Particles Depth Detection using In-Line Digital Holography Configuration

Digital holography (DH) is the direct recording of the Fresnel or Fourier holograms using a CCD or CMOS camera and is considered as an alternative to the optical holography which uses a film or a plate involving wet-chemical or other processes. However, the poor pixel resolution of CCD or CMOS cameras as compared to that of holographic films gives poor depth resolution for images which in turn severely undermines the usefulness of digital holography in densely populated particle fields. The authors in this paper present a technique of depth measurement for the detection of the depth of small tracer particles distributed in 3-D space. This technique is based on in-line holography and the depth of the particles is measured using the numerically reconstructed images obtained from the convolution based Fresnel Reconstruction formula. The particle depth is measured by determining the distribution of light intensity within a proper rectangular sampling window around the test particle. The sample median, mean and minimum value approach is employed as an averaging technique for the light intensity measured at all pixels within that rectangular sampling window and the position on the z-axis where all these values are minimum is regarded as the depth of that particular test particle. The present method is successfully applied to the hologram pattern with overlapping interference fringes which signifies the applicability of the present method to the flow measurements.