Active phase-shifting interferometers for shape and deformation measurements

We have developed active interferometers in which fringes are stabilized by detecting their movement induced by external perturbations such as mechanical vibration or air flow with a spatial filtering detector and by feeding back the signal to a piezoelectric mirror of the interferometer. We also extended the system to active phase-shifting interferometers by which the fringe shifts are monitored in real time to be delivered accurately for automatic analysis even in the presence of external perturbations or temperature drift of the piezoelectric element. By applying these feedback controls to a Twymann-Green interferometer placed on a wooden desk we could measure the flatness of a mirror with almost the same peak-to-valley and root-mean-square values as measured on an optical bench. Only their repeatability was worse, by three times. We also applied the method to a dual-beam speckle interferometer and could measure in-plane displacement even in the presence of air flow generated by a fan, which prevented measurement without feedback. It has proved to be especially useful for measurement of large displacement by integrating incremental displacements. (C) 1996 Society of Photo-Optical Instrumentation Engineers.