An imaging method for analyzing spherical and nonspherical particles.

Abstract

An automatic image processing technique and a comprehensive formulation of depth of field effects for particle analysis are presented. The image processing method operates on original, unenhanced particle images where the background intensity can be non-uniform, and particle size, location, and level of focus are random. Objects that do not meet a standard of focus needed for correct measurements are automatically rejected, and the threshold for each object is determined uniquely based on the particle and local background information. Border points are defined such that a boundary drawn by connecting border points can never have coincident parts, and is smoother. Moreover, the method is capable of recognizing when light is transmitted or reflected through transparent particles makes them falsely appear as if they had holes in them. The method computes various features for non-spherical particles shapes. Depth of field effects due to particle size, shape, and illumination intensity is formulated as an integral equation. The method is adaptable to wide variety of conditions by means of a few functions that are obtained during calibration. Shape factor parameters are introduced to describe particle shapes. The definition of a shape factor is based on the geometric properties of particles such that it is a true measure of the shape effect on particle properties. For spherical particles the domain of particle size, position relative to the object plane and contrast level where valid measurements can be obtained is a three dimensional dynamic envelope. The analogue dynamic envelope for non-spherical particles is an N-dimensional space which includes the size, position relative to the object plane, contrast, and N-3 shape factors. Most general way to correct the depth of field bias is by traversing the object plane along the optical axis. However, conditions are identified that allow proper correction to be made without having to move the object plane.