Interferometric alignment-method in holographic flow measurements

In holographic flow measurements a field of small particles is reconstructed from double-exposure holograms. For this purpose the real image is scanned in three dimensions which requires that both the images – each corresponding to a different state of the flow – overlap exactly in space. The reconstruction of real images is realized with a collimated laser beam and the hologram rotated by 180°. Under practical conditions there usually remains a small offset between the images which has to be minimized because it limits the accuracy of the measurement. This is achieved by a slight modification of the geometry of the reconstructing reference wave. In the poster a new interferometric alignment-method is presented to minimize the remaining offset. The basic concept of the method is to additionally record a fixed point-source of light on the hologram during both the exposures. When reconstructing both holograms at the same time the two point-source light fields interfere in space and generate an out-of-focus interference pattern whose number of fringes is minimized. The accuracy achieved with this method in overlap of the two images in space is in the subpixel domain.