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Counterfeit Detection - Tool Marks at Criminal Scenes

The Application of Portable Digital Microscope Camera for Tool Marks at Criminal Scenes

Abstract

At the scenes of criminal cases, tool marks often exist on the objects which were destroyed by the criminals. Due to the differences among criminal tools, derived tool mark is one of the most common signs in the scene of a larceny case. Larceners use the tools (such as pliers, iron skid, screwdriver, etc.) to damage iron windows, door locks, drawer and safe box, then break into and steal the properties, and repeatedly commit the crimes with the same tools, so these tool marks will become important linking evidences. Tool marks have the features of high emerging probability, easy to find, easy to collect, not easily disappear or destroyed, strong stereoscopic impression, and etc, which can be used for the analysis of crime tactics, criminal history, the authenticity of the case, and types of tools, etc. In accordance with the results of tool marks comparation, the criminal investigators are able to link the larceny cases happened in different places, and increase the number of case solving.

However, the formation of tool marks are derived from the integrated effects of acting force, tools and destructed objects and others, it possesses variability and complexity due to the direction of force, the used tools and the material features of destructed objects. This paper will explore the feasibility of using portable digital microscopic photography to collect tool marks at criminal scene, to allow inspection staff to take rapid and effective collection of tool marks at the criminal scenes, to help the criminal forensics officers to facilitate identification and to enhance the case solving rate of criminal cases.

Foreword

In forensics of criminal case, tool marks are very important linking evidences that criminal tools and criminal scene or every criminal scene can be closely connected together through the identification of tool marks. Dr. Li Changyu pointed out that any act of criminal suspects will lead to the change of the micro matter evidences, and criminal investigators can identify the basic situation of the case, prove the criminal facts and the connection between the criminal suspects and the case through analyzing these changes. Tool marks are most difficult to be destroyed and can reach evidential level in those micro-matter evidences.   However, the tool marks possess diversity and complexity, in the absence of convenient tools to help criminal forensics officers to reduce difficulties of man made collection, these tool marks will always be forgotten and neglected at the criminal scenes.

Tool mark is a trace left after using tools to destruct certain objects, and it is a image of the surface of the tool. At the criminal scenes, traces of tool marks are often be found, especially at criminal scenes of the larceny case, according to the contact differences of criminal tools and destructed objects, there are two types of marks can be produced, namely, linear marks and depression marks. In this paper, we will treat linear marks at the criminal scene as a major object of research, since it is the most man-force-wasting marks in comparing identification currently. When a tool contacts the surface of the destructed object with different acting angles, there will produce different liner marks, therefore, forensic staffs often need to test the patterns of the tool marks with different acting angles. Setting a screwdriver (screwdriver) as an example, forensic staffs must prepare at least four tool marks (test marks) with different angles based on each surface mark, and all test marks shall be compared with the tool marks at the criminal senses.

According to the statistics of varied criminal cases from National Police Agency of Ministry of the Interior, in 1994, there were 555,109 criminal cases, wherein 328,154 were theft cases, accounting for 59.11%, in addition, larceny cases were still holding the majority position of the total criminal cases. When criminals clearly know he should wear gloves to avoid leaving fingerprints as offending the property of the victims at the criminal scenes, collecting fingerprints of criminals at the criminal scenes has become a dream which is more and more unable to reach. However, as criminals using tools to undermine protection equipment(such as bars, locks, etc.) to reach the purpose of invasion or disabling victims’ resistance, in the destruction process, the tool will left surface structure marks on the destructed objects at the scenes. Based on the identification of criminal tool marks, criminal forensics staff can determine the type of crime tools, the authenticity of the scene, the way of crime, possible profession and qualities that criminals may engaged in, so as to analyze the case, identify and narrow the scope of the investigation, and to take the necessary investigation, link the relations of tools and the criminal scene, tools and the criminals, to let criminals take full responsibilities of their crime.

Since the force acting angle, material of tool marks and other problems, will bring difficulties for comparing identification, and that is quite different as the tool marks produced by bullets, therefore, how to effectively digitalize and restore the tool marks of criminal scene for further objective statistic and analysis will substantially increase the willing of using tool marks by inspecting staff at criminal scenes.

Experimental Method

I. instrument and equipment::

1.

Microscopic camera:


This research uses portable X-Loupe G20 microscope camera (such as photo 1) shooting tool marks at the criminal scene, and make comparison with tool marks shot by macro-lens.

Photo 1: X-Loupe G20 potable microscope camera

(a) front view
(b) top view
(c) back view
(d) bottom view
2.

Replaceable Microscopic lens:

 

The microscopic objective lens applicable for portable X-Loupe G20 include 60 X, 150X, 300X, Respective resolution(Resolution) is  90.5 , 181 and 228 lp / mm, all of which have eight low-angle LED lights to change the direction of light sources.

3.

Macro lenses:

 

Canon EF 100 mm F2.8 macro.

4.

Ball type pan-tilt:

 

The ball type pan-tilt can be used to make quickly and roughly adjustment of the lens to be parallel with shooting surface.

5.

Bi-directional fine-tuning pan-tilt:

 

UN-5694 can be finely adjusted about 55 mm between front and back and 50 mm between left to right, it is fitted for macro photography, as shown in photo 2.


Photo 2:UN-5694 bi-directional fine-tuning pan-tilt.

II. Digital image processing:
1. Image (Image Composition):

Since the tool marks are three-dimensional marks, light source from different angles has a decisive influence to the quality of file of tool marks, and light source from different orientation have already caused an obvious the differences of the showing of lines showed by photo 3. (a) Image of 10 o'clock and lateral to the light source, (b) Image of the 4:00 o'clock and lateral to the light source, when the light source is in different direction, the shadowing features of line are also different, while (c), (d) can not record the feature of tool marks.. feature line marks will seriously affect the further comparing  performance of the system, in here, we will apply the achievements of image composition (image composition) technology to the field of forensics, use image composition technology to enhance the quality of feature marks, and research the influence this method of handling applications brings in the field of forensics.

Photo 3:  
(a)Image of 10 o'clock direction light source
(b) Image of 4 o'clock direction light source
(c) Image of 7:00 o'clock light source
(d) Image of 1:00 o'clock light source
(e) the result of composition of (a) and (b) images.
2. Photomerge

Photomerge uses photomerge feature to merge several partial photos together, this will not only exclude other thing other than main image outside the photo, but also enhance image identification features. As shown in photos 4, (a) was shot by the standard-lens, since it is only possible to shoot the whole main object from distant position, so there will be other interfering objects in the image (such as vehicles, trees); (b) is the result of merging several photos, each of them was shot at close distance, so it did not has other interfering objects; (c) the image of ads on the glass window clipped from photo (a); (d ) the same image of ads on the glass window clipped from photo (b),by comparing (c) and (d), it is clear that the merged image has higher resolution.

Photo 4:

 
(a) Image shot by standard lens
(b) the result of photomerge by images from different shooting angles
(c) clipped image of the ads from (a)
(d) clipped image of the ads from (b)

Results and Discussion

I. Differences of microscopic photography and macro photography:

MTF map is the abbreviation of Modulation Transfer Function, an assessment method for testing contrast and sharpness of the lens. Most of lens use - 10 lp / mm and 30 lp / mm as the assessment standards of MTF map, since the resolution of the microscope lens are far greater than the general macro lens, the images of tool marks shot by microscopic lens and macro lens under the circumstance of full aperture will be directly compared. As shown in photo 5, (a) the tool marks shot by macro lens, (b) the tool marks shot by microscopic lens, at the same zooming rate, the tool marks on the same position are displayed in (c) and (d ),from (C) and (d), it can be clearly identified that the image shot by microscopic lens has high-resolution (Resolution).

Photo 5: with same tool marks and same zooming rate

 
(a) shot by macro lens
(b) shot by microscopic lens
(c) Image of tool marks shot by macro lens
(d) Image of tool marks shot by microscopic lens

II. Light source:

If the shooting light source almost parallel to the surface of objects, the minor changes of the height of the object’s surface and the micro features on object's surface all can be clearly recorded. Although there already has "low angle ring light" for sale on the market, but for the easy operation at criminal scenes, this research had used "snake lights," as a low angle light source, and can also get images with good quality. As microscopic cameras have built-in light source, in addition to coaxial light source, the microscopic objective lens also has a low angle light source, the photo 6 shows the  impact caused by shooting the coin (NT50-) under the light source of different orientation, the different orientations of light source direction will cause the shadow of marks to have different positions, when shooting by ring light source, such as photo 6 (i), the main part of marks will be shot down, but more subtle marks will be dim. Photo 6 (j) is the result of image composition of (a) - (b), it has the complete record of the main contours and subtle features, can be used as the main basis for future forensics.


Photo 6: shooting coins with light source from different orientation

(a) the right light source
(b) the bottom light source
(c) the left light source
(d) the top light source
(e) the upper right light source
((f) the lower right light source
(g) the lower left light source
(h) the upper left light source
(i) ring light source
(j) image composition results

III. the problems of shooting: at greater zoom rate:


1. Spherical aberration:

As the focus point of the center and the edge of lens are not consistent, when the light going through spherical surface lens, those light relatively further from the main axis of lens will be affected by mirror reflection –to focus at another point other than focus point, such will cause a dim image, this kind of effect is called spherical aberration. Since the depth of field of high-magnification lens is very short, the dim phenomenon caused by spherical aberration is far more obvious, as shown in photos 8.

Photo 8:The image of the tool marks under the situation of spherical aberration.
2. The difficulty of focusing:

Since the depth of field of microscopic camera lens is very short, together with the influence of spherical aberration, it is very difficult to make one time shooting to get complete and clear image of tool marks, it is suggested that after check the tool marks, several photos of tool marks from different position shall be taken by microscopic camera, then use image composition technology to construct the complete and clear image of tool marks. As shown in photo 9, after check the tool marks, it was found that a clear and unspoiled tool marks belongs to the first half part, respectively, to shoot left and right half of the tool marks using the microscopic camera, and two of the images are required of partial overlapping, after the image composition, a complete and clear image can be derived.

Photo 9:

 
(a) Left half of the tool marks
(b) Right half of the tool marks
(c) Result by image composition of (a) and (b)

Conclusion

The purpose of this research is mainly in discussion of using portable digital microscopy camera to shoot the photo of tool marks which are very important and difficult to take back at the criminal scenes, and use the photo as a basis for future identification of tool marks, for the evidence which can be taken back to the laboratory and observed with microscope, we do not suggest to use microscopic camera. However, for the evidences which require rapid identification at the criminal scenes, microscopic cameras can be used as another powerful weapon by the staff inspecting the criminal scenes, such as traffic accident cases, the recording and analysis of the accumulation direction of the car paint of scratching marks on the car body, quick check of the glass debris, paint debris, breakage of fabric fiber on the clothing of the victims or the troublemakers; inspection of suspicious forged documentations; inspection of suspected counterfeit coins,…….etc.

 
Creative Commons License
The Application of Portable Digital Microscope Camera for Tool Marks at Criminal Scenes by Yu Chiuchung. Fan Chaohsing. Lin Mingfeng is licensed under a Creative Commons Attribution 3.0 United States License.  

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