Forensic examinationJack

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Forensic examinationJack Powered By Docstoc
					The forensic examination of laser printers and photocopiers using digital image
analysis to assess print characteristics
J.S. Tchan, MATAR Research Group, London College of Communication,
Elephant and Castle, London SE1 6SB, England.

Abstract
The work in this paper describes a method that can assist the process of print
identification with respect to the printing machine that produced it. The method used
high spatial resolution and low-noise digital image analysis to measure the size,
intensity and texture characteristics of individual text characters. The relative
variations of these variables were used to identify the machine that produced the print
under examination. The results showed that three machines could be distinguished
and one of these machines also showed differences in the print produced when the
toner cartridge was changed.

1. Introduction
A reason why it is frequently not feasible to prosecute counterfeiters and document
fraudsters is due to the difficulty in establishing links between the counterfeiters or
fraudsters and their printing equipment. This is because of the extremely wide range
of both cheap and expensive laser and inkjet printing machines available. This
problem is exacerbated by the fact that new varieties of printing machine are being
commercially produced on a continual and frequent basis

Also both laser and inkjet printers, which make up most of the cheap office printing
market, have disposable ink and toner cartridges, or the cartridges can be refilled.
This can prevent valid chemical analysis of any similarities or differences in chemical
compositions from the various ink and toner cartridges. Also chemical analysis is a
process that requires the destruction of part of the evidence.

Methods which involve microscopy1 are frequently used by forensic scientists to
determine the production source of digital print. The linking of a document to a
digital printer in these cases usually involves analysing variables such as ink or toner
overspray and assessing alignment, spacing and copy distortion.

Investigations have been carried out by Oliver and Chen2, Tchan, Thompson and
Manning3 and Tchan4,5 using digital image analysis to link documents to printing
machines. Oliver and Chen have studied the relationship between the raggedness of
print and text character distortion of different printers. Tchan, Thompson and
Manning have taken a similar approach but have also used neural networks to link the
contrast, noise and edge characteristics of printed text to the printing machine that
produced it. These attempts at using digital image analysis however could only
provide a positive test for a small range of printing machines.

The analysis of the actual shapes of the text characters produced by different print
engines using digital image analysis is another possible way of fingerprinting printing
machines, Tchan6. However, this method not only suffers from the influence of
replaceable ink and toner components distorting the results of the analysis, but other
drawbacks as well. Firstly, processing huge amounts of image data is time-consuming
due to the large number of fonts and their range of sizes from many different makes
and models of printing machines. Secondly, the problem of ink spread on different


                                           1
kinds of paper or humidity conditions in inkjet printing processes distorts the shapes
of text characters.

An identification methodology for fingerprinting printing machines recently
considered is to use a technique called ESDA7, (Electrostatic Detection Apparatus).
ESDA has been employed to detect and evaluate roller pressure marks on paper8.
These pressure marks are due to the interaction between the paper feeder rollers and
the paper substrate. The original application was for the detection of pen impressions
several layers down in a notepad. It works by charging paper surfaces with a high
voltage. If toner particles are applied to the charged paper surface, imperceptible pen
marks due to writing on a piece of paper placed above this sheet may be revealed.
This means for example, in a notepad, writing can be read from sheets many layers
down from the top sheet that only has the actual writing. As the ESDA technique has
been shown to detect weak imperceptible pressure marks from pens, it might be able
to detect pressure marks from printing rollers.

If the pressure marks can be detected, then to link a document to the printer that
produced it would require comparing the width and spacing characteristics of the
rollers of the machine in question. However, detecting the pressure marks may not
always be possible for the following reasons. Firstly, the pressure exerted by the
rollers is weak, generally much weaker than pen pressure. If the pressure marks are
present, they will sometimes be difficult to detect, even with the most sophisticated
digital image processing systems. Secondly, assuming that the pressure marks are
detectable, heavy handling can destroy them.

A similar technique that has been recently explored, concerns inkjet printers and
indentations on the paper after a printed sheet has been fed through the machine9.
These indentations are caused by the spoke wheels that feed the paper through the
inkjet printer and are most perceptible on moist parts of the paper caused by wet ink.
Due to the heavy pressure imparted by the spoke wheels, the indentations can be seen
using optical microscopy without the aid of ESDA. However not all inkjet printers
have spoke wheels so this technique does not apply to all inkjet printing systems.

Another type of method that has been considered for the identification of laser
printers exploits imperfections in the print known as banding. Banding imperfections
are lines across the printed page when smooth print is required10. The effect has been
attributed to the following two causes. Firstly, fine banding due to the imbalance of
the rotor component of the polygon mirror or mechanical weaknesses of the laser
scanning unit. Secondly, rough banding caused by unsteady motion of the
photoconductor drum or the fuser unit.

Mikkilineni et al11 have devised a system that uses a scanner to analyse relative
texture differences on a printed page caused by banding effects. This system has
shown that 9 out of a set of 10 laser printing machines were successfully identified.

The method described in this paper is an alternative method of measuring banding
effects in laser printers and photocopiers. Instead of scanning images and analysing
the relative texture of text characters, it uses a high resolution and low-noise digital
image analysis system to measure the following variables in printed text. These inter-



                                           2
related variables are sharpness, intensity, size and surface texture. The following
section describes the methodology and the experimental set-up involved.

2. Experimental procedure
When a completely black page was printed out on a photocopier, two different laser
printers and on one of the two laser printers with a different toner cartridge, table 1,
the following effects in figure 1 were seen. These are sketches of the banding lines
seen with an indication of their dimensions and separations. It was observed that some
of the horizontal lines from the HP 4250 for the same toner cartridge were not in fixed
positions and equal in number for different printed sheets. It is unknown whether the
lines were random or followed a complex pattern since further investigation of this
effect has yet to be made. However, for this part of the investigation, only a
confirmation of the existence of the banding effects that are a common feature of
digital printers was required.


                                  Printing System
                      Canon IR 3570 Photocopier
                      HP 1200 Laser Jet Printer
                      HP 4250 Laser Jet Printer Toner Cartridge 1
                      HP 4250 Laser Jet Printer Toner Cartridge 2


Table 1. List of the three printing machines and the printing machine with a change
of toner cartridge used in the investigation.

The experimental procedure can also be separated into three distinct stages. In the
first stage the banding effect was observed for a test page that was entirely covered in
solid toner as stated above. In stage two, a test page of the same text character was
produced and physical differences in the print were investigated for each printing
machine used. This was completed using high-resolution digital image analysis. In
stage three, a page of ordinary text was produced and patterns in the text were again
investigated using high-resolution digital image analysis. The process is illustrated in
figure 2, the flow chart below and will be discussed in greater detail later.

The effects of banding on printed text were investigated using a high-resolution
digital image analysis system, which has been built specifically to analyse the print.
Figure 3 illustrates how the camera was attached to the stand and how the lighting
source was attached to the camera.




                                           3
                                                         . . .




   Canon IR 3570 Photocopier                  HP 1200 Laser Jet Printer




  HP 4250 Laser Jet Printer                   HP 4250 Laser Jet Printer
  Toner Cartridge 1                           Toner Cartridge 2



Figure 1. Sketches of the lines produced by the four different printing samples used
in the investigation




                                          4
            Stage 1
            Look for banding effects on completely printed pages in black solid.
            This was done by visual inspection and using digital image analysis to
            measure print density differences on the printed pages.




           Stage 2
           For a test page consisting of letters of the same text character, the
           image analysis system was used to measure the relative differences in
           physical print characteristics of the individual text letters from
           different printing machines.



           Stage 3
           Sheets of ordinary text from two different toner cartridges used in the
           same laser printer were then analysed by the digital image analysis
           system, again by measuring the relative difference in the physical print
           characteristics of the text letters.




Figure 2. The three different stages of the investigation.




                                           5
                 Polaroid MP4
                 Land Camera
                 Stand


          To Meteor II
          Board and
          Computer


                  Hamamatsu
                  C4742-95
                  CCD
                  Camera


                                             Lens
                                                           LED lighting
                                                           system

               Print
               Sample




Figure 3. The camera, lens, lighting system and stand.

The camera employed in the investigation was a Hamamatsu C4742-95 camera. This
camera had a Peltier cooled CCD chip to increase the signal to noise ratio. The
camera was attached firmly to a camera stand that weighs approximately 30kg. The
lighting unit was firmly screwed onto the lens of the camera. The lighting unit
consisted of a circular array of red LEDS. The LEDS were connected to a laboratory
power supply with low ripple.

Figure 4 shows in block diagram form the different hardware and software
components of the image analysis system. The image data from the camera was
digitised to 8 bit resolution using Matrox Mil software. The data was subsequently
analysed using Visual Basic that was compatible with Matrox Mil. Algorithms were
developed using Visual Basic Active X language that could perform the following
computations on individual text characters.




                                         6
                      Hamamatsu camera digitised images of
                      individual text characters




                      Matrox Mil software interpreted the
                      digitised data so that it could be read
                      by Visual Basic Active X




                      Algorithms were written using Visual
                      Basic so that inter-related sharpness,
                      intensity, surface texture and size
                      measurements could be taken from the
                      text characters



Figure 4. The three stages required to make the necessary measurements for the
analysis.

Five measurements were taken from the text characters. They related to the image
intensity, size and texture of the print under investigation. These are illustrated below
in figure 5. Measurement 1 is the value of the location of the peak in standard 8 bit
greyscale. Measurement 2 is the size of the peak. Measurement 3 averages the overall
intensity of the image by integrating the total intensity of the image and dividing it by
the number of pixels from the text character. Measurement 4 measures the
smoothness of the print by giving the sum of the number of occurrences when
adjacent pixels are equal in greyscale value. Measurement 5 is the area of the printed
text character. Figures 5 and 6 and table 2 illustrate and summarise the measurements
made. The measurements were taken individually from each text character. The image
size, figure 5, was 480 by 483 and had a tonal resolution of 256.




                                           7
            25000
                                                              Background
                                                              Region
            20000
                             Print region
    Pixel   15000
    Count
            10000


             5000


                0
                    50         60        70          80        90        100        110
                                    13

                                         17

                                                21

                                                         25

                                                              29

                                                                    33

                                                                         37

                                                                               41

                                                                                    45

                                                                                          49

                                                                                               53
                    1

                         5

                              9




                                              Relative Intensity



Figure 5. An illustration of the acquired data from the image analysis system.




                                                     8
                                                        If adjacent pixels are
                                                        equal, a counter in the
                                                        algorithm is incremented
                                                        by 1.




Figure 6. A demonstration of how the surface smoothness is calculated. The
rectangle with the differently shaded small squares represents a region of print and its
unevenness.


  Measurement
       1              The value of the peak maximum
         2            Location of the peak maximum for the print region
         3            Integration of the peak area for the print region to determine the
                      average image intensity.
         4            Smoothness measurement for the print region by comparing
                      greyscale values for adjacent pixels. If the pixels of adjacent
                      pixels are equal, an indication of smoothness, a counter in the
                      algorithm is incremented by 1.

         5            The text character area is calculated by counting the number of
                      pixels beyond an arbitrarily selected threshold between the non-
                      image and image peaks figure 5.



Table 2. A summary of the measurements taken.

Figure 7 shows the two print samples used in the investigation. They were both
printed on the same batch of standard laser printer paper in all cases. The font was
Times New Roman and the font size was 22 pts. 22 pts is a large-sized font; it was
used because it facilitated relatively quick measurements to prove that the method is
viable.

A test sheet was produced that consisted of a series of the letter “W” in Times Roman
font and 22 points in size. There were 17 W’s across the page and 30 down the page.
The selection of the letter “W” was arbitrary. However, the size was important to
facilitate ease of measurement when recording the data using the digital image
analysis system.




                                           9
                                                                                   In the case of the page of normal text figure 7 on the right, a cardboard mask, figure 8,
                                                                                   was required to shield the effects of nearby letters influencing the readings since
                                                                                   unwanted parts of letters appeared in the image window. The test set of “W’s” did
                                                                                   not have this effect since the spacing of the “W’s” was designed to eliminate the
                                                                                   requirement for a mask. Figure 9 shows the ‘e’s that were analysed in the page of
                                                                                   normal text.


                                                                                            Text character number
                                                                                   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
                                                                                   WWWWWWWWWWWWWWWWW                                   The proposed project investigates the
30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1




                                                                                   WWWWWWWWWWWWWWWWW                                   possibility of using a computer and artificial
                                                                                   WWWWWWWWWWWWWWWWW                                   intelligence techniques to analyse painted
                                                                                   WWWWWWWWWWWWWWWWW                                   abstract artwork. The computer will then use
                                                                                   WWWWWWWWWWWWWWWWW                                   the results to produce new artwork
                                                                                   WWWWWWWWWWWWWWWWW                                   independently. In other words, the computer
                                                                                   WWWWWWWWWWWWWWWWW                                   learns about the work of artists and produces
                                                                                   WWWWWWWWWWWWWWWWW                                   new work that is original, aesthetically
                                                                                   WWWWWWWWWWWWWWWWW                                   pleasing and in the style of an artist in
                                                                                   WWWWWWWWWWWWWWWWW                                   question.
                                                                                   WWWWWWWWWWWWWWWWW
                                                                                   WWWWWWWWWWWWWWWWW                                   Computers already are used to generate art.
                                                                                   WWWWWWWWWWWWWWWWW                                   One famous example is the use of fractals.
                                                                                   WWWWWWWWWWWWWWWWW                                   Fractals are mathematical number patterns that
                                                                                   WWWWWWWWWWWWWWWWW                                   repeat on a smaller and smaller scale and can
                                                                                   WWWWWWWWWWWWWWWWW                                   be used to produce aesthestically pleasing
                                                                                   WWWWWWWWWWWWWWWWW                                   visual patterns. These patterns are also found
                                                                                   WWWWWWWWWWWWWWWWW                                   in nature in the shapes of leaves or trees for
                                                                                   WWWWWWWWWWWWWWWWW                                   example.
                                                                                   WWWWWWWWWWWWWWWWW
                                                                                   WWWWWWWWWWWWWWWWW                                   The differences between the existing methods
                                                                                   WWWWWWWWWWWWWWWWW                                   and the new proposed method for the
                                                                                   WWWWWWWWWWWWWWWWW                                   production of computer generated art are the
                                                                                   WWWWWWWWWWWWWWWWW                                   following. Firstly, the proposed method uses a
                                                                                   WWWWWWWWWWWWWWWWW                                   computer to learn about human aesthetic
                                                                                   WWWWWWWWWWWWWWWWW                                   preferences and to produce new original works
                                                                                   WWWWWWWWWWWWWWWWW                                   of art based on the preferences. Existing
                                                                                   WWWWWWWWWWWWWWWWW                                   methods d
                                                                                   WWWWWWWWWWWWWWWWW
                                                                                   WWWWWWWWWWWWWWWWW



                                                                                                   W Test Page                                     Normal Test Page



                                                                                   Figure 7. The two sets of test targets that were used to produce the results in this
                                                                                   investigation.




                                                                                                                              10
                            Mask made out of
                            cardboard




hes                                                 hes
 480 x 483 pixel
 window



Figure 8. In the case of the normal text page a mask was required to eliminate the
effect of adjacent text characters.




                                       11
                         The proposed project investigates the
             9           possibility of using a computer and artificial
                         intelligence techniques to analyse painted
                         abstract artwork. The computer will then use          25
                 4       the results to produce new artwork
                         independently. In other words, the computer           18
                         learns about the work of artists and produces
                         new work that is original, aesthetically              19

                 2       pleasing and in the style of an artist in
                                                                                    24
                         question.

                         Computers already are used to generate art.
                         One famous example is the use of fractals.
                         Fractals are mathematical number patterns that
                         repeat on a smaller and smaller scale and can         21
                         be used to produce aesthestically pleasing
                         visual patterns. These patterns are also found        22
                         in nature in the shapes of leaves or trees for
                         example.

                 1       The differences between the existing methods
                         and the new proposed method for the
                         production of computer generated art are the
                                                                               17
                 3       following. Firstly, the proposed method uses a
                         computer to learn about human aesthetic
                                                                               23
                         preferences and to produce new original works
                         of art based on the preferences. Existing

                 5
                             7       10         12        13        16
                                                                          20
                     6
                                 8                             15
                                           11
                                                     14




Figure 9. The letter ‘e’s that where selected from the test page for the classification
          of the two toner cartridges of the HP 4250.




                                                12
                    HP 4250 Toner cartridge 1 comparision between line 1 and 10 down the page


              3.850E+04
                                                                 Line 1
              3.800E+04

              3.750E+04
Area/Pixels




              3.700E+04
                                         Line 10
              3.650E+04

              3.600E+04

              3.550E+04                             11

                                                            13

                                                                 15

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                                                                           19

                                                                                 21

                                                                                      23

                                                                                           25

                                                                                                27

                                                                                                     29
                          1

                              3

                                   5

                                        7

                                                9




                                                         Text character number



                    HP 4250 Toner cartridge 2 comparision between line 1 and 10 down the page


              3.850E+04
                                                                            Line 10
              3.800E+04

              3.750E+04
Area/Pixels




                                       Line 1
              3.700E+04

              3.650E+04

              3.600E+04

              3.550E+04
                                                    11

                                                            13

                                                                 15

                                                                      17

                                                                           19

                                                                                 21

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                                                                                           25

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                          1

                              3

                                   5

                                        7

                                                9




                                                         Text character number




              Figure 10. A comparison of the text character size down line 1 and 10 for the two HP
                         4250 toner cartridges.




                                                          13
                                  Canon IR 3570                                                                Canon IR 3570


              4.1E+04                                                                       4.1E+04

              4.0E+04                                                                       4.0E+04




                                                                              Area/Pixels
Area/Pixels


              3.9E+04                                                                       3.9E+04

              3.8E+04                                                                       3.8E+04

              3.7E+04                                                                       3.7E+04

              3.6E+04                                                                       3.6E+04




                                                                                                                   10

                                                                                                                        13

                                                                                                                             16

                                                                                                                                  19

                                                                                                                                       22

                                                                                                                                             25

                                                                                                                                                  28
                                     10

                                          13

                                               16

                                                    19

                                                         22

                                                               25

                                                                    28




                                                                                                      1

                                                                                                          4

                                                                                                              7
                        1

                            4

                                7

                                    Text character num be r                                                       Text characte r num be r



                                     HP 1200                                                                       HP 1200


              3.6E+04                                                                       3.6E+04

              3.5E+04                                                                       3.5E+04
Area/Pixels




                                                                              Area/Pixels
              3.4E+04                                                                       3.4E+04

              3.3E+04                                                                       3.3E+04

              3.2E+04                                                                       3.2E+04

              3.1E+04                                                                       3.1E+04
                                     10

                                          13

                                               16

                                                    19

                                                         22

                                                               25

                                                                    28




                                                                                                                   10

                                                                                                                        13

                                                                                                                             16

                                                                                                                                  19

                                                                                                                                       22

                                                                                                                                             25

                                                                                                                                                  28
                        1

                            4

                                7




                                                                                                      1

                                                                                                          4

                                                                                                              7
                                    Text character num ber                                                        Text characte r num be r



                            HP 4250 Toner cartridge 1                                                     HP 4250 Tone r cartridge 1


              4.0E+04                                                                       4.0E+04

              3.9E+04                                                                       3.9E+04
Area/Pixels




                                                                              Area/Pixels




              3.8E+04                                                                       3.8E+04

              3.7E+04                                                                       3.7E+04

              3.6E+04                                                                       3.6E+04

              3.5E+04                                                                       3.5E+04
                                     10

                                          13

                                               16

                                                    19

                                                         22

                                                               25

                                                                    28
                        1

                            4

                                7




                                                                                                                   10

                                                                                                                        13

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                                                                                                                                             25

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                                                                                                      1

                                                                                                          4

                                                                                                              7




                                    Text character num ber                                                        Text characte r num be r



                            HP 4250 Tone r cartridge 2                                                    HP 4250 Tone r cartridge 2


              4.0E+04                                                                       4.0E+04

              3.9E+04                                                                       3.9E+04
                                                                              Area/Pixels
Area/Pixels




              3.8E+04                                                                       3.8E+04

              3.7E+04                                                                       3.7E+04

              3.6E+04                                                                       3.6E+04

              3.5E+04                                                                       3.5E+04
                                                                                                                   10

                                                                                                                        13

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                                                                                                                                  19

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                                     10

                                          13

                                               16

                                                    19

                                                         22

                                                               25

                                                                    28
                        1

                            4

                                7




                                    Text characte r num be r                                                      Text characte r num be r


                   Figure 11. A comparison of the text character area down line 1 of the W test pages.
                   Left, for 3 adjacent pages, right their average.


                                                                         14
                                   Canon IR 3570                                                                 Canon IR 3570


               4.0E+04                                                                        4.0E+04

               3.9E+04                                                                        3.9E+04
Area/Pixels




                                                                               Area/Pixels
               3.8E+04                                                                        3.8E+04

               3.7E+04                                                                        3.7E+04

               3.6E+04                                                                        3.6E+04

               3.5E+04                                                                        3.5E+04




                                                                                                                                   11

                                                                                                                                         13

                                                                                                                                              15

                                                                                                                                                   17
                                                                                                        1

                                                                                                             3

                                                                                                                  5

                                                                                                                        7

                                                                                                                              9
                                                      11

                                                           13

                                                                15

                                                                     17
                         1

                              3

                                    5

                                          7

                                                  9
                                    Text character num ber                                                         Text character num ber



                                        HP 1200                                                                     HP 1200


               3.8E+04                                                                        3.8E+04

               3.7E+04                                                                        3.7E+04
 AreaP/ixels




                                                                                Area/Pixels
               3.6E+04                                                                        3.6E+04

               3.5E+04                                                                        3.5E+04

               3.4E+04                                                                        3.4E+04

               3.3E+04                                                                        3.3E+04




                                                                                                                                   11

                                                                                                                                         13

                                                                                                                                              15

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                                                                                                        1

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                                                      11

                                                           13

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                                                                     17
                         1

                              3

                                    5

                                           7

                                                  9




                                    Text character num ber                                                         Te xt character num ber



                             HP 4250 Tone r cartridge 1                                                     HP 4250 Toner cartridge 1


               3.8E+04                                                                        3.8E+04

               3.7E+04                                                                        3.7E+04
Area/Pixels




                                                                               Area/Pixels




               3.6E+04                                                                        3.6E+04

               3.5E+04                                                                        3.5E+04

               3.4E+04                                                                        3.4E+04

               3.3E+04                                                                        3.3E+04
                                                      11

                                                           13

                                                                15

                                                                     17




                                                                                                                                   11

                                                                                                                                         13

                                                                                                                                              15

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                         1

                              3

                                   5

                                          7

                                                  9




                                                                                                        1

                                                                                                             3

                                                                                                                  5

                                                                                                                        7

                                                                                                                              9




                                    Te xt character num ber                                                        Text characte r num be r



                             HP 4250 Toner cartridge 2                                                      HP 4250 Toner cartridge 2


               3.9E+04                                                                        3.9E+04

               3.8E+04                                                                        3.8E+04
Area/Pixels




                                                                                Area/Pixels




               3.7E+04                                                                        3.7E+04

               3.6E+04                                                                        3.6E+04

               3.5E+04                                                                        3.5E+04
               3.4E+04                                                                        3.4E+04
                         1

                               3

                                    5

                                           7

                                                  9

                                                      11

                                                           13

                                                                15

                                                                     17




                                                                                                                                   11

                                                                                                                                         13

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                                                                                                        1

                                                                                                             3

                                                                                                                  5

                                                                                                                        7

                                                                                                                              9




                                     Text character num be r                                                       Text character num ber


                     Figure 12. A comparison of the text character area across line 1 of the W test pages.
                     Left, for 3 adjacent pages, right their average.


                                                                          15
                                   Canon IR 3570                                                                            Canon IR 3570


Relative average   55.4                                                                               55.4




                                                                               Relative average
                   55.2                                                                               55.2
    intensity




                                                                                   intensity
                    55                                                                                     55

                   54.8                                                                               54.8

                   54.6                                                                               54.6




                                                      11

                                                           13

                                                                15

                                                                     17




                                                                                                                                               11

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                          1

                              3

                                    5

                                          7

                                                  9




                                                                                                                1

                                                                                                                        3

                                                                                                                            5

                                                                                                                                    7

                                                                                                                                           9
                                     Text character number                                                                      Text character number



                                        HP 1200                                                                                  HP 1200


                   55.5                                                                             55.5




                                                                               Relative intensity
Relative average




                   55.3                                                                             55.3
    intensity




                   55.1                                                                             55.1

                   54.9                                                                             54.9

                   54.7                                                                             54.7




                                                                                                                                               11

                                                                                                                                                    13

                                                                                                                                                         15

                                                                                                                                                              17
                                                      11

                                                           13

                                                                15

                                                                     17




                                                                                                            1

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                                                                                                                                  7

                                                                                                                                        9
                          1

                               3

                                    5

                                           7

                                                  9




                                     Text character number                                                                  Text character number



                              HP 4250 Toner cartridge 1                                                             HP 4250 Toner cartridge 1


                   55.2                                                                               55.2
                                                                               Relative average
Relative average




                    55                                                                                     55
                                                                                   intensity
    intensity




                   54.8                                                                               54.8

                   54.6                                                                               54.6

                   54.4                                                                               54.4
                                                                                                                                               11

                                                                                                                                                    13

                                                                                                                                                         15

                                                                                                                                                              17
                                                                                                                1

                                                                                                                        3

                                                                                                                            5

                                                                                                                                    7

                                                                                                                                           9
                                                      11

                                                           13

                                                                15

                                                                     17
                          1

                              3

                                    5

                                          7

                                                  9




                                     Text character number                                                                      Text character number



                              HP 4250 Toner cartridge 2                                                             HP 4250 Toner cartridge 2

                   55.4                                                                               55.4
                                                                               Relative average
Relative average




                   55.2                                                                               55.2
                                                                                   intensity
    intensity




                    55                                                                                     55

                   54.8                                                                               54.8

                   54.6                                                                               54.6
                                                      11

                                                           13

                                                                15

                                                                     17




                                                                                                                                               11

                                                                                                                                                    13

                                                                                                                                                         15

                                                                                                                                                              17
                          1

                              3

                                    5

                                          7

                                                  9




                                                                                                                1

                                                                                                                        3

                                                                                                                            5

                                                                                                                                    7

                                                                                                                                           9




                                     Text character number                                                                      Text character number




                          Figure 13. A comparison of the relative average intensity across the W test pages.
                          Left, for 3 adjacent pages, right, their average.


                                                                          16
                                        Canon IR 3570                                                                      Canon IR 3570

 Size of peak/Pixels




                                                                                   Size of peak/Pixels
                       10100                                                                              10100

                        9600                                                                              9600

                        9100                                                                              9100

                        8600                                                                              8600




                                                          11

                                                               13

                                                                    15

                                                                         17




                                                                                                                                             11

                                                                                                                                                  13

                                                                                                                                                       15

                                                                                                                                                            17
                               1

                                   3

                                        5

                                              7

                                                      9




                                                                                                                  1

                                                                                                                      3

                                                                                                                           5

                                                                                                                                 7

                                                                                                                                         9
                                         Text character number                                                              Text character number



                                            HP 1200                                                                            HP 1200
 Size of peak/Pixels




                                                                                    Size of peak/Pixels
                       10100                                                                              10100

                        9600                                                                               9600

                        9100                                                                               9100

                        8600                                                                               8600




                                                                                                                                             11

                                                                                                                                                  13

                                                                                                                                                       15

                                                                                                                                                            17
                                                                                                                  1

                                                                                                                      3

                                                                                                                           5

                                                                                                                                 7

                                                                                                                                         9
                                                          11

                                                               13

                                                                    15

                                                                         17
                               1

                                   3

                                        5

                                              7

                                                      9




                                         Text character number                                                              Text character number



                                   HP 4250 Toner cartridge 1                                                          HP 4250 Toner cartridge 1
Size of peak?pixels




                                                                                   Size of peak/Pixels




                       10100                                                                              10100

                       9600                                                                                9600

                       9100                                                                                9100

                       8600                                                                                8600
                                                          11

                                                               13

                                                                    15

                                                                         17
                               1

                                   3

                                        5

                                              7

                                                      9




                                                                                                                                             11

                                                                                                                                                  13

                                                                                                                                                       15

                                                                                                                                                            17
                                                                                                                  1

                                                                                                                      3

                                                                                                                           5

                                                                                                                                 7

                                                                                                                                         9



                                         Text character number                                                              Text character number



                                   HP 4250 Toner cartridge 2                                                          HP 4250 Toner cartridge 2
                                                                                   Size of peak/Pixels
Size of peak/Pixels




                       10100                                                                              10100

                       9600                                                                                9600

                       9100                                                                                9100

                       8600                                                                                8600
                                                                                                                                             11

                                                                                                                                                  13

                                                                                                                                                       15

                                                                                                                                                            17
                                                                                                                  1

                                                                                                                      3

                                                                                                                           5

                                                                                                                                 7

                                                                                                                                         9
                                                          11

                                                               13

                                                                    15

                                                                         17
                               1

                                   3

                                        5

                                              7

                                                      9




                                         Text character number                                                              Text character number




                               Figure 14. A comparison of the text character peak intensities across the W test
                               pages. Left, for 3 adjacent pages, right, their average.



                                                                              17
                                                        HP 4250 Toner cartridge 1


              1.615E+05
              1.610E+05

Area/Pixels   1.605E+05
              1.600E+05
              1.595E+05
              1.590E+05




                                                                    11


                                                                           13


                                                                                    15


                                                                                          17


                                                                                               19


                                                                                                    21


                                                                                                           23


                                                                                                                25
                           1


                                    3


                                            5


                                                    7


                                                             9
                                                                 Text character num ber



                          HP 4250 Toner cartridge 1. Average readings for the chart im m ediately above.


              1.605E+05
              1.603E+05
Area/Pixels




              1.601E+05
              1.599E+05
              1.597E+05
              1.595E+05                                             11


                                                                           13


                                                                                    15


                                                                                          17


                                                                                               19


                                                                                                    21


                                                                                                           23


                                                                                                                25
                           1


                                    3


                                            5


                                                    7


                                                             9




                                                                 Text character num ber



                                                        HP 4250 Toner cartridge 2


              1.620E+05
              1.615E+05
Area/Pixels




              1.610E+05
              1.605E+05
              1.600E+05
              1.595E+05
                                                                    11


                                                                           13


                                                                                    15


                                                                                          17


                                                                                               19


                                                                                                    21


                                                                                                           23


                                                                                                                25
                           1


                                    3


                                            5


                                                    7


                                                             9




                                                                 Text character num ber



                      HP 4250 Toner cartridge 2. Average readings for the chart im m ediately above.


              1.608E+05
              1.606E+05
Area/Pixels




              1.604E+05
              1.602E+05
              1.600E+05
              1.598E+05
                            1   2   3   4   5   6   7    8   9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
                                                                 Text character num ber




         Figure 15. The area differences using the page of text for the HP 4250 laser printer
         for the two toner cartridges.



                                                                    18
3. Results
Firstly, the accuracy of the system was established by assessing variations in the
intensity and area measurements for a single text character sampled 50 times. A
maximum value from the mean of +/-0.05% was observed when the mask was not
employed and +/- 0.03% when the mask was used for the intensity measurements.
These error values were tripled in size for the area measurements.

Secondly, two W test pages from the HP 4250 printer, one from each of the two toner
cartridges were analysed. This, as in all of this investigation, required individual
sequential manual alignment and subsequent measurement from each text character
on a line of text. Figure 10 shows how the size of the letter ‘W’ for line 1 and 10 of
the grid changes down the page for the HP 4250 printer using the two different toner
cartridges. The consistent patterns indicate that the digital image analysis system has
recorded meaningful results.

Thirdly, a demonstration was made of how the four printing samples could be
distinguished using the W template. This was achieved by using three adjacent W test
sheets from each of the four printing samples in a print run of 10. The reported results
in this section used only the first horizontal and vertical lines of the W test sheets
because of the time-consuming nature of recording the measurements and associated
time constraints of the researcher. The time problem was only discovered during the
experimental phase and centred on alignment difficulties of the text characters in the
image window. The left hand graphs figures 11 to 14 show the individual data from
the three sheets and on the right hand side their averages.

It was shown that the peak size, the average intensity and the size measurements
yielded useful information for the classification process. The smoothness values were
inconsistent and produced no useful classification information. The reasons for the
surface smoothness variable not yielding conclusive results were probably a
combination of toner inconsistency, a low volume of processed data and the nature of
the algorithm employed. The position of the peaks represented in figures 5,6 also
provided no useful data, as they did not vary significantly in position. However, the
most useful data came from the size measurements, which produced the clearest
patterns between the printer samples and their production source.

The experimental results using the W template show that all four printing samples
could be differentiated by a combination of the analysis of the area variation of the
text characters down the page and the average intensity, peak size and area variations
of the text characters across the page, figures 11 to 14.

Finally, the masking technique was employed, figure 8, with the printed page of
normal text shown in figures 7,9 to find differences from the different printing
examples. In this case from the HP 4250 printer when the toner cartridge was
changed. Figure 15 shows a shallow well or an inverted sharp spike at position 15 for
toner cartridge 1 and an inverted sharp spike for toner cartridge 2 at position 14. This
result was obtained from print runs of 201. The graphs. figure 15, show the results
from sheets 1 of 101 and 201. The measurements were also sampled for other sheets
in the print run at 20 sheet intervals (numbers 1,21,41,61,81,101,121,141,161,181 and
201 in total) and were 100 percent consistent in the fact that the inverted sharp spike



                                          19
at position 14 only appears for all print samples from toner cartridge 2 and not at all
for toner cartridge 1.

4. Conclusions
The results of the investigation thus far demonstrate the potential of the method for
the forensic analysis of print; both in linking a machine to a particular document and
to show whether a document has been tampered with.

It has been shown that a small number of toner based printing systems can be
classified using high-resolution image analysis to measure the relative changes in the
physical properties of individual text characters both along and across pages of
printed text. Even at this stage of its development the system has potentially useful
forensic applications. Also, these results correlate with the work carried out by
Mikkilineni et al on the measurement of surface texture by scanning the text
characters of laser printers.

In the case of this investigation more work is required on smaller text characters and
many more different text pages at much larger volumes. This is necessary since the
analysis of smaller text characters at greater numbers will improve the precision of the
system. The reliability of the method must also be tested for test runs of thousands of
printed sheets. Potential changes in the patterns of the measured values for the print
characteristics can then be observed for a sequence of a large number of pages.

However, the further work stated above requires better measurement techniques and
precision. Also the current masking method is difficult to carry out for small text
characters and needs to be improved. More crucially, since the method is time-
consuming, an automated system that uses a high-precision scanning device is
needed. This would greatly improve the volume of print that can be processed in a
given time. This should enable a greater number of machines to be classified with
greater precision. In addition, it would be worthwhile to try the techniques on inkjet
printing, which have the additional problems of ink spread and very uneven print.



References

1. B.S. Lindbolm, R. Gervais, Scientific Examination of Questioned Documents,
Taylor and Francis, 2006.

2. J. Oliver, J. Chen, “Use of Signature Analysis to Discriminate Digital Printing
Technologies” IS&T NIP 18, 2002, pp. 218-222.

3. J.S. Tchan, R.C. Thompson, A. Manning, "The Use of Neural Networks in an
Image Analysis System to Distinguish Between Laser Prints and their Photocopies"
The Journal of Imaging Science and Technology 44, (2) pp. 2000 132-144.

4. J.S. Tchan "Classifying Digital Prints According to their Production Process
Using Image Analysis and Artificial Neural Networks" Proceedings of SPIE; San
Jose, California: Optical Security and Counterfeit Deterrence Techniques III, 2000,
3973 pp. 105-116.


                                          20
5. J.S. Tchan “The Development of an Image Analysis System that can Detect
Fraudulent Alterations Made to Printed Images.” Proceedings of IS&T and SPIE
Optical Security and Counterfeit Deterrence Techniques V, 2004, 5310 pp.151-159.

6. J.S. Tchan “Forensic Analysis of Print Using Digital Image Analysis”
Proceedings of SPIE; Santa Clara, California: Human Vision and Electronic Imaging
VIII. 2003, 5007, pp. 61-72.

7. J. Levinson, Questioned Documents A Lawyers Handbook, Academic Press,
London 2001.

8. G. M. Laporte, “The Use of an Electrostatic Detection Device to Identify
Individual and Class Characteristics on Documents Produced by Printers and Copiers-
A Preliminary Study” J Forensic Sci, 49, (3), pp.610-620 (2004).

9. Y. Akao, K. Kobayashi, Y. Seki “Examination of Spur Marks Found on Inkjet
Printed Documents” J Forensic Sci 50, (4), pp. 915-923 (2005).

10. J. You, “Banding Reduction in an Electrophotographic Printer” J. Imaging Sci
and Technol 49, (6), pp. 635-640 (2005).

11. A. K. Mikkilineni, P. Chiang, G. N. Ali, G.T.C. Chiu, J. P. Allebach, E. J. Delp,
Printer “Identification Based on Graylevel Co-occurrence Features for Security and
Forensic Applications” Proceedings of Spie, 5681, 2005, pp. 430-440.




.




                                         21

				
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