Kenro Kuroki

Methods for identification of images acquired with Digital cameras

From the court we were asked whether it is possible to determine if an image has been made with a specific digital camera. This question has to be answered in child pornography cases, where evidence is needed that a certain picture has been made with a specific camera. We have looked into different methods of examining the cameras to determine if a specific image has been made with a camera: defects in CCDs, file formats that are used, noise introduced by the pixel arrays and watermarking in images used by the camera manufacturer.

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.

Visualizing Latent Fingerprints on Color-Printed Papers Using Ultraviolet Fluorescence

Abstract:  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.

Study on height measurement from a single view

A simple expression between the ratio of the length of two collinear line segments in a 3D-world space and that in an image plane is presented when the line is normal to the ground or the reference plane. This expression provides a correction term to the linear relation between the ratio of two line lengths, which holds only when the camera axis is parallel to the ground or the reference plane. The correction term is denoted by the inverse of the ratio of X-coordinates in the image plane coordinate system. We examine the accuracy of the method using a lattice pattern images videotaped by a digital video camera. It is recognized that the height of an object is obtained with a good degree of accuracy. We applied our method to height measurement in an actual scene and obtained an unknown height by means of homogeneous coordinate. However there remains some uncertainty to specify points in actual scenes, which leads to errors in height measurements.

An Approach to Individual Video Camera Identification

This paper presents a method that can be used to determine whether or not questioned video images were recorded with a specific video camera. This type of identification can be made because the nonhomogeneous nature of dark currents in charge coupled devices (CCDs) can be detected by integrating multiple images and the distribution pattern of the nonhomogeneous dark currents is unique and intrinsic to a specific camera. The distribution patterns of the dark currents in nine cameras representing four different types were examined. In eight of the nine cameras (three types), unique detectable patterns were identified in recorded blank images, indicating that it should be possible to identify whether or not a given image had been recorded with a given camera. The method presented was used in an actual case to determine whether or not questioned video images of a criminal scene were recorded with the suspects camera, and the results of that effort are reported.

CCD fingerprint method for digital still cameras

We have reported the Charge Coupled Device (CCD) fingerprint method for identification of digital still cameras. The CCD fingerprint method utilizes the nonhomogeneous nature of dark currents in CCDs. In this study, we have measured CCD defects patterns of various digital still cameras including professional cameras and cheap ones with various resolution and compression rates. As a result, CCD defect pattern was detected in all cameras except for a low-resolution cheap camera using only one image. Resolution mode change of digital cameras did not affect the position of defect points in general but in some cases, relative pixel intensity varied. Image compression did not affect the pixel position for blank images within normal compression rate, but when there existed light in the background, the pixel position was blurred as the compression rate became high. In conclusion, it is recognized that the CCD fingerprint method can be applied in principle to digital still cameras, that is, individual camera identification can be achieved in principle by using images taken with the camera.

Case studies and further improvements on source camera identification

Actual case examples and further improvements on source camera identification are shown. There are three specific topics in this paper: (a) In order to improve performance of source camera identification, the hybrid identification scheme using both dark current non-uniformity (DCNU) and photo-response non-uniformity (PRNU) is proposed. The experimental results indicated that identification performance would be improved by properly taking advantage of their features; (b) Source camera identification using non-uniform nature of the CCD charge transfer circuit is proposed. The experimental results with twenty CCD modules of the same model showed that individual camera identification for dark images was possible by the proposed method. Furthermore, it was shown that the proposed method had higher discrimination capability than the method using pixel non-uniformity when the number of recorded image was small; (c) The authors have been performed source camera identification in the five actual criminal cases, such as homicide case, and so on. The analytical procedure was a sequential examination of hot pixel coordinates validation followed by the similarity evaluation of sensor noise pattern. The authors could clearly prove that the questioned criminal scenes had been recorded by the questioned cameras in four cases of the five.

Individual Camera Identification Using Correlation of Fixed Pattern Noise in Image Sensors

Abstract:  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.

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.

Wide-field time-resolved luminescence imaging and spectroscopy to decipher obliterated documents in forensic science

Abstract. We applied a wide-field time-resolved luminescence (TRL) method with a pulsed laser and a gated intensified charge coupled device (ICCD) for deciphering obliterated documents for use in forensic science. The TRL method can nondestructively measure the dynamics of luminescence, including fluorescence and phosphorescence lifetimes, which prove to be useful parameters for image detection. First, we measured the TRL spectra of four brands of black porous-tip pen inks on paper to estimate their luminescence lifetimes. Next, we acquired the TRL images of 12 obliterated documents at various delay times and gate times of the ICCD. The obliterated contents were revealed in the TRL images because of the difference in the luminescence lifetimes of the inks. This method requires no pretreatment, is nondestructive, and has the advantage of wide-field imaging, which makes it is easy to control the gate timing. This demonstration proves that TRL imaging and spectroscopy are powerful tools for forensic document examination.