A Fourier-transform method to measure concentration during solidification of a transparent metal model

A 1-D implementation of the Fourier-transform method of fringe pattern analysis was designed to measure phase and concentration changes that take place in the diffusion layer during vertical directional solidification of the metal model material, ammonium chloride and water. Fringe patterns were acquired with the laser optical techniques of interferometry and holographic interferometry.

The Fourier-transform method uses Fourier domain processing to decode phase from a pattern of tilt fringes. Manual fringe location is bypassed, and outside information regarding phase is not required. Unlike intensity techniques, the Fourier-transform method filters unwanted irradiance variations and produces a complete phase field. The Fourier-transform method is more automated than many intensity methods. FORTRAN programs written for this project featured the use of a Hanning window for low pass filtering, accurate determination of the carrier frequency, phase-unwrapping via a recurrence relationship, and base phase subtraction.

Phase and concentration plots of the expected form were produced with the Fourier-transform method. Consistent results were obtained for similar experimental conditions, and the method performed well in comparison with an intensity method. The Fourier-transform method will be used to analyze holograms for an upcoming microgravity flight experiment. The method was successfully applied to fringe patterns produced with holographic interferometry, but poor fringe visibility led to significant noise in the output.