|Year : 2015 | Volume
| Issue : 2 | Page : 93-98
Research on Several Inspection Methods to Determine the Sequence of Photocopying and Stamping
Collaborative Innovation Center of Judicial Civilization; Department of Forensic Science, Key Laboratory of Evidence Science ( University of Political Science and Law), Ministry of Education, China
|Date of Web Publication||27-Nov-2015|
Collaborative Innovation Center of Judicial Civilization, China. Key Laboratory of Evidence Science (China University of Political Science and Law), Ministry of Education
Source of Support: None, Conflict of Interest: None
Determining the sequence of document production and stamping is one of the most important issues in the field of questioned document identification. In China, whether a contract or a document is legal or not, highly depends on the relationship between the generation time of the document's content and its stamping. Usually, a formal (legal) document would become effective once a seal is affixed, and normally, the content of the document is generated first and then subsequently stamped after being approved by the relevant personnel. Therefore, correctly identifying the sequence used to produce a document is necessary to determine whether it is authentic or whether it may have been forged. However, because of the interference of many factors, the identification of such kind of forged documents has long been considered one of the most difficult technical issues in the field of questioned document identification. In this work, four nondestructive approaches to determine the order of photocopying and stamping were investigated: The stereomicroscope method, fluorescence microscopy, the three-dimensional pseudo-color method, and the contour ring method. Each method is associated with its own advantages and disadvantages, but all have been shown to produce some useful results relevant for determining the sequence of photocopying and stamping.
Keywords: Contour ring, fluorescence microscope, stereomicroscope, three-dimensional pseudo-color
|How to cite this article:|
Wang J. Research on Several Inspection Methods to Determine the Sequence of Photocopying and Stamping. J Forensic Sci Med 2015;1:93-8
The IDMH.Z780 system was developed and assembled by the Beijing
Langyuechuan company. This system combines hardware (including a CCD
obtained from the Germany Kappa company and a microscope from the Leica
Company) with 3D.processing software. This system is mainly used in the
field of document examination.
| Introduction|| |
The use of stamps in China has a long history. Stamps have been produced since the age of Shang Zhou, approximately 3600 years ago, but at that time, people used stamps for social purposes only. By the Qin and Han dynasties, approximately 2000 years ago, a complete stamping system was established, and beginning at that time, stamps began to symbolize social status and power. As time passed, the role of stamps in Chinese people's social life became increasingly important in areas ranging from personal exchanges and trade to politics, economics, culture, and all aspects of life. Even in modern Chinese society, without stamps, people cannot exercise power, sign contracts, release official documents or announcements, or engage in banking activities. Thus, stamps have occupied a very important position in Chinese society.
Generally, photocopying generates a copy of an original document using an electrostatic copier. Since the invention of copiers in the 1960s, these devices have been widely used in people's daily lives and work because of its convenience, low cost, and rapidity. Electrostatic toner is a type of dry ink and is paper permeable, only adhering to the surface of paper and forming a thin layer of pigment.
Photocopying and stamping result in three layers with overlapping areas: The paper fiber layer on the bottom, the first pigment layer (stamp impression or copy toner) in the middle, and the last pigment layer (copy toner or stamp impression) on the top. Therefore, regardless of the method used, identifying the photocopying and stamping sequence should be mainly achieved by comparing the appearance and microscopic morphology of the strokes on both the overlapping and nonoverlapping areas.
This work pursued the following research objectives:
Because of the widespread use of electrostatic copiers, people can not only copy a file's contents quickly but also acquire simulated images. Although people benefit from the convenience provided by these machines, criminals can also take advantage of them. We found that the number of cases involving photocopying is increasing annually, especially with regard to the sequence of photocopying and stamping. Photocopying has some similarities to laser printing, but some subtle differences do exist. Therefore, it is important to analyze the various methods of identifying the sequence of photocopying and stamping.
In China, when producing a formal document or contract, the content must be generated first, and then the content must be confirmed by the relevant person, and finally, the document must be stamped. In other words, the normal order of a document should be content before stamping; if a different sequence were followed, the document should be considered abnormal or fake.
Overcoming the difficulty of this analysis
Because it can be influenced by many parameters, identifying the sequence of content generation and stamping has remained a very difficult issue in the field of document examination. All three components – paper, stamp, and toner – strongly and directly influence the conclusion. In addition, the stamping strength and the medium's degree of hardness or softness also play important roles.
Multiple complementary methods
Currently, numerous methods of examining the sequence of content and stamping exist, but no complete solution exists. Many researchers are working to develop better methods, but until a complete solution is found, a variety of methods should be used to confirm the results.
| Materials and Methods|| |
This research investigated three different brands of copiers: RICOH MP3350B, CANON imageRUNNER 2520i, and Brother MFC 74700. These are all-in-one machines and represent commonly available brands. The three different brands of photocopy paper were found to exert no obvious impact on determining the sequence of photocopying and stamping.
Generally, the components of stamp ink are dyes, pigments, synthetic resins, surfactants, vegetable oil or mineral oil, high-boiling point solvents, and other substances. Most stamp impressions are made in red, and the ink can infiltrate into the paper fiber. In this work, four different kinds of stamps were investigated: Quick-drying inkpad, traditional stamp pad, atomic stamp pad, and photosensitive stamp pad. All of them are commonly available.
The three different copiers and the four different types of stamp ink were combined under a variety of different conditions: Seal pressure: Heavy, medium, and light; underlying surface: Soft, medium (approximately 1 cm), and hard; stamp ink concentration: Thick, moderately thick, and weak. In addition, the sequence of photocopying and stamping was varied. Two hundred experimental samples were generated.
Changes in the medium
This mainly refers to the sequence of stamping first and copying after. Because of the different toner-adsorption abilities of blank paper and the impression, the appearance of the toner stroke may vary between these two areas.
Superposition of the layers
After photocopying and stamping, three layers are formed in the overlapping areas: The paper fiber layer on the bottom, the first pigment layer (stamp impression or copy toner) in the middle, and the last pigment (copy toner or stamp impression) on the top. The existence of these three layers allows the examination of the sequence in which the toner and stamp ink were applied.
Differences in pigment thickness
Copy toner is typically paper impermeable, whereas stamp ink is permeable. When copying occurs first, the superposition of paper, toner, and ink is complete, but when stamping occurs first, the stamp ink may permeate into the paper fiber, resulting in incomplete superposition.
Equipment and methods
Stereomicroscopy is one of the most commonly used methods in the field of questioned document examination. The working principle of stereomicroscopy is the transmittance of two groups of optical images with different angles to peoples' left and right eyes to generate a binocular parallax, improve the image depth information in the brain, and thus form an image with a three-dimensional (3D) appearance.
The overlapping areas of toner and impression tend to form an additional 3D layer. By examining these areas, the levels at which each pigment is located can be determined, allowing the elucidation of the sequence in which they were applied. Using stereomicroscopy to examine the copying and stamping sequence mainly involves observations of the distribution and changes in the surface pigment in the overlapping areas. The changes can be determined by comparing the same strokes or lines in both the overlapping and nonoverlapping areas to identify whether the strokes in the overlapping areas exhibit abnormal characteristics, such as becoming thinner, broken, or diffuse. The presence of these characteristics indicates that the examined line was formed recently.
Fluorescence microscopes are high-end configuration microscopes in the field of questioned document examination. These instruments are equipped with continuous excitation sources and rely on the emission of visible fluorescence when subjected to excitation by one or more specific wavelengths to distinguish between different materials.
Stamp ink is a strong fluorescent material. It strongly absorbs ultraviolet and visible light with short wavelengths, and its fluorescence intensity is maximized when excited by green light with a wavelength of approximately 470 nm. In contrast, copy toner does not absorb this wavelength at all and thus emits no fluorescence. Using fluorescence microscopy to determine the copying and stamping sequence mainly involves observing or measuring the changes in the fluorescence intensity.
The IDMH-Z780 system is a commonly used instrument in document examination in China. It consists of a high-magnification microscope, a charge-coupled device, a computer, and software [Figure 1]. This system is capable of generating a 3D image from a two-dimensional image and displaying it as a pseudo-color image or a contour ring image according to the height.
|Figure 1: The IDMH-Z780 system. Left: High-magnification microscope. Right: The charge-coupled device. Center: The computer and software|
Click here to view
The 3D pseudo-color method is mainly used to describe the microscopic states of the overlapping areas using the colors available in the configuration software. If the pigment thickness or concentration differs, the displayed color will also differ. In this method, a higher 3D image height indicates a higher pigment concentration.
Contour refers to the closed coils that are connected by the points at equal height on a topographic map. Contour has been widely used in many fields, including geography, meteorology, and geology, and has broad applicability. In the contour ring method, images are shown in different colors and different closed coils to describe the pigment concentration in detail. The contour ring method focuses on the toner stroke's edge. By comparing the edge's shape and its completion state in both the overlapping and nonoverlapping areas, the copying and stamping sequence can be determined. This method is mainly used as an effective supplement to the 3D pseudo-color method.
| Results|| |
Feature 1: Differences in toner stroke integrity
In the sequence of stamp first and copy after, toner strokes in the overlapping areas may appear incomplete. This is mainly because the smoothness difference between the surface of the impression and that of the paper [Figure 2].
|Figure 2: Stereomicroscope method results. Left: The characteristics of the toner strokes when stamping occurred first in the sequence. Right: The characteristics of the toner strokes when copying occurred first in the sequence|
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The adsorption of the toner on the impression surface is less than that on the paper surface. Therefore, when copy toner is applied to an impression, the stroke may appear incomplete. This phenomenon can be observed at the edge of the strokes and in the middle of the strokes as well. The latter is referred to as the hollow-out phenomenon. This is one of the major features indicating that the sequence consisted of stamp first and copy after.
In contrast, in the sequence of copy first and stamp after, the toner stroke is relatively complete. In this sequence, the toner particles are applied to the paper surface only. Because the absorption ability of the paper surface is constant, the appearance of the strokes is consistent between the overlapping and nonoverlapping areas.
Regarding strokes that appear incomplete, some causal factors may result in toner strokes taking on a broken or incomplete appearance even when copying occurs first in the sequence. Therefore, before using this feature to determine the sequence, the same stroke must be compared in both the overlapping and nonoverlapping areas. If this characteristic is consistently observed throughout the stroke, it may be attributable to the use of a poor-quality copier. Conversely, if the characteristic is only observed in the overlapping area, it is a key indicator that stamping occurred first in the sequence. Therefore, in practice, every stroke must be examined to determine the regularity of the characteristics and exclude the interference of accidental factors.
Feature 2: Differences in toner stroke appearance
This characteristic usually occurs when stamping occurs after copying. In this case, the toner strokes may show infiltration and diffusion phenomena. This is because when stamping occurs first, the impression has usually existed on the paper for a long time and thus, most of the oil from the stamp has been volatilized or diffused into the paper fiber. As a result, when a copy is made using the stamped paper, the miscibility of the stamp ink and toner will be minimal. In contrast, if copying occurs before stamping, the oil from the stamp will dissolve the toner particles, thereby spreading the toner on both sides of the stroke and within the paper [Figure 2].
The phenomena of infiltration and diffusion may sometimes occur when stamping is performed before copying, particularly when copying immediately follows stamping. This is because that after a stamp is applied, if the ink is still very fresh when it is exposed to copy toner, the miscibility of the inks may occur as in the sequence in which copying occurs first. However, when stamping is performed first, the infiltration and diffusion phenomena may affect the whole toner stroke in both the overlapping and nonoverlapping areas. This difference can be very useful in distinguishing the applied sequence.
Feature 3: Differences in toner stroke color
This characteristic occurs when copying occurs first in the sequence. When red ink covers black pigment, green and yellow colors can sometimes be observed. That is to say, if the color is different in the overlapping areas, the document is produced using the correct sequence [Figure 2].
Feature 4: Differences in toner stroke height
This characteristic also occurs when copying occurs first in the sequence. Because the toner particles accumulate on the paper's surface, they exhibit a characteristic height. In some cases, if high pressure is used to apply the stamp, the height of the toner may be decreased. This phenomenon can be observed under a microscope [Figure 2].
The experiment revealed that the stereomicroscope method was useful to observe both the inkpad and traditional stamp pad, but its performance for the other two types of stamps was not as good.
Fluorescence microscope inspection
The fluorescence microscopy results clearly show differences between the two sequences. When stamping occurred first, the black strokes absorb the red fluorescence of the stamp ink and appear completely black [Figure 3], left side]. In contrast, when copying occurred first in the sequence, the red fluorescence can be seen clearly that covers the toner layer, and the characteristics are very clear and stable [Figure 3], right side].
|Figure 3: Fluorescence microscopy results. Left: Pictures of four types of stamps when stamping occurred first in the sequence. Right: Pictures of four types of stamps when copying occurred first in the sequence|
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Fluorescence microscope inspection is a very effective method for all four types of stamp investigated and is a nondestructive and simple technique. However, in some circumstances, when copying occurs after stamping, some red fluorescence may be observed from the toner strokes. This is because of the large gap between the toner particles. The red fluorescence of the stamp in the bottom layer can therefore be emitted through these gaps. This important issue should be considered when analyzing real samples. There are significant differences between fluorescence from below and above the toner. The former appears discontinuous and shows a weak intensity, whereas the latter is continuous and strongly intense.
3D pseudo-color method
In the 3D pseudo-color method, the change in the toner stroke color density is the key characteristic revealing the sequence used to generate the document. When stamping occurred first, the color density of toner stroke in the overlapping areas may exceed that in the nonoverlapping areas [Figure 4], left side]. Conversely, when copying was performed first, the color density of toner stroke in the overlapping areas should be almost the same as that in the nonoverlapping areas [Figure 4], right side]. However, this difference was only obvious for the quick-drying inkpad; the difference was less obvious for the other three types of stamp analyzed.
|Figure 4: Three-dimensional pseudo-color method results. Left: Pictures of four types of stamps when stamping occurred first in the sequence. Right: Pictures of four types of stamps when copying occurred first in the sequence|
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It is clear that this method produces beautiful and directly interpretable images, but has good performance for the inkpad only. Because this method is subject to the effects of many factors, including stamping strength, stamp color, penetration ability, and index choice, it requires further research.
Contour ring method
When stamping occurs before copying, the edge of the toner strokes may become blurred in the overlapping areas. The contour rings of the toner stroke and stamp may penetrate each other. However, in the nonoverlapping areas, the edge of the toner strokes should appear as clearly solid lines. Therefore, the strokes in the two areas appear substantially different [Figure 5], left side].
|Figure 5: Contour ring method results. Left: Traditional inkpad when stamping occurred first in the sequence. Right: Traditional inkpad when copying occurred first in the sequence. Green cross strokes are from stamping; yellow vertical strokes are from photocopying|
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When copying occurs before stamping, the edge of the toner strokes in the overlapping areas is flat and unbroken. Although the edge may appear slightly lighter in the overlapping areas than in the nonoverlapping areas and exhibit a slight deformation, the overall appearance should be the same in both areas [Figure 5], right side].
When using the contour ring method, the selection of the testing area is very important. For toner, complete strokes should be analyzed in both the overlapping and nonoverlapping areas. For stamp ink, locations where the red ink is relatively complete and uniform should be analyzed. These sampling locations are chosen to obtain results that are as close to reality as possible.
| Discussion|| |
In this study, all the four tested methods were found to exhibit different characteristics for some or all of the four types of stamps analyzed. From the stamp perspective, inkpads and traditional stamp pads are more likely to be examined, and almost all methods are useful for testing these two types of stamps. Among the methods, fluorescence microscopy is simple to operate and nondestructive and produces good test results for all of the types of stamp tested. This method is highly recommended.
This study shows that the 3D pseudo-color image and contour ring methods are currently able to provide some useful results for only one or two types of stamp. This is because they are limited by certain parameters, including the choice of the tested area and the operation index. Therefore, an expert may be needed to operate these systems.
Determining the copying and stamping sequence is a very difficult task, and multiple complementary methods are needed. Indeed, when more methods are used, the results are closer to reality. Therefore, methods to determine the copying and stamping sequence must continue to be developed and analyzed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]