Single-Frame-Real-Time-Interferometry for Measuring State Changes of Technical Objects
For measurement of changes in state of technical objects with interferometric methods, phase-shifting interferometry is a commonplace procedure. Yet, at least 3 interferograms have to be taken before and after the change. For phase-shifting being done mechanically and therefore taking a couple of hours, this procedure can only be applied to slowly changing surfaces and is very sensitive to outer interferences. In industrial measuring, e.g. in vibration analysis, changes have to be covered in periods of a few milliseconds though.
Example for a speckle interferometer. Source: MIKOS, FAU Erlangen-Nürnberg
This project's target was the development and realization of a single-frame-realtime-interferometer to measure state changes of technical objects, which enables a complete measurement within a single cameraframe through simultanous reception of three phaseimages. By utilization of pulsed lasers for illumination it is therefore possible, to also detect rapid objectdeformations. For practical operation a compact architecture of the interferometer is necessary, wherefore the neasuring head shall be developed in microsystem-technique. The coverage of the speckle-interferograms is done with a hires CCD-camera, to achieve a sufficiently big effective range.
At FORWISS Passau, a specially adjusted evaluation algorithm has been developed, to process the cameraimages and to compute a reconstruction of the deformation. Special difficulties result from ambiguities referring to the wavelength of the utilized laserlight (wrapping-effect) as well as the high noiserate in the phaseimages. To solve the problem, a hybrid phase-unwrapping-algorithm has been developed that determines the underlying deformation by polynomial-, Fourier-, and spline-based approximation.
Rawphase image of the deformation of an ironplate. Visible: High noiserate as well as the wrapping-effect.
Resulting 3D-deformation profile (Generated by the hybrid phase-unwrapping algorithm)
A comparison of some phase unwrapping approaches can be found here.