The dispersion of some optical properties of vitrinite

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Abstract
SUMMARY: The dispersion characteristics of some optical properties of vitrinites from a number of coals of differing coal rank and type were investigated using a microscope photometer. The properties studied were air and oil reflectances, refractive index and absorption index.The technique employed involved the measurement of air and oil reflectances at a number of wavelengths in the visible spectrum for selected points on the coal, as opposed to averaging the reflectances obtained in each immersion medium for each wavelength. The values of air and oil reflectances were then used to calculate the corresponding values of refractive index (n) and absorption index (κ) using a rearrangement of Beer's equation. Smooth dispersion curves were obtained for each selected field of vitrinite using this procedure. This method was preferred to the more widely used method of calculating n and κ from the average air and oil reflectances at each wavelength, because refractive and absorption indices are non‐linear functions of reflectance, which makes the arithmetic mean of the reflectances unsuitable for the calculation of n and κ.The values of all parameters for the vitrinites were found to increase towards the blue region of the visible spectrum and to increase at all wavelengths with increasing coal rank. Some negative values of κ2 were obtained and the significance of these values is discussed.