Abstract:
A method and an apparatus to obtain a surface mapping of a ballistic piece of evidence (BPOE) under examination, such as a bullet or a spent cartridge case, that can be used thereafter as a 3D signature for identifying purpose during ballistic test comparison. The method comprises providing a measurement unit adapted to acquire a relief map of the surface of the BPOE and acquiring with the measurement unit the relief map of the surface to thereby obtain the mapping of the surface of the BPOE. Preferably, the measurement unit of the present invention comprises a confocal sensor such as confocal microscope. Also, the present invention includes acquiring the relief map of the bullet surface or of the cartridge case surface by acquiring and assembling a mosaic of regional reliefs that are partly overlapping with their surroundings regional reliefs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. Ser. No. 10/836,315 filed May 3, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a system and a method for the analysis and comparison of ballistic pieces of evidence such as fired bullets and spent cartridge cases. More specially, it relates to a computerized system and method for ballistic piece of evidence (BPOE) analysis based on a 3D imaging of the BPOE surface.  
       BACKGROUND OF THE INVENTION  
       [0003]     In the field of forensic science, investigations of crimes involving firearms use ballistic comparison test to determine if a bullet or a spent cartridge case found on the crime scene has been fired by a firearm in question. Ballistic comparison tests rely on the fact that when a bullet is fired by a firearm, striations are created on the bullet surface and these striations have enough unique features to represent a signature of the firearm. With regards to spent cartridge cases when a firearm is fired unique identifiable features (marks) are impressed or striated onto various areas of the cartridge case. These unique marks are transferred from the firearm to the cartridge case each time the firearm is discharged; these marks also represent the signature of the firearm. Therefore by comparing the striations or impressed characteristics of two bullets or two cartridge cases, it is possible to conclude if they have been fired by the same firearm.  
         [0004]     Most existing automatic ballistic comparison systems use 2D imaging techniques to obtain images of the striations or impressed marks on the ballistic piece of evidence (i.e. the bullet or the cartridge case) under test. They then compare these images to other images from a database of known firearms. The firearm that was used to fire the ballistic piece of evidence (BPOE) under test can be identified when a match is obtained between the images of the BPOE under test and the images of the database corresponding to a BPOE fired by the same firearm.  
         [0005]     However, ballistic matching techniques based on 2D-imaging present many drawbacks. In particular, it is found that the technique lack robustness: the images resulting from a 2D imaging are very dependent on the exact way the BPOE is illuminated and imaged as well as on the surface conditions of the BPOE, therefore affecting the performance of the technique.  
         [0006]     Recently, Bachrach et al. in their U.S. Pat. No. 6,505,140 B1 have proposed to use a confocal technique, also referred to as a 3D-imaging technique, to better study and identify the features of a bullet surface. A confocal sensor enables one to measure the striation structure and this leads to a more reliable way than a 2D-imaging technique to characterize the striations on a bullet surface. However, the striation characteristics are resolved along particular direction of a bullet striation, to obtain what is called a depth profile of the bullet striation. Therefore, although a 3D-imaging technique is used, no real 3D bullet signature is established and used for further ballistic comparison.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, an object of the present invention is to provide a method and an apparatus to obtain a surface mapping of a BPOE under examination that can be used thereafter as a 3D signature for identifying purposes during ballistic test comparison.  
         [0008]     More specifically, in an embodiment, the present invention provides a method of mapping a surface of a BPOE. The method comprises acquiring a plurality of regional reliefs, each regional relief corresponding to a region of the plurality of regions, and assembling the plurality of regional reliefs into a mosaic for obtaining a relief map of the surface to thereby obtain a mapping of the surface of the BPOE.  
         [0009]     Preferably, the present invention provides a method for acquiring the relief map of the surface of the BPOE by acquiring and assembling a mosaic of overlapping regional reliefs. The method comprises selecting at least one region that is partly overlapping at least another region for thereby acquiring at least two overlapping reliefs, each having an overlapping area; and aligning the at least two overlapping reliefs such that the overlapping areas are substantially overlapping.  
         [0010]     Advantageously, the overlapping area of an overlapping relief represents approximately 50% of the surface of the overlapping relief, when the BPOE that is under examination is a bullet.  
         [0011]     Advantageously, the overlapping area of an overlapping relief represents approximately 50% of the surface of the overlapping relief, when the BPOE that is under examination is a cartridge case.  
         [0012]     Preferably, a confocal sensor is provided to perform the mapping of the surface of the BPOE.  
         [0013]     In another preferred embodiment, the present invention provides an apparatus for mapping a surface of a BPOE. The apparatus comprises a holder for holding the BPOE and a measurement unit for measuring a light intensity from the BPOE as a function of a measurement position, wherein the measurement position is a relative position between the BPOE and the measurement unit. The apparatus also comprises a displacement unit for providing a series of different measurement positions and a controller unit to control the displacement unit for mapping the surface of the BPOE.  
         [0014]     Preferably, the measurement unit comprises a confocal sensor such as a confocal microscope. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description and accompanying drawings wherein:  
         [0016]      FIG. 1A  is schematic view of a computerized system for bullet ballistic analysis based on 3D imaging in accordance with a first embodiment of the present invention;  
         [0017]      FIG. 1B  is schematic view of a computerized system for spent cartridge case ballistic analysis based on 3D imaging in accordance with a second embodiment of the present invention;  
         [0018]      FIG. 2A  is a flow chart of a surface mapping method of a bullet in accordance with one embodiment of the present invention;  
         [0019]      FIG. 2B  is a flow chart detailing the steps of acquiring the relief map of  FIG. 2A , in the case where a mosaic of regional reliefs is acquired;  
         [0020]      FIG. 3  is a flow chart detailing the steps of acquiring a mosaic of regional reliefs of  FIG. 2B  when a confocal microscope is used;  
         [0021]      FIG. 4  is a flow chart detailing the steps of measuring a series of light intensities of  FIG. 3 ;  
         [0022]      FIG. 5  is a flow chart for a method of acquiring and assembling a mosaic of overlapping reliefs in accordance with one embodiment of the present invention  
         [0023]      FIG. 6A  is an example of a regional relief obtained with the surface mapping method of a bullet in accordance with one embodiment of the present invention;  
         [0024]      FIG. 6B  is a sectional view taken along cross-section line A-A of  FIG. 6A  showing the striation structure;  
         [0025]      FIG. 7  is an example of a mosaic of overlapping reliefs, after they have been overlapped using the surface mapping method of a bullet in accordance with one embodiment of the present invention,  
         [0026]      FIG. 8A  is an example of a regional relief obtained with the surface mapping method of a cartridge case in accordance with one embodiment;  
         [0027]      FIG. 8B  is a sectional view taken along cross-section line A-A of  FIG. 8B  showing the striation structure;  
         [0028]      FIG. 9  is an example of a mosaic of overlapping reliefs, after they have been overlapped using the surface mapping method of a cartridge case in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     We will describe a system for mapping a surface of a BPOE  9 . The system can be equally used to map a bullet  12  or to map a spent cartridge case  112 .  
         [0030]     System for mapping a bullet surface: In a first embodiment of the present invention, a computerized system  10  equipped with a confocal microscope is used to obtain a surface mapping of a bullet. This embodiment is illustrated in  FIG. 1A  and can be described as follow. The system  10  comprises as the main elements a bullet holder  11  to hold a bullet  12 , a confocal microscope sensor  16  to acquire a relief map of a surface  17  of the bullet  12 , and a displacement system controlled by a computer  22 , to vary the relative position between the bullet  12  and the sensor  16 . It is worth mentioning that although, in certain embodiments of the present invention, a confocal microscope is used as the mapping sensor, other sensors could also be used (Moiré, Laser Confocal Scanning Microscopy, Pixel Contrast).  
         [0031]     In this first embodiment, the bullet holder can rotate the bullet  12  about the rotation axis  13  shown in  FIG. 1A , via a motor  14 . Using sensor  16 , this rotation movement of the bullet  12  permits the measurement of contiguous regions of the bullet  12  surface  17 , to form later on a relief map of a peripheral band of the bullet  12 . In this embodiment, a regional relief of a region of the bullet  12  is obtained, and then the bullet  12  is rotated by an angle to present another surface region, that will include a 50% overlap with the previous region, to the field of view of the sensor  16 , and so on, until a complete peripheral band is measured. The exact angle of rotation is calculated and is inversely proportional to the bullet caliber. Obviously, other measurement configurations may also be implemented without departing from the scope of this invention.  
         [0032]     In this first embodiment, the rotation of the bullet  12  is controlled via the controller  24  which is itself linked to the computer  22 . An encoder  27  measures the rotation angle of the bullet  12  and relays this information to the computer for further analysis.  
         [0033]     The relative position between the bullet  12  and the sensor  16  is changed by the micro-positioners  19  and  20 . One micro-positioner  20  permits the variation of the height of the sensor  16  whereas the other micro-positioner  19  enables to vary the relative distance and tilt between the sensor  16  and the bullet  12 . All micro-positioners  19 ,  20  are controlled by the computer  22  so that a series of measured intensities as a function of a series of distances may be obtained in a controlled manner, and that several such series may be obtained for several different regions of the bullet  12 .  
         [0034]     In this first embodiment, the confocal microscope  16  comprises an analog to digital converter  21  which relays to the computer  22  the digitalized microscope measurements. The computer  22  is used to control and adjust the parameters of the sensor  16  such as for example the focal distance of the microscope, its field of view, etc. In this first embodiment, the computerized system  10  also comprises a display unit  28  to visualize the measurements and the experiments parameters, and is also linked to a database  29  in which the measurements will be stored. The database  29  also gives access to measurements of other bullets for ballistic comparison studies. An analysis program  37  is used for processing the experimental data of the bullet  12  under examination and a comparison program  38  is used to compare the present bullet  12  relief map to other relief maps found in the database  29 . In a preferred embodiment, the methods of the present invention may be embodied in computer  22 .  
         [0035]     System for mapping a cartridge case surface: In a second embodiment, a system  110  similar to system  10  just described, is used to obtain a surface mapping of a spent cartridge case  112 . This second embodiment is illustrated in  FIG. 1B . The main differences between  FIG. 1B  and  FIG. 1A  are related to the ways the cartridge case  112  is held and displaced by system  10  while acquiring the mosaic of relief regions.  
         [0036]     In this second embodiment, the cartridge case holder  111  can displace the cartridge case  112  about the X and Y axis shown in  FIG. 1B , via two motors  14 . Using sensor  16 , this displacement of the cartridge case  112  permits the measurement of contiguous regions of the cartridge case  112  surface  117 , to form later on a relief map of a composite region of the cartridge case  112 . In this second embodiment, a regional relief of a region of the cartridge case  112  is obtained, and then the cartridge case  112  is displaced first in the X direction and then in the Y direction to present another surface region, that will include a 50% overlap with the previous region, to the field of view of the sensor  16 , and so on until a complete array of regional reliefs, forming, when assembled, an image of the full head of the cartridge case  112 , is obtained. The exact displacement is constant. Obviously, other measurement configurations may also be implemented without departing from the scope of this invention.  
         [0037]     In this second embodiment, the rest of the system  110  is the same as for the system  10 , and therefore will not be further described.  
         [0038]     A surface mapping method: We will now describe in detail a method  50  of mapping a surface of a BPOE  9 . The method applies equally well to a bullet  12  or to a cartridge case  112 .  
         [0039]     The main steps of a surface mapping method  50  of a BPOE according to an embodiment of the present invention are listed in  FIG. 2A  and are described in more detail in the following paragraphs.  
         [0040]     A BPOE  9  under examination is provided at step  52  and a measurement unit adapted to acquire a relief map of a surface of the BPOE is provided at step  54 . Then, at step  56 , the measurement unit acquires the relief map to thereby map the surface of the BPOE  9 . In one embodiment, this surface is a peripheral band of a bullet  12  corresponding to a surface region of the bullet  12  expected to contain significant striations  23  produced when the firearm is fired. In another embodiment, this surface corresponds to a surface region of the cartridge case  112  expected to contain significant impressed marks  23  produced when the firearm is fired. Often, but not exclusively, this surface region is located using a 2D-imaging technique, this permits examination of the bullet  12  or the cartridge case  112  prior to the 3D-imaging measurement, in order to identify what surface region of the BPOE  9  contains the striations or impressed marks  23  that should be analyzed with the present invention. In an embodiment, the previously described computerized systems  10  and  112  comprise the measurement unit used in step  54 . Thus according to the method  50 , a BPOE  9  under examination is installed in the bullet holder  11  of the computerized system  10  (when the BPOE is a bullet) or in the cartridge case holder  111  of the computerized system  110  (when the BPOE is a cartridge case). The interface program  37  embedded in the computer  22  is used to program the sensor  16  measurement parameters, and to control the sensor displacements via the micro-positioners  19 ,  20 . When the BPOE  9  is a bullet  12 , the computer  22  also controls the holder rotating motor  14 . The computer thus sends a signal to the rotating motor  14  to rotate the bullet  12  until the surface, that has been chosen to be mapped, is in the field of view of the sensor  16 , and the encoder  27  sends back a signal to inform the computer of the value of the measurement angle. The computer  22  also sends signals to the micro-positioners  19 ,  20  to adjust the height of the sensor  16  relatively the bullet  12  via the micro-positioner  20 , and a relative distance, d (not shown), between the bullet surface and the sensor  16  via the micro-positioner  19 . When the BPOE  9  is a cartridge case  112 , the computer  22  controls the transversal displacement unit  113  that permits the displacement of the cartridge case head in the field of view of the sensor  16 . The computer sends a signal to transversal displacement unit  113  to displace the cartridge case  112  until the surface, that has been chosen to be mapped, is in the field of view of the sensor  16 , and the encoder  27  sends back a signal to inform the computer of the value of the measurement angle. The computer  22  also sends signals to the micro-positioners  19 ,  20  to adjust the height of the sensor  16  relative to the cartridge case  112  via the micro-positioner  20 , and a relative distance, d (not shown), between the cartridge case surface and the sensor  16  via the micro-positioner  19 .  
         [0041]     Turning now to  FIG. 2B , in a preferred embodiment of step  56 ′, a surface region of a BPOE  9  susceptible to contain significant impressions and/or striations  23  is mapped. When the BPOE  9  is a bullet  12 , this surface region corresponds to a peripheral band of the bullet surface containing significant striations  23 . When the BPOE  9  is a cartridge case  112 , this surface corresponds to a full region of the cartridge case head surface containing significant impressions and/or striations. This mapping is performed by acquiring a mosaic of regional reliefs, each regional relief corresponding to a region among a plurality of regions (step  60 ), where the plurality of regions forms, in the case of a bullet, the peripheral band or forms, in the case of a cartridge case  112 , the full region of the cartridge case head surface. Then, at step  67 , this mosaic of regional reliefs is assembled for obtaining the relief map of the surface region of a BPOE  9  under examination. This assembling is performed, for example, with the analysis program  37  of the computer  22 .  
         [0042]     Now referring to  FIG. 3 , a flow chart details the step of acquiring a mosaic of regional reliefs from  FIG. 2B  (step  60 ) when a confocal microscope is used as the measurement unit (step  54 ′). First, at step  61 , a region is selected among the plurality of regions that form the mosaic. Then, at step  70 , the system  10  makes a series of measurements with the confocal microscope to obtain a series of light intensities I d (x,y), corresponding to the selected region as a function of the variable distance d, where d is the distance separating a reference surface of the BPOE  9  and the confocal microscope and where x and y are the surface coordinates of the selected region. This step  70  comprises several intermediate steps that are detailed in  FIG. 4  and will be described herein below.  
         [0043]     When step  70  is completed for the selected region, the regional relief z(x,y)  80  corresponding to the selected region is determined in step  63  using the well-known analysis principles of the confocal method. The result of this analysis is a regional relief map z(x,y)  80 , where z is the depth value at the coordinates (x,y) of the selected region, and this result can be simply stored in the computer  22  for further analysis.  
         [0044]     Then, steps  61 ,  70 , and  63  are repeated until the regional reliefs of all the regions of the plurality of regions have been determined, therefore obtaining a complete mosaic of regional reliefs (step  67 ).  
         [0045]     Turning now to  FIG. 4 , the intermediate steps that are comprised in step  70  of  FIG. 3  are detailed. The goal is to measure with the confocal microscope the light intensity distribution, I d (x,y), for a series of different distances d, where x and y are the surface coordinates of the region under examination. A first distance is selected (step  71 ) via the computer  22 , then the sensor  16  position is adjusted (step  73 ) accordingly via the micro-positioners  19 . Then a first intensity distribution is acquired (step  75 ) with the sensor  16 , digitized, and sent to the computer  22  for storage (step  77 ). These steps are repeated for all the distances of the series of distances, so that a complete series of light intensities is obtained (step  79 ).  
         [0046]      FIG. 6A  is an example of a regional relief  80  of a bullet surface obtained with the surface mapping method just described, where an enlarged striation  23  can be seen. In this preferred embodiment, a regional relief corresponds to a physical area of 1.5 mm×1.5 mm. The regional relief is mapped into an array of 512×512 pixels providing, in an exemplary embodiment, a lateral resolution of approximately 3 microns. Typical depth range of a regional relief depends on the bullet caliber, the firearm and the region. The biggest range would be found on a small caliber on the shoulder area and would be around 50 microns. The smallest range would be found on a .50 caliber in the middle of a groove engraved area and would be around 5 microns. The depth resolution of the sensor is, in an exemplary embodiment, 10 nanometres. Obviously, other physical characteristics of the regional relief can be obtained without departing from the scope of this invention, by tuning the system  10  differently.  
         [0047]      FIG. 6B  is a sectional view taken along cross-section line A-A of  FIG. 6A  showing the striation structure  93  along this cross-section line. It can be seen on this figure that this cross-section line possesses profile characteristics and features that can be used as a firearm identification signature.  
         [0048]      FIG. 8A  is another example, this time of a regional relief  180  of a cartridge case obtained with the surface mapping method just described, where an impression and/or striation  23  can be seen. A regional relief corresponds to a physical area of 1.5 mm×1.5 mm. The regional relief is mapped into an array of 512×512 pixels providing a lateral resolution of approximately 3 microns. Typical depth range of a regional relief depends on the cartridge case caliber, the firearm and the region. The biggest range would be found on a large caliber on the firing ping impression area and would be around 250 microns.  
         [0049]      FIG. 8B  is a sectional view taken along cross-section line AA of  FIG. 8A  showing the impression and/or striation structure  93 , which can be used, as already mentioned, as a firearm identification signature.  
         [0050]     Turning now to  FIG. 5 , a preferred embodiment of the mapping method  50  comprises acquiring a mosaic of overlapping reliefs  56 ′, each overlapping relief corresponding to an overlapping region among a plurality of overlapping regions, wherein each overlapping relief has at least one overlapping area in common with an overlapping area of another overlapping relief (step  62 ). The advantage of acquiring overlapping reliefs (partly overlapping) is that they can be more easily aligned when comes the time to assemble them to form the final image of the BPOE. This assembling can be performed by finding the best overlapping degree between overlapping areas that are in common (step  64 ).  
         [0051]     It is worth mentioning that the relief maps may suffer of distortions due, for example, to the curved surface of the bullet. The present invention provides an algorithm correcting these distortions effects in order to eliminate such distortion in the final assembling step. Also other distortion effects due to optical effects or due to misalignment measurement errors are also corrected via the algorithm provided by the present invention.  
         [0052]      FIG. 7  gives an example of a peripheral band  100  of a bullet  12  that was formed by a mosaic of aligned overlapping reliefs  88  where an exploded close-up of three consecutive overlapping reliefs of the mosaic is shown. As can be seen, each overlapping relief  88  has at least one overlapping area  89  in common with an overlapping area  89  of another overlapping relief. In this example, the overlapping areas  89  are chosen such that each of the overlapping regions of this mosaic overlaps its neighbor regions by about 50%. Naturally other overlapping degree could also vary without departing from the scope of this invention.  
         [0053]      FIG. 9  gives an example of surface array  200  of a cartridge case  112  that was formed by a mosaic of aligned overlapping reliefs  88 . As can be seen, each overlapping relief  88  has at least one overlapping area  89  in common with an overlapping area  89  of another overlapping relief. In this example for a cartridge case head, the overlapping areas  89  are chosen such that each of the overlapping regions of this mosaic overlaps with its neighbor regions by about 50%. Naturally other overlapping degree could also be varying without departing from the scope of this invention.  
         [0054]     It will be understood that numerous modifications methods and apparatus described herein will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense. It will further be understood that it is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure that come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features herein before set forth, and as follows from the scope of the appended claims.