Using Synthetic Infrared Spectra Derived from N/k Optical Constants for Standoff Detection of Chemical Deposits
Abstract
We report results from a recent field experiment to test the validity of using physics-Based synthetic infrared spectra to serve as endmembers in a spectral database targeted at chemical deposits. Specifically, the optical constants n and k, (the real and imaginary part of the refractive index) were used to first model infrared reflectance spectra for different thicknesses of chemical layers (e.g. acetaminophen, methylphosphonic acid - MPA, etc.) on various conducting and insulating substrates such as aluminum, wood, and glass. in the experimental portion of the research, thin films of the solid and liquid analytes were deposited onto such substrates to form micron-thick layers of the analytes at different thicknesses: Standoff data from an imaging instrument were then recorded and analyzed to not only identify the different analytes, but also quantify the layer/deposit thickness. to gauge success, the detection results using the synthetic data were compared to the results from laboratory hemispherical reflectance (HRF) spectra that were collected for the same sample planchets measured in the field via standoff methods. Preliminary results indicate good agreement between the synthetic reference data as compared to the lab-measured HRF data in terms of their ability to quantitatively reduce longwave infrared data. Specifically, modeled IR spectra for acetaminophen on an aluminum planchet at various thicknesses (1, 2, 5, 10, 15, and 20 μm) were synthesized and compared with standoff field reflectance data as well as HRF laboratory reflectance spectra for two samples: A 5.2 μm- and 12.8 μm-thick layer of acetaminophen on aluminum. using a first-order approximation, analysis of the field data estimates the thicknesses of the samples to be 2 and 10 μm for the two samples, respectively, while the HRF laboratory data yields thickness estimates of between 5-10 μm and 10 μm, respectively. Both yield reasonable estimates, with the uncertainty most likely due to factors yet to be accounted for in the synthetic spectra such as light scattering.
Recommended Citation
C. E. Lonergan and J. D. Erickson and M. R. Kelly-Gorham and O. M. Primera-Pedrozo and M. J. Wilhelm and B. M. Forland and K. D. Hughey and B. E. Bernacki and T. J. Johnson and T. L. Myers, "Using Synthetic Infrared Spectra Derived from N/k Optical Constants for Standoff Detection of Chemical Deposits," Proceedings of SPIE - The International Society for Optical Engineering, vol. 12541, article no. 1254109, Society for Photo-Optical Instrumentation Engineers, Jan 2023.
The definitive version is available at https://doi.org/10.1117/12.2663579
Department(s)
Materials Science and Engineering
Keywords and Phrases
acetaminophen; complex refractive index; hyperspectral imaging; methylphosphonic acid; optical constants
International Standard Book Number (ISBN)
978-151066198-1
International Standard Serial Number (ISSN)
1996-756X; 0277-786X
Document Type
Article - Conference proceedings
Document Version
Citation
File Type
text
Language(s)
English
Rights
© 2025 Society for Photo-Optical Instrumentation Engineers, All rights reserved.
Publication Date
01 Jan 2023
Comments
U.S. Department of Energy, Grant DE-AC05-76RL01830