Application of Hyperspectral Imaging in Forensic Science

Abstract

The aim of this research was to study and compare two different configurations of hyperspectral imaging (HSI) systems, an area-scan hyperspectral imaging system and a line-scan hyperspectral imaging system, and investigate their applicability in forensic science. Compared to the area-scan HSI system, the line-scan system required a stable and continuous movement of the sample and suffered from more noise due to its higher spectral resolution and lower light throughput. However, the line-scan HSI system was extremely useful for resolving chromophores and fluorophores which have spectral features that had small widths or were close together, whereas the lower spectral resolution of the area-scan HSI system resulting in partial or complete loss of these spectral features. The line-scan HSI system also performed better for an example of structural colour, porous silicon, which has closely-spaced interference fringes in its spectral dimension. The slight change in polarity of fingermarks caused by ozone exposure was also studied using HSI systems. Charged fingermarks deposited on glass were either exposed to ozone or left unexposed, and then all fingermarks were stained with the solvatochromic dye Nile Red and then imaged. The area-scan HSI system was better for this fingermark imaging and in most cases, the slight shift in wavelength of the Nile Red fluorescence could be detected by this system. Different fluorescence behaviour was found for fingermarks that had been stained with Nile Red suspended in the fluorous solvent HFE-7100 compared to with methanolic Nile Red, and these differences were analysed using principal component analysis (PCA). The aging of fingermarks deposited on polyethylene was also examined using HSI, and this study shows that fingermarks tend to penetrate into the polyethylene over a period of a few days. The use of the area-scan HSI system for iodine detection on a polyethylene substrate was studied. Since iodine has a broad absorption in the blue and near-UV, the HSI system did not give improved performance compared to traditional imaging. A new application of HSI for the discrimination of polypropylene lids was then investigated. Nine polypropylene lids were imaged using the area-scan HSI system with a crossed polarised filter. The image cube acquired with this system were processed with a fast Fourier transform to determine the optical retardation over the lids. Each polypropylene lid had a unique retardation map due to the slightly different thermochemical experience during the injection moulding process used in lid fabrication. This research has shown that HSI is a powerful tool in the detection of chromophores, fluorophores, and structural colour in forensic science. HSI allows investigation of both spatial properties and spectral characteristics, especially when HSI is combined with principal component analysis or multivariate curve resolution.