Fired Cartridge Examination Method and Imaging Apparatus

The present invention relates to a fired cartridge examination imaging apparatus for use during forensic analysis of spent firearm cartridges. The invention relates further to a method of comparing fired cartridges from firearms.

Fired bullets and bullet cartridges are left with markings from the firearm from which they come. These markings result from forced contact with metal parts in the firearm and are unique to the firearm since the metal parts have engraved surfaces which are themselves unique due to the machining process during manufacturing. Using microscopic examination of fired bullets and cartridges (casings or shells), firearms experts have traditionally been able to compare markings to identify whether fired bullets or spent cartridges originate from a given firearm.

The process of bullet examination has been successfully automated using apparatus as set out in PCT/CA92/00216 (published as WO 92/20988). Bullet illumination is described in U.S. Pat. No. 5,379,355.

In manual comparison of fired cartridges, the cartridge under investigation is placed under a microscope along with a test cartridge from a known firearm. The impression of the breech face on the primer and the firing pin impression in the primer surface have markings that are compared. Conventionally, side illumination is used which provides an image dependent on the direction of the illumination. To compare the markings on cartridge bases, the angle of illumination must be the same. For this purpose, illumination direction protocols are used. When two cartridges originate from the same firearm, the markings will have the same characteristics. Since the appearance of the primer breech face impression and firing pin impression is different using different angles of illumination, the firearms examiner will usually confirm a match using illumination from more than one angle.

The need to use an illumination direction protocol and possibly additional images of each cartridge using different illumination angles is a problem for automating the process of imaging and analysis.

It is an object of the present invention to simplify and therefore to improve the automated process of fired cartridge imaging and analysis. In brief, the object of the present invention is achieved by using axially symmetric illumination for obtaining images of the fired cartridges. These images can be rotated with respect to one another and compared to obtain a correlation factor.

It is a further object of the present invention to provide such an apparatus which can switch between a magnification suitable to view the firing pin impression and a magnification suitable to view the breech face impression on the primer surface.

According to the invention there is provided a fired cartridge imaging apparatus comprising a fired cartridge mounting device for holding the cartridge substantially aligned with a longitudinal axis, a primer surface of the cartridge being substantially perpendicular to the axis, a cartridge microscope having an optical axis and mounted with the optical axis substantially parallel to the longitudinal axis, focusing means for focusing the microscope to image a breech face impression on the primer surface and a firing pin impression surface in the primer surface, and an axisymmetric light source mounted to project axially symmetric light onto the breech face impression and the firing pin impression surface about the longitudinal axis. In this way, images of the breech face and the firing pin impressions can be used for comparative analysis independently of an angular orientation of the cartridge held by the mounting device.

According to the invention, there is also provided a method of comparing fired cartridges from firearms comprising the steps of:

illuminating a base of a first fired cartridge from a first firearm using axially symmetric light;

obtaining using a microscope a first image of a breech face impression on a primer of the first base with the breech face impression in focus;

illuminating a base of a second fired cartridge from a second firearm using axially symmetric light;

obtaining using a microscope a second image of a breech face impression on a primer of the second base with the breech face impression in focus;

rotating the first and the second images relative to one another; and

obtaining a maximum correlation value for the rotated first and second images.

The method according to the invention preferably further comprises steps of: adjusting a focus and magnification of the microscope to obtain a first image of a firing pin impression on the first primer with the firing pin impression in focus; adjusting a focus and magnification of the microscope to obtain a second image of a firing pin impression on the second primer with the firing pin impression in focus; rotating the first and the second firing pin images relative to one another; and obtaining a maximum correlation value for the rotated first and second firing pin images. Of course, the invention also contemplates obtaining a combined correlation value based on the maximum correlation value for the breech face impression images and the maximum correlation value for the firing pin impression images, as well as a relative rotation orientation of the first and second breech face impression images with respect to the first and second firing pin impression images.

Preferably, the illumination is automatically adjusted by analyzing each image to determine if the illumination is right. In the case that it is not right, an adjustment is made and the image is taken over again.

The invention will be better understood by way of the following detailed description of a preferred embodiment with reference to the appended drawings, in which:

FIG. 1 is a perspective view of the apparatus according to the preferred embodiment except for the image processing computer showing the cartridge mounting device attached to a main microscope stage and the cartridge microscope and camera mounted to the optics of the main microscope;

FIG. 2 is a detailed view showing the cartridge microscope and mounting device of FIG. 1;

FIG. 3 is a detailed plan view of the cartridge mounting device according to the preferred embodiment; and

FIG. 4 is a schematic illustration of a primer from a fired cartridge.

As shown in FIG. 1, the preferred embodiment has the cartridge microscope and camera optics 10 mounted to complementary (main) microscope optics 12 for analyzing fired bullets. The cartridge 21 is viewed using a microscope 14 and images are taken using a CCD (charge coupled device) camera 20. Axially symmetric light is provided by a fiber optic ring light 16. The cartridge 21 is securely held by a mounting device 18, which itself is adjustably mounted on an arm 30 attached to the stage 22 of the main microscope 12.

The microscope 14 is mounted by arm 15 to the microscope optics of the main microscope 12 so that the focusing system 24 (fine vertical height adjustment) can be used. The stage 22 is moveable in the so-called x-y horizontal plane, although such adjustment is not normally used from one cartridge to the next. The focus and x-y movements are computer controlled, as is known in the art.

With reference to FIGS. 2 and 3, the mounting device 18 will be described in greater detail. The cartridge 21 is held in a V-shaped groove 38 between two plates 34 and 36. Plate 34 is connected to arm 15, and plate 36 is slideably connected to plate 34 by pins 40 and springs 42. A handle may be provided as shown. The device 18 is adjustably connected to arm 30 and can be moved along direction 33 as bolts 35 are repositioned in slot 32. The vertical adjustment along direction 33 is a rough adjustment, with finer adjustments being achieved by focusing system 24 (FIG. 1). The horizontal edge member (not shown in the figures for clarity) is mounted to plate 34 and extends over the top of the cartridge 21 to act as an abutment. The edge member fixes the vertical height of the cartridge 21 with respect to the mounting device 18. To insert the cartridge 21 into the device 18, the plate 36 is pulled away from plate 34, and the cartridge 21 is placed from below into the groove 38 until reaching the horizontal edge member. The plate 36 is then released and the cartridge is held securely in place.

A zoom lens 17 is provided to adjust the magnification of the image seen by microscope 14. The useful images gathered using camera 20 are from the primer 31. The primer is the malleable or mobile central part of the rear of the cartridge 21, and is the part struck by the firing pin of the firearm. As shown in FIG. 4, the firing pin itself leaves its mark 41 in the primer 31, and this is one useful image. The primer also contains markings from the breech face of the firearm which are imprinted during firing. The breech face impressions are another useful image. The contour 45 of the firing pin (which can be obtained from either of the previous two images) is another piece of useful information.

When a cartridge is to be tested using the present invention, it is placed in the mounting device 18 as set out above. The image from camera 20 is usually seen on a monitor. Using the zoom lens 17, the magnification is adjusted to view the primer. The zoom adjustment could be manual, but is preferably automatic. The breech face impression surface of the primer 31 is place in focus using the automatic focusing system 24, and an image of the breech face impressions is obtained and stored in the computer. An adjustment of the stage 22 may be necessary to bring the primer near the center of the optic axis 11, but this is not necessary when the system was last used for cartridge imaging. An adjustment of the illumination may also be required. The image is analyzed in the computer for contrast. If insufficient contrast is present, or if the image is too bright or too dim, the level of illumination in ring light 16 is adjusted. Next, the magnification is increased to bring the firing pin impression into view. The image is focused as before, and the firing pin impression image is stored. Again the image is checked for proper illumination, and if necessary, the illumination is adjusted and the image is taken over.

Two cartridges will never produce exactly the same image, even if they are from the same firearm. In the analysis of a cartridge, the object is to locate which cartridge or cartridges resemble the test cartridge the most. A numerical correlation factor is computed for each set of images (breech face and firing pin impression images) with respect to the images for the test cartridge 21. The closest images are then compared in greater detail to determine whether the test cartridge was fired from the same weapon as the cartridge whose images are stored in the computer's memory (disk or tape storage).

The process of correlation involves rotation of the images to obtain the correlation value from the relative rotational position which gives the best correlation. Clearly, this relative rotational position should be approximately the same for the breech face impressions (on surface 31 in FIG. 4) as for the firing pin impressions (on surface 41 in FIG. 4), at least with firearms in which the firing pin is not free to rotate. Also, the relative rotational position of the contour 45 of the firing pin (round, oval, triangular, and irregular shapes similar thereto) must correspond with the firing pin impression orientation. The computer may recognize the shape of contour 45 using line or object recognition techniques known in the art. Preferably, the contour shape is extracted when the magnification is larger, for greater precision. During correlation, the breech face and firing pin markings and contour shape are analyzed. The markings may comprise a series of broken lines as shown in FIG. 4. The breech face of the firearm may have markings other than milling lines which are imprinted onto the primer 31. The same is true for the firing pin impressions. Thus, the impressions may include characteristics which are not segments of ordered lines. The correlation software takes into account width, depth, length or other dimensional characteristics of the markings, as well as any order in the markings, and calculates a numerical correlation value between the test images and each of the stored images. The correlation value for a given stored image is taken as the one from the best relative rotational orientation between the test and stored images. As can be appreciated, the axially symmetric light gives a single image which contains information about all surface markings. When direct side lighting is used, better contrast may be obtained, but some information is lost.

The invention allows a single cartridge to be compared with hundreds or thousands of other cartridges using the computer and its cartridge image database in a relatively short time. The result is a small number of closest correlated cartridges which can quickly be analyzed further to determine whether a "match" has been found. The further analysis may be done by computer analysis, however, the final judgment is preferably carried out by a ballistics or firearms expert manually using special equipment for comparing for comparing the microscopic images.