Revealing Fraudulent Documents in a New Light with Hyperspectral Imaging
In a world of perpetual forward motion in forensic examination tools and technology, hyperspectral imaging flexes its capabilities in the analysis of black ballpoint pen inks.
By Sara Nedley
|Hyperspectral imaging of black ball-point pen inks.
The tools necessary for successful forensic document examination are ever-evolving, with each new case presenting challenges to the examiner that pushes them to employ the latest and greatest technologies available to forensic scientists. Although not an entirely new technology, hyperspectral imaging (HSI) is beginning to show its analytical prowess when applied to forensic examinations. So what is HSI? It’s a combination of digital imaging and spectroscopy that can be employed as a powerful means of determining chemical similarities and differences.
Ask any forensic document examiner what is of the utmost importance to them when they are examining evidence and most likely they will tell you they need nondestructive, objective, and reliable methods to discriminate between samples made by different inks. In fact, spectral analyses have proven to be just that kind of tool. Visible/near-infrared (NIR) reflectance and luminescence can be a major asset to the forensic document examiner, providing the means for both a nondestructive and chemical-based analysis. What does HSI add to the mix? By pairing digital imaging with spectral analysis, HSI is taking document examination to the next level by increasing the power of discrimination available for ink differentiation.
Putting the Proof into Writing with Black Ballpoint Pens
ChemImage has been active in the advancement of hyperspectral imaging analysis of various types of forensic evidence. Recently, ChemImage has turned their attention to the differentiation of black ballpoint pen inks. In a previous study, as published by Derek Hammond in the Journal of Forensic Sciences 1, Video Spectral Analysis (VSA) and Lab Color Mode (LCM) were both used to analyze 990 black ballpoint pen pair ink samples in order to determine the differentiation capabilities of both analytical techniques. During the initial study 187 pen pairs were found to be indistinguishable using VSA. By choosing to continue this study at ChemImage, the pen pair samples were further analyzed using HSI and the results then compared to the original VSA results. A subset of 99 samples was sent to ChemImage to be analyzed in a blinded manner.
HSI, like VSA, is a nondestructive solution in the evaluation of the composition of pen inks. Unlike VSA, however, HSI uses an electro-optically controlled liquid crystal tunable filter (LCTF) that is capable of achieving discrete wavelength settings as compared to an integrated signal from various interference filters. As a direct result of using LCTF technology, HSI has an increased spectral resolution of 4-10nm, which allows for increased discrimination of chemically similarly samples. Another benefit is that it employs a spectral camera that has a higher sensitivity in the NIR range (700-1100nm), detecting smaller chemical differences beyond the visible portion of the electromagnetic spectrum.
The preliminary results of this study have shown HSI has an increased power of discrimination in the analysis of the samples in comparison to VSA. Using HSI, an additional 29 samples were successfully differentiated. The study has also helped to provide ChemImage with vital information on the capabilities of spectral processing, when applied to ink samples, and has served as guide in the overall method development of HSI analysis of questioned documents.
In order to successfully determine and quantify the capabilities of HSI in the analysis of inks, all 990 samples will be analyzed and compared with the results of the original study. Other future goals for the study include the use of luminescence imaging (visible and NIR) to further analyze the samples, inter-operator comparisons to better understand the interpretation of HSI data as well as evaluation of quantitative methods that can aid examiners in making objective conclusions when utilizing HSI.
As the study continues, ChemImage is striving to provide the forensic document community with a powerful technology capable of solving the most difficult of challenges faced by today’s examiners. To date, HSI has repeatedly demonstrated that it is capable of producing useful data in the discrimination of various inks. HSI may just prove to be the tool of choice for forensic document examiners when it comes to nondestructive ink analysis, so stay tuned for future developments.
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 Hammond, D. L., Validation of LAB Color Mode as a Nondestructive Method to
Differentiate Black Ballpoint Pen Inks, J. For. Sci., Vol. 52, No 4., pp. 967-973, July 2007.