A Self-Aligning Point Diffraction Interferometer For Fluid Studies

We desribe a self-aligning scheme for a Point Diffraction Interferometer (PDI) to be utilized in connection with strongly aberrated wavefronts, like those customarily encountered in the analysis of stressed fluids. In spite of the fact that the focal plane intensity di-stribution is speckle type, a peak chaser maintains a bright speckle on top of the PDI pinhole, so that enough power is available in the reference beam and fringe visibility remains quite acceptable. We will show typical results obtained for thermal stresses in air and liquid mixtures.