Spectral methods in criminalistics
• documentation expertise;
• identification of different material evidence;
• expertise of medicinal, narcotic and poisonous substances, foodstuffs;
• expertise of fibers, hair, plastic products;
• expertise of paint-and-lacquer coatings;
• expertise of oil products falsification;
• expertise of falsification of wines and spirits;
• expertise of precious stones and jewelry.
Spectral methods are used during the forensic medical examination: spectral determination of blood, sperm stains. Detection of washed out blood traces and blood traces located on multicolored, dark and dirty objects. Spectral methods allow detecting the authenticity of works of art (paintings) eliminating the falsification. But this expertise represents the individual field of study.
PRINCIPLES OF IMPLEMENTATION OF SPECTRAL ANALYSIS METHOD
The following spectral ranges are usually used for forensic processing: UV A-ultraviolet, long waves (UV-A) – 320–400 nm; visible range – 400–780 nm; near infrared (IR) range (IR-A, Near-IR, NWIR) – 780–1 400 nm [1–6].
Two main methods of spectral analysis can be marked out: point spectral analysis and two-dimensional spectral analysis. In the first case, illuminating certain point on the surface of tested object the spectrum of radiation reflected in this point is recorded. In the second case, the whole surface of object is illuminated, and then photographic recording (two-dimensional recording) of radiation reflected from the object surface is performed.
Also, the methods of spectral analysis can be divided into two classes: microspectral analysis (spectral analysis of objects is performed on the basis of observations using microscope) and macrospectral analysis (spectral analysis of macro-objects is performed).
Let us consider the method of two-dimensional macrospectral analysis. The spectral analysis of objects in this paper means recording of object radiation within various spectral ranges. At the same time, it is assumed that the object is illuminated by standard light sources (solar illumination, halogen or fluorescent lamps). Usually recording of object image is performed with the help of digital cameras. The structure of sensing array of recording device is executed in such a way that the coatings of three types transmitting the colors in three different spectral ranges (blue, red and green) are applied in staggered order on sensing elements. Respectively, the object image is recorded within three spectral ranges: blue, green and red (Fig. 1).
Traditionally, in order to obtain images within different spectral ranges the narrow-band interference filters are used. And the designers of measurement systems use several schemes of device arrangement depending on the color filter position in the unit of illumination source or image detector. The standard method of image recording with the help of digital camera does not use the special color filters in source or detector.
Other option of the scheme provides the allocation of special color filters in the unit of illumination source. The scheme is implemented using the color filters, which emit the required spectral range in broad-band radiation of light source (sunlight, lamp), or using the narrow-band radiators (color light-emitting diodes). But in order to record the image it is desirable to use black-and-white camera.
In the third variant, broad-band light source is used, and the necessary spectral range is detected using the color filter installed in front of recording device (Fig. 2). In such case, in the same manner as in previous case, in order to record the signal it is desirable to use black-and-white camera.
There is configuration of measurement system when the color filters are installed in the unit of radiation source and in front of recording device. Such configuration is the basis of universal recording complexes which are equipped with the sets of color filters (Fig. 3).
In order to record the multispectral photographs within the visible region of spectrum, use of common digital cameras is also possible. In order to record the images within UV and IR region, it is necessary to use special cameras developed for the registration of images within the relevant spectral ranges.
OF DOCUMENTATION EXPERTISE
Spectral method allows determining by which method the tested document was made: using the technology of graphic art, Xerox machine, dot-matrix printer, inkjet, laser or sublimation printer. For this purpose, it is sufficient to analyze the structure of dye distribution in printed characters.
For example, how to determine the authenticity of seal (original seal or seal applied by scanning and printing with color printer). For this purpose, the seal is examined with high-power magnification, and it allows distinguishing the original seal from the seal executed with the help of desktop publishing. The fact is that the picture obtained on computer consists of the set of colored dots. Thus, we see the mosaic printing. And original seal represents the picture consisting of coloring particles with the same color.
In order to determine the modifications made in the document after its execution (etching, adscript, correction), it is required to examine the image of suspicious fragment created in UV or IR beams. Corrections, which were later made in the document, are clearly seen within these ranges (Fig. 4). The spectral method grants the opportunity to identify the structure of pen ink. Examination of reflection spectrum of the trace of pen filled with the inks, which are different by the chemical structure, is performed on the basis of analysis of lettering.
The spectral method allows determining the presence of special marks, stamps, microfonts and other protection elements on documents and securities [7–10].
USE OF IR SPECTROMETRY
FOR DOCUMENTATION EXPERTISE
IR spectroscopy allows restoring dimly-visible and spilled-on texts at the expense of differentiation of spectral properties of different text fragments. The efficiency of methods of IR spectroscopy is based on the fact that the values of reflection factor for various materials within IR region significantly differ. Detection of hidden text on documents takes place at the expense of the fact that various types of dyes differ by the degree of absorption and reflection of IR beams. Therefore, the method is used not only for the detection of erased traces but also for the identification of securities. IR spectroscopy is used for the expertise of banknotes in order to determine their authenticity at the expense of visualization of graphic elements applied with special dye.
In order to obtain the object photograph within IR rays, it is illuminated by incandescent lamp creating the intense beam of IR rays. And the color filter, which cuts off the visible light, is located in front of the lens of digital camera. Photographing must be performed with high exposure in order to obtain bright photograph.
LUMINESCENT METHODS FOR DETECTION OF COUNTERFEIT BANKNOTES
Of course, the higher level of protection of banknotes is provided by holographic marks applied on the surface and read at the certain wavelength and certain viewing angle. Use of holo-pixels demonstrates the dependence of brightness stereogram element on observation direction. But application of holographic elements requires the use of special holographic printers. There is quite simple technological method of banknote protection when the special fibers or marks, which begin luminesce during illumination, are included into the structure of paper stock intended for the production of securities or banknotes. It is well known that the luminescence spectrum is shifted to the region of longer waves in comparison with the light of probing radiation. Therefore, when irradiating with UV light such fragments start emitting the light within visible range (Fig. 5, 6) and in case of irradiation with visible light they start emitting within IR range.
UV luminescence is also used for the search of traces of fingerprints on smooth surfaces at the expense of illumination by UV lamp and detection of traces of gunpowder around gunshot wound on skin surface.
EQUIPMENT FOR SPECTRAL STUDIES
In Russia the equipment for spectral studies is produced by the following companies: "Rastr" (Veliky Novgorod), "EVS" (Saint Petersburg), "Spektr" (Moscow), "Vildis Tekhnologii" (Moscow) (Fig. 7–9).
Types of studies supported by TV complex "Expert-K" produced by CJSC "EVS" include studies of the character and color of luminescence of objects in UV illumination from built-in sources with the central wavelength of 365 nm; multispectral studies within narrow-band subranges of wavelengths 570, 610, 645, 695, 780, 850 and 950 nm using 14 cutting color filters; studies of IR luminescence of dyes; studies of anti-Stokes luminescence.
The complex "Distekh-VSK" produced by "Vildis Tekhnologii" includes two special digital video cameras with 3.2 million pixels each, 16 illumination sources with the wavelength of 254 to 940 nm, various geometry, 16 optimally selected filters [11–18].
Equipment for spectral studies is also produced abroad by the following companies: Projectina (Switzerland), Froster&Freeman (UK), MS MacroSystem (Nederland), CRAIC Technologies (USA) (Fig. 10, 11). Comparison of equipment parameters is given below.
COMPACT SPECTRAL DEVICES
Some companies produce compact devices for documentation studies within different spectral ranges. Such devices are usually called video-magnifier, video-mouse (Fig. 11–13).
The light sources based on light emitting diodes are used in television magnifier "Videomysh VM-2": blue (470 nm), green (567 nm), yellow (590 nm), red (655 nm), IR (810 nm) (see Fig. 12).
Multimode colored megapixel television magnifier USB 2.0 "BTP-1332" is intended for the check of documents, banknotes and securities for the presence and compliance of security elements (relief, microprinting, peculiarities of lines of background grids and ornaments, metamerism of dyes, IR, UV luminescence of fragments and fibers etc.) with the visualization of magnified image on display screen (see Fig. 13).
PROCESSING OF MULTISPECTRAL DATA
Multispectral data processing requires use of the special software. And more importantly, the special methods of multispectral image processing are required. Earlier, such methods were used during processing of multispectral space photographs. Processing of multispectral data can be simulated using the program Photoshop. Every colored image consists of three monochromatic images (channels) – red, green and blue. When processing the images, operations are performed simultaneously for three channels. But using the function "Image-Adjustment-Levels", each channel can be corrected independently. Channels can be reviewed separately using the function "Window-Channels". The program allows copying every channel into separate image and processing independently from other channels.
Two-Dimensional Spectral Histogram
When processing black-and-white images, the histogram of brightness distribution is used. It constructs the dependence of number of dots in image on the level of brightness. When processing colored images, three histograms are plotted, for each one of three introduced components (blue, green, red). When processing multi-zone components, the histograms for each recorded range of wavelengths can be plotted.
Task of Segmentation of Multi-Zone Images
One of important tasks of processing of multispectral data consists in the segmentation task (object selection). For segmentation it is important to form the criterion of proximity for two image points. If the measure of proximity is lower than some threshold, then these image points refer to one object. Intensity of the element located in row X and column Y and recorded at the wavelength λ of image can be described by the function F (x, y, λ). Segmentation can be performed on the basis of spectral maximum. This model of segmentation works well in case when there are several different areas (components) on multispectral image, and each area has its maximum of absorption. Let us assume that there is multispectral image or several photographs obtained in different spectral ranges (Fig. 14). Let us consider some point on the photographs. Let us analyze the value of intensity of this point in different photographs. The number of spectral photograph, in which the value of point intensity was maximal, must be assigned to this point. This operation should be performed for all points in image. As a result, we will obtain segmented image. Image points which have maximum in the same spectral photograph will be given identical numbers. Therefore, the points with similar spectral characteristics will have the same number. Obviously, the number of point types (number of segments) will not exceed the number of spectral ranges.
Boundary Detection in Multi-Zone Image
Several methods of trace boundary detection in multi-zone image can be mentioned here. The brightness jump is calculated in every point of component image, and the difference between brightness values is calculated in adjacent points.
The first one of them relies upon brightness multi-zone imaging on the basis of all components. The brightness of every point in brightness image is equal to the sum of brightnesses of all images. If in some point the brightness jump is higher than threshold value, then this is the boundary point.
In the second method, every component of multi-zone image is analyzed separately. The point is considered to be boundary if the brightness jump in this point is higher than threshold value. In resulting image the point is considered to be boundary if it is boundary point at least for one component. The method of analysis when every component of multi-zone image is analyzed separately can be used. The point is considered to be boundary if the brightness jump in this point is higher than threshold value. In resulting image the point is considered to be boundary if it is boundary point for all components.
Thus, the methods of spectroscopy used in the practice of forensic inquiry expand the analysis capabilities. Further development and implementation of standardized measurement methods will allow using the results of spectral analysis in the capacity of evidence basis.