A Re-Evaluation of Refractive Index and Dispersion Measurements Using the Hot-Stage Immersion Method for Additional Discrimination of Automotive Glass

THE MICROSCOPE
2024, Volume 71:4, pp. 175–186
DOI
https://doi.org/10.59082/LNRA8132
AUTHORS
Gwenith Zydlewski and Patrick Buzzini
ABSTRACT
The refractive index (RI) is a fundamental property of glass, and the hot-stage immersion method is an established method for its examination in a forensic setting. Motivated by a discordant body of literature regarding the contribution of dispersion and the use of alternative monochromatic filters compared with the commonly used sodium D-line at 589 nm, this study aimed to evaluate the discriminating capabilities of the three monochromatic Fraunhofer F, D, and C-lines at 486 nm, 589 nm, and 656 nm, respectively, as well as the dispersive index, using four sample sets from a total of 36 automotive glass samples known to have close-range RI values. The study revealed that, in general, assessing and calculating the dispersive index (ν) from the refractive index values of the three Fraunhofer lines did not provide any further differentiation among the automotive glass samples examined in this research. Additionally, analysis of the individual filters revealed that the traditional D-line provided the lowest discrimination capabilities. In contrast, using the blue filter (F-line) and, in some instances, the red filter (C-line) considerably improved these capabilities, prompting a re-evaluation of the practice of measuring glass refractive index in forensic laboratories.
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