The characteristics of the particle position along an optical axis in particle holography

Particle diagnostics involving three-dimensional distributions are important topics in many engineering fields. The holographic particle velocimetry system is a promising optical tool for measuring three-dimensional particle velocities. One of the inherent limitations of particle holography is the very long depth of field of particle images, which causes considerable difficulty in the determination of particle positions along the optical axis. In this study, diffused illumination holography was adopted as a tool for analysing the characteristics of particles. The prerequisites for a more precise determination of the velocities of particles are a profound understanding of image properties and of the focusing parameter used to determine the particle position along an optical axis. For this purpose, the characteristics of speckles and particle images were investigated with respect to variations in hologram aperture and particle position along the optical axis. Based on our findings, we introduced three auto-focusing parameters corresponding to the sizes of particles, namely, correlation coefficient, sharpness index and depth intensity, to determine the focal plane of a particle along the optical axis. To investigate the suitability of these parameters, the plane image of a dot array screen containing different dot sizes was recorded and the positions of each dot along the optical axis were evaluated. In addition, the effect of particle distance from the holographic film was examined by changing the distance between the screen and the holographic film. All measurement results verified that the evaluated positions throughout the suggested auto-focusing parameters remain within acceptable error ranges. These research results may provide fundamental information for the development of a holographic velocimetry system based on automatic image processing.