Deriving the infrared complex refractive indices of organic powders for optical modeling: comparison of methods

Kelly A. Peterson; Bruce E. Bernacki; Danielle L. Saunders; Jeremy D. Erickson; Schuyler P. Lockwood; Timothy J. Johnson; Tanya L. Myers

doi:10.1117/12.3013843

7 June 2024 Deriving the infrared complex refractive indices of organic powders for optical modeling: comparison of methods

Kelly A. Peterson, Bruce E. Bernacki, Danielle L. Saunders, Jeremy D. Erickson, Schuyler P. Lockwood, Timothy J. Johnson, Tanya L. Myers

Author Affiliations +

Proceedings Volume 13031, Algorithms, Technologies, and Applications for Multispectral and Hyperspectral Imaging XXX; 130310E (2024) https://doi.org/10.1117/12.3013843
Event: SPIE Defense + Commercial Sensing, 2024, National Harbor, Maryland, United States

Abstract

Identification of solids via infrared reflection spectroscopy requires a spectral library of all solids likely to be encountered. A confounding factor in populating such a spectral library is that the reflectance spectra of solids vary with their form, including particle size, film thickness, and substrate. To reduce the efforts of experimentally constructing such a library, an alternate strategy is to use the wavelength-dependent optical constants, n and k, of a solid to calculate a series of reflectance spectra corresponding to each scenario or morphology. Because most n/k measurements are best performed on mm-sized crystals, however, the challenge of determining the optical constants increases when a solid is only readily available as a powder, as is often the case. Some organic solids, such as caffeine, are both unavailable in large crystals and difficult to press into pellets. In this study, the infrared optical constants, or complex refractive indices, of caffeine were determined using three different methods: single-angle reflectance, infrared spectroscopic ellipsometry, and quantitative absorbance measurements of KBr pellets. The n and k values derived through each method were used to model the hyperspectral imaging reflectance spectrum of a caffeine film on a steel planchet. Over 1,110 – 870 cm^-1, the single-angle reflectance-derived n and k had the best correlation with the experimental spectrum. These results suggest different organic solids may require different methods to determine the most accurate infrared complex refractive indices for synthetic spectral libraries.

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(2024) Published by SPIE. Downloading of the abstract is permitted for personal use only.

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Kelly A. Peterson, Bruce E. Bernacki, Danielle L. Saunders, Jeremy D. Erickson, Schuyler P. Lockwood, Timothy J. Johnson, and Tanya L. Myers "Deriving the infrared complex refractive indices of organic powders for optical modeling: comparison of methods", Proc. SPIE 13031, Algorithms, Technologies, and Applications for Multispectral and Hyperspectral Imaging XXX, 130310E (7 June 2024); https://doi.org/10.1117/12.3013843

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