Combined Imaging and Chromatic Confocal Microscopy Technique to Characterize Size and Shape of Ensembles of Cuboidal Particles

The presence of needle- and plate-like particles has detrimental consequences on their downstream processing in the fine chemicals sector. Therefore, the ability to accurately characterize the particle size and shape of the powder is essential to quantify and predict their impact on the product processability. Nonetheless, tools able to characterize the size and -- most importantly -- the shape, of ensembles of cuboidal crystals are seldom available. Thus, the overarching goal of this work is to provide a fast and accurate offline size and shape characterization tool. To this aim, we have designed and experimentally validated a combined imaging and chromatic confocal microscopy technique. We propose two modes of operation: one that facilitates the 3D reconstruction of particles at the expense of characterization time; and the other that facilitates rapid characterization without the need to 3D reconstruct the particles. We evaluate and validate the performance of our technique, using a commercial technique as a reference, by characterizing particles that exhibit differences in size and shape characteristics and optical properties. We show that our technique can be used to accurately obtain three characteristic lengths (length, width, and thickness) for thousands of particles, making it a valuable addition to existing process analytical technology.