurement, phase shifting technique or Fast Fourier transform method is applied to extract the phase values related to the deformation or the OPD change. In our laboratory, we have developed several optical systems for the quantitative analyses of vibration modes of solid structures and for the measurement of surface shapes.
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As mentioned above, the major advantage of the ASPM-SI technique is its high degree of noise immunity. This features depicts its application to industrial settings where various types of environmental noises are of concern, where conventional holography or ESPI technique has been found inadequate. Here are some typical quantitative measurement results.
(a) A fringe pattern obtained from conventional
ESPI;
(b) A fringe pattern obtained from the same test condition
as in (a) but using ASPM- SI;
(c) A phase map obtained from ASPM-SI depicting a object vibration amplitude;
(s) A perspective plot the measurement results.
We have also developed a method of applying the ASPM-SI technique to
quantitative non- destructive inspection of composite repair patches (delaminations).
For an example of the test results, click here (400Kbyte).
here (600Kbyte).