A Two-Camera Depth From Defocus Imaging System And Sensitivity Analysis

Abstract

A single lens, two-camera imaging system using backlight photography has been previously proposed based on Depth from Defocus (DFD) for the measurement of size and three-dimensional position of particles or drops. It makes use of two images with different degrees of defocus blur captured simultaneously at different imaging planes. In the present study, the influence of system and particle parameters on the calibration functions and measurement uncertainties of such a system have been comprehensively investigated and experimentally verified. The parameters varied include the distance between the imaging planes, the image bit resolution, pixel resolution and the random image noise. Furthermore, the intensity and position of the backlighting and the type of particle and its relative refractive index have been varied. Experimental verification was conducted using calibration dot targets, polystyrene latex spheres, and pulverized coal powders. A sensitivity analysis was performed using relative errors or differences for the three quantities - calibration function, particle size and particle position. The study concludes with explicit recommendations on which quantities are most influential in determining measurement accuracy and therefore, must be carefully controlled. Using this imaging system, measurements were conducted for a mono-sized standard latex sphere clouds suspended in a water container, and the results of particle size distribution were compared with that obtained using a common single lens, one-camera imaging system with thresholding image processing. Subsequently, the system is used for measurement of a sparse spray. Comparison measurements using the laser diffraction technique and the analysis of focused images are provided,with relatively good quantitative agreement. The measurement depth range based on the DFD method exhibits a simple proportional relationship with particle size, which can be used for bias correction of the particle size distribution and number density. This leads to a highly resolved measurement volume depth; hence, an accurate estimation of the overall measurement volume, allowing very accurate volume number density distributions to be estimated. This constitutes a decisive advantage over existing optical techniques for particle size measurements.