Signs of authenticity of banknotes and determination of the method of manufacturing counterfeit banknotes..
Signs of authenticity of banknotes and determination of the method of manufacturing counterfeit banknotes
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There are several approaches to solving the problem of checking banknotes for authenticity. Recently appeared currency authenticity testers have defined one of them: unlimited trust in the results of machine verification. However, as practice shows, testers are capable of detecting only the crudest types of counterfeits, since usually one or two parameters are checked, in the best case three. Judging the authenticity or counterfeit of a banknote by only one or two parameters is practically impossible. Today, unfortunately, a machine cannot yet replace a person in this area. But skillful use of testers and some simple devices in combination with human skills and knowledge of the characteristic features of banknotes allows for the detection of counterfeits quite effectively. It is from this point of view that all further material is presented.
Banknotes are products of printing industry with some specific properties. The main materials used in printing are paper and paints. But the materials used for making banknotes have a number of characteristic properties.
Paper
The paper used to produce banknotes is different from the consumer paper used to produce regular printed products. It does not contain optical brightener and therefore appears dark in filtered ultraviolet light. General-purpose paper will luminesce with blue or bright blue light. This is clearly visible even if you illuminate the side of a banknote pack. However, it should be noted that if a banknote gets into a solution of washing powder (for example, during accidental washing), the paper will absorb the optical brightener and will luminesce in UV light. Paper, with very rare exceptions, has a two-tone watermark (contains alternating darker and lighter areas that differ from the rest of the banknote), clearly visible when held up to the light. It must have slightly blurred, fuzzy contours. This is due to the fact that the thickness of the paper changes smoothly. If a banknote has a watermark with clear contours and consisting only of dark or light areas, then it is necessary to check the presence of other security elements. Often, colored security fibers are introduced into the paper of banknotes, which, as a rule, luminesce under ultraviolet light. In addition, plastic, metallized and metallic threads are quite common, sometimes coming to the surface of the banknote from the front side, the so-called «floating».
Paints
The paints used in the production of banknotes differ from conventional printing paints. They are more resistant to various chemicals and do not change their color. A ferromagnetic pigment is introduced into the paints, which triggers various testers. The use of pigments that luminesce under ultraviolet radiation (red, green, and yellow glow) is very common. Under the influence of various chemicals (washing powders, solutions used in dry cleaning, solvents), paints can partially change their original color, and sometimes components that glow under UV radiation are washed out.
Printing reproduction of images
The clearest and most contrasting images on banknotes are made using the intaglio printing method, which allows for images with a fairly thick paint layer. At low magnification, minor paint bleeds between the paper fibers are visible along the edges of the strokes. The thickness of the paint layer in the middle of the stroke is greater than at the edges. The protective grid and rosettes on the unprinted field are printed using the iris printing method, which allows for smooth transitions from one color to another. This printing method is one of the varieties of flat offset printing. At low magnification, it is clearly visible that the paint layer is thin. Paper fibers are visible through it. The serial number is applied using a numerator, which is a letterpress printing form. If you carefully examine the serial number at low magnification (8—10 5x 0), you can notice a number of features:
— a clearly visible paint border along the edges of the strokes;
— the edges of the images are even and clear;
— the presence of paper deformation in the form of traces of pressure from the printing elements of the numberer.
All images on banknotes are made only with strokes of varying width. If the guilloche frame has a solid paint fill instead of thin strokes, or the images consist of small dots of three colors, this should alert you. In certain places on the banknote, there is microprinting — small repeating texts that in most cases can be seen with a small increase. Drawings for alignment are widespread. One part of the image is on the front side, and the other on the back. When viewed against the light, they must match exactly and (or) form some image. In the thin lines of the guilloche frame, there is sometimes a latent (hidden) image that is visible only in oblique light. Relatively rare are kinegrams, which are colorful, iridescent images that can only be seen in parts, turning the banknote at certain angles. These images are almost impossible to counterfeit.
How to determine the authenticity of a banknote
If you receive a banknote that is in doubt about its authenticity or you do not have information on this type of banknote, you need to do the following:
— check that the portrait on the banknote matches the designated denomination;
— check for a two-tone watermark (there are no watermarks on US dollars);
— find security fibers, which should be located not only on the surface, but also inside the paper pulp;
— check for a security thread. If it is “floating”, then be sure to examine the banknote in transmitted light (the thread should look like a solid dark strip);
— check the quality of reproduction of small elements;
— the most contrasting images should be in relief;
— check the exact match of the drawings for alignment;
— all details should be made only with strokes of different widths;
— all images on the banknote should be clear, color transitions should be smooth;
— using a magnetic sensor, determine the presence of ferromagnetic pigment in the paint (serial number, dark-colored images);
— make sure that the banknote paper does not glow under ultraviolet radiation;
— under UV radiation, fibers and/or circles (plates), as well as individual images that were not visible under normal lighting, should glow.
Types of counterfeit banknotes
In practice, there are two methods of counterfeiting banknotes. One of them is partial counterfeiting. In this case, some of the details indicating the denomination of a banknote are replaced in some way with the details of a banknote of higher denomination. The alteration of banknotes is carried out by counterfeiters manually using commonly available tools and materials. The details indicating the denomination of a banknote are removed mechanically or painted over. New images are drawn by hand, printed using letterpress or screen printing. In some cases, missing images are cut out of one banknote and pasted onto another. This type of counterfeiting is easy to recognize, since counterfeiters cannot achieve satisfactory results in this way.
Another method is to produce completely counterfeit banknotes. In these cases, unregulated materials and technological processes are used. This can be either a crude counterfeit or one made at a fairly high technical level, with an imitation of a watermark, protective fibers and threads and tactile properties similar to a genuine banknote. The quality of counterfeit banknotes depends on the technical equipment, professional skills of the counterfeiter and the conditions in which the sale is expected.
Printing methods used to produce counterfeit banknotes.
Letterpress printing forms have a spatial separation of the printing and non-printing elements: the relief printing elements are in the same plane, and the non-printing areas are recessed. Since all the printing elements are located in the same plane, during the printing process they are covered with a layer of paint of uniform thickness and are brought into contact with the printed surface (pressed into the paper), as a result of which the following characteristic features remain on the print:
— the presence of traces of pressure of the printing elements, expressed in the deformation of the paper in places of colorful images;
— the paper fibers are completely covered with a layer of paint;
— along the contours of the images there is a colorful border, which appeared as a result of paint spreading.
Flat offset printing
This printing method involves the use of industrial equipment and therefore allows for a fairly large amount of «products». The printing process is based on the selective wetting of non-printing elements with water (or aqueous solutions), and printing elements with oil-based paint. Metal plates are used as forming materials for the production of printing forms. The suitability of a particular metal is determined by its physical, chemical and mechanical properties. Depending on the technology of manufacturing printing forms, printing elements can be created directly on the forming material or on an intermediate (copy or varnish) layer. Direct and offset printing can be done from flat printing forms. The direct method, often called lithographic, is low-productivity and is almost never used today. In offset printing, paint is transferred to paper through an intermediate rubber web, due to which the pressure on the printing form is sharply reduced, which reduces its wear and allows for an increase in printing speed. A wide variety of printing forms are used, which, according to the manufacturing method, can be divided into two large groups: photomechanical and electrophotographic. Photomechanical forms are obtained by copying a negative or positive (depending on the type of copying layer and the type of printing form) onto a form material coated with a light-sensitive layer. Electrophotographic printing forms are obtained directly — by directly copying the original onto the form material, or indirectly — by transferring the image from an intermediate plate onto the form material.
The most characteristic features of the flat offset printing method:
— uniform filling of the stroke with paint;
— the thickness of the paint layer is small, paper fibers are visible through it;
— there are no traces of pressure of the printing elements (no traces of paper deformation);
— the edges of the strokes have a smooth, slightly wavy border.
A rather rare method of intaglio printing is used to obtain images with a sufficiently thick layer of paint, forming a relief in the places of the images. Printing forms have a spatial separation of non-printing and printing elements. Printing elements of different depths, in most cases, are small cells separated by thin partitions. Non-printing elements are raised and are in the same plane. During the printing process, paint is applied in excess to the entire surface of the printing form, and then removed from the non-printing elements with a special device. Depending on the depth of the printing elements, the strokes of the image have different thicknesses of the paint layer. The following features are present on the print obtained by this printing method:
— the paint is unevenly distributed across the width of the stroke; in the middle it lies in a thicker layer than at the edges;
— the images are on the convex surface of the paper;
— the strokes have smooth edges.
Genuine banknotes are made using the intaglio printing method, which is a type of intaglio printing. The edges of the strokes have an uneven border due to the paint spreading between the paper fibers (viscous paints are used during printing, the pressure between the printing and form cylinders is about 2 tons). Screen printing is very rarely used to make counterfeit banknotes. Screen printing forms are grids stretched over a frame. The blank elements are covered with a layer through which the printing ink does not pass. The printing elements are open and the paint is pressed through them onto the surface to be printed using a special device.
This printing method has the following characteristic features:
— all images consist of colorful bumps;
— the paint layer is quite thick;
— images appear three-dimensional;
— small texts and images are reproduced with significant distortion or are missing.
Inkjet printing
The colorful image is formed directly on the printed surface by drops of paint from a system of nozzles. Currently, peripheral devices for personal computers operating on this principle are widely used. More common are printers using water- or alcohol-soluble paints, which are copied well. There are models that use liquid printing dyes and wax-based paints heated to a liquid state before starting work. Under a microscope, it is visible that all images consist of small, randomly located dots of three (yellow, purple and blue) or four (the same + black).
Electrophotography
Electrophotography is based on the property of photoconductors to change their electrical parameters under the influence of light. The surface of the photoconductor is uniformly charged with static electricity and an image is projected onto it using an optical system or it is recorded line by line by a laser beam. The areas of the photoreceptor that were exposed to light lose their electrical charge. The charge is retained in the illuminated areas. The latent electrostatic image is revealed by electrographic powder (toner). To do this, the powder particles are given a charge of opposite polarity compared to the charge of the latent image. The powder particles are attracted to the charged areas of the photoreceptor, making the image visible. The powder image is transferred to paper and fixed there.
Photoreceptor characteristics. Electrophotographic devices can be identified by the photoreceptor material (selenium, cadmium sulfide, or organic photoconductor). The line of the image obtained on the selenium layer has poor filling of the central part with powder in wide areas (edge effect). On the surface of the copy there are a large number of dots — marshes. Copies obtained on an organic photoconductor or cadmium sulfide have a small number of dots — marshes and uniform distribution of powder along the line.
Characteristics of latent image development methods. The quality of the copies obtained depends to a large extent on the development method, in addition, each of the methods has its own characteristic features revealed during microscopic examination of the object.
Cascade method:
— poor reproduction of halftone images;
— presence of traces from the media in the form of point dents, or the media itself (glass balls).
Magnetic brush:
— the copy is clean, there are no dots or smudges;
— halftone images and large solid areas of images are reproduced well;
— the magnetic media can transfer with the toner to the copy and become fixed on it, which triggers the sensors for ferromagnetic pigment.
Aerosol method (dust cloud):
— no edge effect;
— high resolution;
— no dots — smudges on the copy.
Methods of fixing the image.
Thermal fixing of images: where the paper is completely covered with a layer of toner, there are areas with a smooth, matte surface. The areas located below these areas have a smooth, shiny surface. This is explained by the fact that to fix the image, the copy passes between two heated rollers. The electrographic toner melts and is fixed on the surface of the paper.
With thermal fixing, the copy passes under an IR lamp. In this case, the toner melts and hardens without any mechanical action, forming a smooth, shiny layer on all areas.
Color electrophotography.
On an analog color xerox, images are obtained by the same principle as in a regular black-and-white one. The difference is that the original is exposed three times through different filters. Each color-separated image is developed with toner of one of the three primary colors (yellow, purple and blue), and when they are superimposed, a full-color image is obtained. The copy retains all the features characteristic of a regular black-and-white xerox copy. On a device with digital image processing, three color-separated images are obtained using filters. The difference is that the image is read line by line (for example, with a step of 60 microns) and information about the illumination of individual points of the image is sent to the microprocessor. Then the image in digital form is sent to the processing unit, from there to the printing laser system, as in a regular laser printer (currently, there is no technique for distinguishing a digital xerox from a color laser printer). In the printing unit, the laser beam is deflected by a rapidly rotating polygon (multifaceted) mirror and exposes the light-sensitive cylinder line by line. The intensity of the laser beam changes in accordance with the brightness information of the original. After the image recording process is complete, a cassette with toner of the corresponding color approaches the cylinder and the usual process of developing and transferring the image to paper begins. By superimposing single-color images, a full-color copy is obtained. At a slight increase, it is visible that all images consist of separate lines. Some models of color copying and duplicating machines with a laser reproducing unit leave a hidden mark of yellow toner on their copies. This mark is clearly visible in UV light. Canon, Kodak, and Agfa machines are manufactured by the same company, are produced at the same plant, and differ only in software. They leave a pattern on their copies that resembles a protective grid of small dots made with yellow toner.
According to the available verified information, the mark is placed by Canon CLC-350, CLC-550; Kodak ColorEdge 1525+ and 1550+. Rank Xerox devices leave a mark in the form of local groups of dots (matrix of approximately 8 x 15 dots) located in a checkerboard pattern. According to verified information, the mark is left by Xerox 5765, Xerox 4235 models. According to information from a fairly reliable source, Minolta color copying and duplicating machines place a mark similar in structure to the mark of Rank Xerox devices. It was possible to find out that Canon produces a device that determines the model, brand and factory number of the device that made the copy by the mark. Rank Xerox produces software that allows you to decipher the information contained in the mark.
Let's hope that in the foreseeable future, forensic departments will adopt this new product and, with the help of this equipment, will be able to solve issues related to deciphering the coded information contained in the tag.