Depth-of-field model for size measurement using a borescope imaging technique under high object concentrations

The direct imaging technique has been a common technique for size distribution measurement in multiphase systems. A statistical reconstruction method was developed in a previous study to determine the actual size distribution from image size measurements. The method accounts for the change in magnification scale within the depth-of-field (DOF) of the imaging device. It is anticipated that the DOF will reduce at high object concentrations. Since accurate DOF information is necessary for the application of the reconstruction method, the effect of object concentration on the DOF is examined in this study. A DOF model is developed to predict the DOF under various object concentrations. The applicability of the DOF model in the statistical reconstruction method is verified experimentally. A theoretical analysis is also performed to discuss the effect of having objects whose image sizes are larger than the maximum viewable limit of the imaging device.