Depth from Defocus Technique: A Simple Calibration-Free Approach for Dispersion Size Measurement

TL;DR

This paper introduces a simple, calibration-free 'Depth from Defocus' method using a single camera for volumetric measurement of dispersed spherical particles, enabling in-situ sizing in complex, hard-to-measure systems.

Contribution

It presents a novel, easy-to-deploy DFD technique that estimates particle size and position without calibration, suitable for diverse applications and natural phenomena.

Findings

Effective for sparse dispersions including droplets and pollen

Simple optical setup with semi-autonomous calibration

Applicable to in-situ measurements in complex environments

Abstract

Particle size measurement is crucial in various applications, be it sizing droplets in inkjet printing or respiratory events, tracking particulate ejection in hypersonic impacts, or detecting floating target markers in free surface flows. Such systems are characterised by extracting quantitative information like size, position, velocity and number density of the dispersed particles, which is typically non-trivial. The existing methods like phase Doppler or digital holography offer precise estimates at the expense of complicated systems, demanding significant expertise. We present a novel volumetric measurement approach for estimating the size and position of dispersed spherical particles that utilises a unique 'Depth from Defocus' (DFD) technique with a single camera. The calibration free sizing enables in-situ examination of hard to measure systems, including naturally occurring phenomena like pathogenic aerosols, pollen dispersion or raindrops. The efficacy of the technique is demonstrated for diverse sparse dispersions, including dots, glass beads, spray droplets, and pollen grains. The simple optical configuration and semi-autonomous calibration procedure make the method readily deployable and accessible, with a scope of applicability across vast research horizons.