The Calibration of Dispersion Staining Colors

Dispersion staining (DS) is an effective and versatile technique for measuring the refractive index (RI) of non-opaque materials by polarized light microscopy (PLM) and has been widely applied in the analysis of asbestos minerals. DS converts the RI difference between an object and its surrounding liquid medium with known RI into an observable color, i.e., the DS color, in the visible spectrum, resulting from the corresponding matching wavelength λm between the RI of the object and its surrounding liquid. Based on the relationship between DS color and λm, λm can be quantitatively evaluated from the observed DS color. Once λm is known, the RI difference between the object and liquid can be calculated. From the liquid’s RI value, the RI value of the object is then measured. The accuracy of RI measurement is directly and primarily dependent on the accuracy of the DS color estimation. Therefore, it’s paramount to obtain an accurate estimation of DS color. In order to improve the accuracy of DS color estimation, it must be carefully calibrated against the specific polarized light microscope used and the specific eyes of the analyst performing the measurement. This paper presents a practical, step-by-step procedure for the calibration of DS color with necessary RI-λm conversion tables at different working temperatures.