PZT calibration

One important source of error in phase-shift interferometry consists of a nonuniform application of phase steps, due, for example, to a nonlinear piezoelectric effect in the phase shifter. We developed a method for inline PZT calibration using a video camera for acquiring interferograms and an ATI Radeon X1650 video card for frame processing. The PZT was calibrated over a voltage range of 40 V and a displacement range of 11 μm. We acquired 801 interferograms in a Twyman-Green interferometer and a 801 × 801 matrix computed from the subtraction of each interferogram from every other interferogram was obtained and used for calibration. The speed and accuracy of frame processing of the video card was compared to the results obtained by an Intel QuadCore processor. In terms of accuracy there are no significant differences, but the processing using the video card was ∼5 times faster than the processing using the processor. The processing took ∼5 min and we obtained a dispersion of ∼10 nm when we fitted the displacement of the PZT to a polynomial of degree 5 in voltage.