Evaluation of a standardised procedure to assess the shape of pellets using image analysis

The influence of threshold definition, number of pellets counted, image magnification and lightning technique on the assessment of pellet shape has been investigated using three batches of pellets and an image analysis system. The pellet parameters measured were 'aspect ratio', 'circularity', 'projection sphericity', 'e(R)' and 'Feret diameter.' The methodical error, reproducibility and repeatability of the results were chosen as statistical test parameters. The position of the light source is crucial in providing an accurate particle size value. Top light was identified as the illumination technique that gave a mean pellet size similar to the true pellet size. The use of a light table produced significantly larger pellet size values. A minimum pixel resolution appears necessary for an accurate shape parameter definition. One pixel should not cover more than 30 microm for pellets of an average particle size of 1.2 mm. Shape descriptors, which are based on a multiple combination of area and perimeter data such as the circularity, are greatly dependent on the number of pellets counted. Shape factors, which do not (aspect ratio) or only as a single value do involve an area or perimeter measurement (e(R), projection sphericity) are, however, nearly independent of the number of pellets counted, as long as the magnification is sufficiently large and the pellets are randomly drawn from the batch. For nearly spherical particles, the methodical error is below 1%, but for elongated particles this error can reach 5%. The repeatability is also very good for nearly spherical particles (<2%), but increases to very large values if the particles are clearly elongated. The limiting values for the various shape factors should be reconsidered. An upper value for the aspect ratio of 1.1 and a lower value of 0.6 for e(R) are recommended. The circularity should not be used as the shape factor to characterise spheres, because errors in image recognition can affect strongly the applicability of this shape factor. The projection sphericity has only a limited sensitivity to variations in particle shape.