RESUMO
This study presents a simple technique for the approximation of retardation, thickness and mass of birefringent particles with a retardation from 8 to 231 nm retardation. Tuning of the imaging system (standard light microscope equipped with a left and a right circular polarizer) to match grey values of polymer retarder films of known retardation with rendered grey values allows for a robust calibration and accurate approximation of retardation. In addition, a technique for accurate particle segmentation using a Canny-Deriche algorithm was used to minimize the bias on mass estimated from different thresholding techniques. The technique was tested using microscopic calcitic plates called coccoliths produced by the marine algal group coccolithophores, and the results compare well with published coccolith mass estimates obtained from volumetric analysis. LAY DESCRIPTION: Material with certain optical properties display interference colours when observed in a light microscope under circular polarized light. This study presents a simple technique for measuring the thickness and retardation of small particles within the 8 to 231 nm retardation range based on the grey values of their interference colours. Retardation is a measure of the distance between waves of two mutually perpendicular polarized light waves after passing through material. The technique involves the tuning of a standard light microscope system equipped with a left and a right circular polarizer and a digital camera to match grey values of polymer retarder films with a known retardation with grey values of a digitially rendered Michel-Lévy chart. A technique for accurate isolation of particles from the image background using a Canny-Deriche algorithm is also described, which avoids possible biased results from thresholding. The techniques were tested using microscopic calcitic plates called coccoliths produced by the marine algal group coccolithophores, and the results compare well with published estimates obtained from volumetric analysis.