Visualization of latent fingerprints using fluorescence lifetime imaging on paper emitting strong fluorescence.

Latent fingerprints were successfully visualized using fluorescence lifetime imaging (FLIM) on paper which emits strong fluorescence with a lifetime close to that of fingerprints and thus from which it is difficult for time-resolved spectroscopy to visualize fingerprints. Latent fingerprint samples on paper were excited using a 450 nm or 532 nm nanosecond pulsed-laser, and time-resolved fluorescence images were obtained at a delay time of 6-16 ns in intervals of 1 ns, to the excitation pulse. The excitation beam was expanded using a lens, and the fluorescence from the fingerprints was captured using an intensified CCD camera. Because of the large fluorescence intensity of the background paper of approximately two to four orders of magnitude larger than that of the fingerprint, the fingerprint was not visualized on each fluorescence image by time-resolved spectroscopy. However, the fingerprint was visualized in a FLIM image constructed using a series of the fluorescence images for the case with the fluorescence intensity of the background paper being four orders of magnitude larger than that of the fingerprint. The difference in fluorescence lifetime in the FLIM image of the visualized fingerprint and background paper was in the order of 0.1 ns, which was an order of magnitude smaller than the inherent fluorescence lifetime of a few nanoseconds for the fingerprints and paper. It was demonstrated that, at a background fluorescence intensity with a certain order of magnitude larger than that of fingerprints, FLIM has the potential to visualize latent fingerprints which cannot be visualized by time-resolved spectroscopy.

Application of mist to fingermark detection: Misting with high-boiling-point liquid containing p-dimethylaminocinnamaldehyde and cyanoacrylate.

In order to detect latent fingerprints that could be damaged by liquid or powder reagents, non-destructive processes such as gaseous reagents have been developed. In this report, we propose the use of fine mist generated when hot vapor of high-boiling-point liquids is rapidly cooled by surrounding air for fingermark detection. Octyl acetate (OA), 2-phenoxyethanol (2PE), and methyl decanoate (MD) were found to efficiently produce mist when heated to 230°C. By combining these liquids with p-dimethylaminocinnamaldehyde (DMAC) and cyanoacrylate (CN), our team demonstrated effective fluorescence staining of cyano-treated fingermarks using DMAC/OA misting or DMAC/2PE misting, and one-step fluorescence detection of latent fingermarks without cyanoacrylate treatment using DMAC/OA/CN misting or DMAC/MD/CN misting. Fingermark fluorescence was efficiently observed by excitation with a blue LED light (max. wavelength 470 nm) equipped with an interference filter and passing through a 520 nm long-pass filter. We successfully obtained fluorescent images from fingermarks on several substrate materials using the developed misting method.

Independent component analysis of hyperspectral data measured from overlapping latent fingermarks: Forensic potential of independent component images.

Overlapping fingermark images are sometimes discarded because fingermark collation for the individual fingermarks is difficult. Fluorescence hyperspectral data (HSD) measured using the models of double overlapping fingermarks obtained under the excitation of a high-power, continuous wave, green laser is suitable for obtaining individual fingermark images. However, there are limitations such as the problems on each spectrum of the individual fingermark and the forensic value of the obtained images. In this study, independent component analysis (ICA) was applied to the fluorescence HSD obtained from the models of doubly overlapping fingermarks, to obtain independent component (IC) spectra and the corresponding IC images. Forensic value of the obtained IC images was examined, considering the possibility of fingermark collation in masked fashion to the model sample information. The IC images obtained from the HSD had enough potential to enable extracting twelve minutiae required for fingermark collation if the image quality was good.

Separation of overlapping fingerprints by principal component analysis and multivariate curve resolution-alternating least squares analysis of hyperspectral imaging data.

Overlapping fingerprints are often found at crime scenes, but only individual fingerprints separated from each other are admissible as evidence in court. Fingerprint components differ slightly among individuals, and thus their fluorescence spectra also differ from each other. Therefore, the separation of overlapping fingerprints using the difference of the fluorescence spectrum was performed with a hyperspectral imager. Hyperspectral data (HSD) of overlapping fingerprints were recorded under UV LED excitation. Principal component analysis (PCA) and multivariate curve resolution-alternating least squares (MCR-ALS) were applied to the HSD to determine the optimal method for obtaining high-contrast images of individual fingerprints. The results suggested that MCR-ALS combined with PCA-based initialization is capable of separating overlapping fingerprints into individual fingerprints. In this study, a method for separating overlapping fingerprints without initial parameters was proposed.

Visualization of Aged Fingerprints with an Ultraviolet Laser.

Detection of aged fingerprints is difficult because they can degrade over time with exposure to light, moisture, and temperature. In this study, aging fingerprints were visualized by time-resolved spectroscopy with an ultraviolet-pulsed laser. Fingerprints were prepared on glass slides and paper and then stored under three lighting conditions and two humidity conditions for up to a year. The fluorescence intensities of the fingerprints decreased with time. Samples were stored in the dark degraded less than in sunlight or under a fluorescent lamp. Samples were stored under low humidity degraded less than under moderate humidity. As the storage period increased, a fluorescence emission peak appeared that was at a longer wavelength than the peak visible in earlier spectra. This peak was used for visualization of an aged fingerprint over time. An image of the fingerprint was not initially visible, but an image appeared as the time since deposition of the fingerprint increased.

Differentiation of black writing ink on paper using luminescence lifetime by time-resolved luminescence spectroscopy.

The time-resolved luminescence spectra and the lifetimes of eighteen black writing inks were measured to differentiate pen ink on altered documents. The spectra and lifetimes depended on the samples. About half of the samples only exhibited short-lived luminescence components on the nanosecond time scale. On the other hand, the other samples exhibited short- and long-lived components on the microsecond time scale. The samples could be classified into fifteen groups based on the luminescence spectra and dynamics. Therefore, luminescence lifetime can be used for the differentiation of writing inks, and luminescence lifetime imaging can be applied for the examination of altered documents.

Portable hyperspectral imager with continuous wave green laser for identification and detection of untreated latent fingerprints on walls.

Untreated latent fingerprints are known to exhibit fluorescence under UV laser excitation. Previously, the hyperspectral imager (HSI) has been primarily evaluated in terms of its potential to enhance the sensitivity of latent fingerprint detection following treatment by conventional chemical methods in the forensic science field. In this study however, the potential usability of the HSI for the visualization and detection of untreated latent fingerprints by measuring their inherent fluorescence under continuous wave (CW) visible laser excitation was examined. Its potential to undertake spectral separation of overlapped fingerprints was also evaluated. The excitation wavelength dependence of fluorescent images was examined using an untreated palm print on a steel based wall, and it was found that green laser excitation is superior to blue and yellow lasers' excitation for the production of high contrast fluorescence images. In addition, a spectral separation method for overlapped fingerprints/palm prints on a plaster wall was proposed using new images converted by the division and subtraction of two single wavelength images constructed based on measured hyperspectral data (HSD). In practical tests, the relative isolation of two overlapped fingerprints/palm prints was successful in twelve out of seventeen cases. Only one fingerprint/palm print was extracted for an additional three cases. These results revealed that the feasibility of overlapped fingerprint/palm print spectral separation depends on the difference in the temporal degeneration of each fluorescence spectrum. The present results demonstrate that a combination of a portable HSI and CW green laser has considerable potential for the identification and detection of untreated latent fingerprints/palm prints on the walls under study, while the use of HSD makes it practically possible for doubly overlapped fingerprints/palm prints to be separated spectrally.

Visualizing latent fingerprints on color-printed papers using ultraviolet fluorescence.

Laser detection of latent fingerprints on a white paper has been performed, previously. Ultraviolet fluorescence from various kinds of printer toner and ink used for home printers were measured to study fluorescence imaging of fingerprints on a color-printed white paper. The experimental system consisted of a nanosecond pulsed tunable laser and a cooled CCD camera. Excitation wavelengths are 230 and 280 nm. Fourteen printers consisting of three color laser printers, three color inkjet printers, five monochrome laser printers, two monochrome copy machines, and a color copy machine were tested. Toner and ink of most printers exhibited fluorescence in the region from 360 to 550 nm. In most cases, clear fluorescence images were obtained by time-resolved imaging with a band-pass filter and 280-nm excitation. However for toners from laser color printers that showed strong fluorescence, better results were obtained with 230-nm excitation. Latent fingerprints on a photograph page and a black-character page of a newspaper were also imaged.

Individual camera identification using correlation of fixed pattern noise in image sensors.

This paper presents results of experiments related to individual video camera identification using a correlation coefficient of fixed pattern noise (FPN) in image sensors. Five color charge-coupled device (CCD) modules of the same brand were examined. Images were captured using a 12-bit monochrome video capture board and stored in a personal computer. For each module, 100 frames were captured. They were integrated to obtain FPN. The results show that a specific CCD module was distinguished among the five modules by analyzing the normalized correlation coefficient. The temporal change of the correlation coefficient during several days had only a negligible effect on identifying the modules. Furthermore, a positive relation was found between the correlation coefficient of the same modules and the number of frames that were used for image integration. Consequently, precise individual camera identification is enhanced by acquisition of as many frames as possible.

Fluorescence spectra and images of latent fingerprints excited with a tunable laser in the ultraviolet region.

Fluorescence spectra of sebum-rich latent fingerprints were studied with a tunable laser for non-destructive fingerprint detection without chemical treatment. The tunable laser consists of a nanosecond pulsed Nd-YAG laser and an optical parametric oscillator (OPO) crystal. The fluorescence spectra and images were measured at various excitation wavelengths in the ultraviolet region by the time-resolved fluorescence method. We have previously reported that a typical fluorescence spectrum of fingerprints consists of two peaks located at c. 330 and 440 nm. In order to determine the wavelength of optimal excitation, excitation spectra were measured at wavelengths ranging from 220 to 310 nm. The fluorescence intensity of the 330 nm peak became maximal with excitation at 280 nm. The images of latent fingerprints on white papers were also measured and the clearest image was obtained with excitation at 280 nm. The influence of continuous irradiation on the fluorescence of fingerprints was measured at the optimal excitation wavelengths. The 330 nm peak was strong at first and decreased with continuous irradiation, whereas the 440 nm peak, which was weak at first, increased gradually.

Ultraviolet fluorescence imaging of fingerprints.

We studied fluorescence imaging of fingerprints on a high-grade white paper in the deep ultraviolet (UV) region with a nanosecond-pulsed Nd-YAG laser system that consists of a tunable laser and a cooled CCD camera. Clear fluorescence images were obtained by time-resolved imaging with a 255- to 425-nm band-pass filter, which cuts off strong fluorescence of papers. Although fluorescence can be imaged with any excitation wavelength between 220 and 290 nm, 230 and 280 nm are the best in terms of image quality. However, the damage due to laser illumination was smaller for 266-nm excitation than 230- or 280-nm excitation. Absorption images of latent fingerprints on a high-grade white paper are also obtained with our imaging system using 215- to 280-nm laser light. Shorter wavelengths produce better images and the best image was obtained with 215 nm. Absorption images are also degraded slightly by laser illumination, but their damage is smaller than that of fluorescence images.

Ultraviolet fluorescence spectra of fingerprints.

We have studied inherent fluorescence spectra and imaging of fingerprints in the deep ultraviolet (UV) region with a nanosecond-pulsed Nd-YAG laser system that consists of a tunable laser, a cooled CCD camera, and a grating spectrometer. In this paper, we have studied UV fluorescence spectra of fingerprints under 266-nm illumination. Fluorescence spectra of fingerprints have two main peaks, around 330 nm (peak A) and 440 nm (peak B). At first, when a fingerprint has just been pressed, peak A is dominant. However, its intensity reduces as the total illumination time increases. On the other hand, peak B is weak at first. It appears after enough 266-nm illumination and its intensity increases as time elapses. After 3 h of illumination, peak A almost diminishes and peak B becomes dominant. By leaving the fingerprint under a fluorescent lamp in a room without laser illumination, peak A can be restored partly, while the intensity of peak B still increases. Time-resolved fluorescence spectra were also measured for these two peaks. The lifetime of each peak is 2.0 nsec (peak A) and 6.2 nsec (peak B) on average. Both peaks seem to consist of several components with different lifetimes. In the case of peak A, the 330-nm peak decays fast and a new component at 360 nm becomes dominant when the delay time exceeds 20 nsec. In the case of peak B, unlike peak A, no clear peak separation is observed, but the peak position seems to move from 440 to 460 nm when the delay time becomes larger.