Image Analysis Methods
The extreme progress in image capturing and exceptional increase of the computational power within the last few years have revolutionized microscopic methods and made image analysis methods very popular for the characterization of particles. Especially as in addition to size also relevant shape information becomes available by the method. Currently, mainly instruments creating a 2D-image of the 3D-particles are used. Two methods have to be distinguished.
Static image analysis is characterized by non-moving particles, e. g. on a microscope slide. The depth of sharpness is well defined resulting in a high resolution for small particles. The method is well established and standardized [ISO 13322-1:2004 Particle size analysis - Image analysis methods - Part 1: Static image analysis methods], but can only handle small amounts of data. The particles are oriented by the base, overlapping particles have to be separated by time-consuming software algorithms, and the tiny sample size creates a massive sampling problem resulting in very low statistical relevance of the data. Commercial systems reduce these effects by using large or even stepping microscopic slides and the deposition of the particles via a dispersing chamber. As all microscopic techniques can be used, the size range is only defined by the microscope used.
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FIG. 1 Set-up of static (left) and dynamic (right) image analysis for particle characterization.
Dynamic image analysis images a flow of moving particles. This allows for a larger sample size. The particles show arbitrary orientation and the number of overlapping particles is reduced. Several companies offer systems which either operate in reflection or transmission, with wet dispersion or free fall, with matrix or line-scan cameras. The free fall systems are limited to well flowing bulk materials. Systems with wet dispersion only allow for smallest samples sizes and slow particles. This type of instruments is going to be standardized by [ISO/FDIS 13322-2: Particle size analysis - Image analysis methods - Part 2: Dynamic methods]. As currently visible light is used for imaging, the size range is limited to about 1µm at the fine end.
Common to all available instruments are small particle numbers which result in poor statistics. Thus recent developments have yielded a combination of powerful dry and wet dispersion with high speed image capturing. Particle numbers up to 107 can now be acquired in a few minutes. Size and shape analysis is available at low statistical errors [W. Witt, U. Köhler and J. List, Direct Imaging of Very Fast Particles Opens the Application of the Powerful (Dry) Dispersion for Size and Shape Characterization, PARTEC 2004, Nürnberg].
