We built a hyperspectral imaging apparatus using middle-infrared light of 8–14 μm, which has a strong ability to identify organic materials, and attempted visualization of the distribution of organic materials that could not be identified by a naked eye. For this purpose, we utilized a low-cost bolometer camera (Nippon Avionics co., ltd. C100V, Japan) for its easy availability rather than an expensive mercury cadmium telluride (MCT) array sensor. To compensate for the low sensitivity of this bolometer, we adopted a Fourier-type spectroscopic system (Aoi Electronics co. ltd., Japan) using an imaging interferometer devised by the Kagawa University, Japan; this interferometer has higher light-utilization efficiency than Michelson interferometers, which are used in popular interferometry techniques.
In this study, 4 types of adhesives, 9 types of varnishes and more than 50 types of inks were put on Al plates of size 10 cm × 10 cm and were used as samples. Glossy paper for printing photos with an inkjet printer was also used as a sample. A 300 °C black body of size 15 cm × 15 cm was used as a light source. Spectra of 320 × 240 points were measured at a wavelength resolution of approximately 9 cm−1. The mirror was scanned only once. The measurement time was approximately 30 s.
Hyperspectral images of adhesives, varnishes and inks on Al plate and paper were successfully measured. Spectra over a 5 × 5-pixel neighborhoods were averaged, and the averaged spectra were compared with those measured by a commercially available Fourier transform infrared (FTIR) spectroscopy. The averaged and measured spectra had absorption peaks at the same wavelengths. Furthermore, by analyzing the measured spectra, the distribution of substances invisible to the naked eye was visualized. Our results show that if low-absorbance organic materials are put on a high-reflectance surface such as an Al plate, the middle-infrared hyperspectral imaging could be measured using a bolometer. Additionally, hyperspectral imaging of high-reflectance paper, such as glossy paper, could also be measured. Because a bolometer camera is much cheaper than an MCT array, hyperspectral imaging with such a camera has many potential applications. Moreover, an imaging interferometer, with its high efficiency of light utilization, is very suitable for the purpose.
Obliterated writing is writing that has been obscured by different-colored materials. There are obliterated writings that cannot be detected by conventional methods. A method for deciphering such obliterated writings was developed in this study. Mid-infrared spectroscopic imaging in the wavelength range of 2.5–14 μm was used for deciphering because the infrared spectrum differs among different brands of colorants. Obliterated writings were made by pressing information protection stamps onto characters written by 4 kinds of colorants. The samples were tested for deciphering by the Fourier-transform infrared imaging system. Two peak areas of two specific wavenumber regions of each reflectance spectrum were calculated and the ratio of the two values is displayed as a unique gray scale in the spectroscopic image. As a result, the absorption peak at various wavenumbers could be used to decipher obliterated writings that could not be detected by the conventional methods. Ten different parameters for deciphering obliterated writing were found in this study.
A new method of deciphering obliterated writing is proposed in this paper. Inks with a peak at 1620 cm-1 were used for writing characters on paper, and information protection stamps were then used to obliterate them. Conventional detection methods consisting of visible and near-infrared spectroscopic photography and fluorescent photography of wavelength between 0.4–1.0 μm were compared with a novel method using infrared spectroscopic imaging with a 16-element mercury cadmium telluride array detector. The samples of obliterated writing could not be detected by the conventional methods, but could by the new method. Therefore, this new method is very useful for deciphering obliterated writing.
In this paper, discrimination method for inkjet printed counterfeits and feature extraction method of spur marks are proposed. Spur marks are tool marks formed on a printout by paper conveyance gears of inkjet printers, and inkjet printers are classified into some groups by type of spurs and their arrangement. Spur marks were visualized and clearly distinguished from background by infrared oblique lighting and gradient image processing. As the characteristic values of spur marks, the pitch and mutual distance were introduced. By radon transform of spur mark images, direction of spur mark lines and the mutual distance were estimated. Spatial frequency components of spur marks were analyzed by two-dimensional fast Fourier transform to estimate the pitch. The pitch and mutual distance were extracted almost automatically. Proposed methods were applied to samples printed by a color inkjet printer. Measurement results were compared with those obtained by conventional method using x-y stage, and the accuracy was proved. By detecting existence of spur marks on printouts, discrimination of inkjet printed material was achieved. This technique will be applicable to model and manufacturer identification of printers, and significant not only for counterfeit detection but also in the field of forensic document examination.
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