Real-time fringe-pattern analysis

Abstract

The task is to learn the phases of real positive intensity fringes from an optical testing interferometer. The introduction of substantial tilt into the interferometer establishes a field of finely spaced fringes that serve as a spatial heterodyne carrier. Sequential pixel values from a TV video signal of the picture are distributed among three separate signal channels, every third pixel going to the same channel. The distribution rate is set at ~3 pixels/fringe so that each channel senses one phase of a three-phase stroboscope or moiré. Complex weighting of the channel signals eliminates the common mode to provide in-phase and quadrature analog fringe signals. A direct analog-to-digital arctangent converter, with that analog signal pair as input, provides 4-bit (¹/₁₆-cycle resolution) fringe phase at a 5-MHz sampling rate. The converter is coupled to a turns counter that automatically registers unwrapped phase. The similarity of the signals to ntsc color TV encoding is noted along with certain other applications.