1. Measurement of particle size by direct imaging, including defocus effects Julian Kashdan, John Shrimpton Thermofluids Section, Department of Mechanical Engineering, South Kensington Campus, Imperial College London, SW7 2AZ INTRODUTION Particle/droplet image analysis (PDIA) is an image-based sizing technique which uses an automated processing algorithm for analysing digital images of two-phase flows. As with other single droplet counter sizing methods such as phase Doppler anemometry (PDA), digital image analysis can be used to determine the properties of individual droplets or particles such as velocity, size, shape and particle concentration over a finite region of interest in the flow. The development of image-based droplet sizing techniques has intensified significantly in recent years and this has mainly been due to technological advances which have improved the resolution, stability and reduced the cost of CCD imaging systems. In addition to the greater simplicity of imaging methods over light-scattering techniques such as PDA, another major advantage is that a visual record of the spray under investigation is available, providing a simple means to verify what is, and perhaps more importantly, what is not being measured. METHOD The method works by comparing the area of the dark inner area with the mean gradient of the halo. The latter is calculated by comparing the areas and relative displacements of the two grey halo areas in the schematic. For a given optical configuration there exists a unique relationship between the total area and the halo areas on one hand, and true object diameter and distance of object from plane of focus on the other TABLE 1 Comparison of spatial PDA and PDIA data volume distribution and effect of varying the sphericity parameter, S on D 30 (open symbols) and normalised valid sample number (closed symbols), T adaptive =0.85, N o =5818 at Z/d o =36. Composite image of Patterson globe calibration graticle showing objects of increasing diameter (rows) and at increasing defocus distances (columns) for the determination of a ‘usable’ depth of field. Schematic representation of droplet shadow image and intensity profiles across X-X for (a) an in-focus droplet and (b) a defocused droplet. Schematic diagram showing PDIA set-up for spray measurements with diode laser. Variation of halo area, A H as a function of distance from plane of best focus, D F for 18<D<145  m. Graph comparing PDIA threshold corrected (T corrected ) pdf versus uncorrected pdf and PDA data for Z/d o =36, R/d o =15. REFERENCES JT Kashdan, JS Shrimpton, A Whybrew, “Two phase flow characterisation by automated Digital Image Analysis, Part 1: Fundamentals Principles and Calibration of the technique’,Particle and Particle Systems Characterization, 20, 6, pp 387-397, Feb 2004 JT Kashdan, JS Shrimpton, A Whybrew, “Two phase flow characterisation by automated Digital Image Analysis, Part 2 : Application of PDIA for Sizing Sprays’. Particle and Particle Systems Characterization, 21, pp. 15-23, 2004