Particles Depth Measurements Using In-Line Digital Holography Configuration

摘要

Digital holography is now being accepted as a very promising technique in the 3-D particle image velocimetry. 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. Two different novel techniques for the detection of the depth of small tracer particles distributed in 3-D space is presented in this paper. These techniques are based on in-line holography and the depth of the particles in both cases is measured using the numerically reconstructed images obtained from the convolution based Fresnel reconstruction formula. In the first case, the particle depth is measured by the statistical measures of light intensity within a proper rectangular sampling window around the test particle whereas in the second case, the geometric measure of the particle intensity peak is used for the determination of the particle depth. Both methods are applied to the different hologram patterns with overlapping interference fringes and the measurement results are compared and discussed in some detail.