Data processing: measuring – calibrating – or testing – Measurement system – Dimensional determination
Reexamination Certificate
2000-01-18
2003-01-07
Hilten, John S. (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Dimensional determination
Reexamination Certificate
active
06505140
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a computer aided ballistic analysis system, and particularly, to a computerized ballistics matching system using the 3D information of a bullet's surface.
More particularly, the present invention relates to a computerized system and method for bullet ballistic analysis based on measurements of depth profiles of striations on the bullet surface, set-up for depth profile acquisition, and software for data acquisition, processing and comparison.
The present invention not only relates to the system for matching bullets fired by known or unknown guns, but it also relates to the system for matching a bullet under investigation to a gun in question by two methods. In particular, to a first method developed for creating a unique “signature” of the gun in question based on depth profiles of control bullets fired from the gun in question, and to a second method based on comparisons of the degree of similarity between the profiles of said control bullets among themselves, and the comparisons of the profiles of said control bullets and the bullet under investigation.
Further, the present invention relates to a software developed for normalizing the acquired 3D data by compensating the same for measurement (coaxiality) errors.
Furthermore, the present invention relates to software developed for the acquisition and matching of the bullet under investigation to another bullet or to a gun in question.
BACKGROUND OF THE INVENTION
The scratches (striations) formed on a bullet by a gun barrel through which the bullet is fired create a signature with enough unique features that it may be matched with other bullets fired by the same gun. The matching process has been manually accomplished for many years using an optical instrument called a comparison microscope. Manual comparisons of bullets can be quite time consuming and such technique is used sparingly unless there is some reason to believe that a bullet from a crime scene was in fact fired from a gun in question.
Recent machines have been built which attempt to automate the process of ballistics analysis. The goal is to enter bullet images into a database and to allow a computer to search the database for matches. Due to the fact that a computer can make such comparisons many times faster than a human, searching large databases is, at least in principle, feasible. The digitized images of bullets and cartridge cases can also be used to provide additional tools which assist firearms examiners in their manual comparison.
For example, U.S. Pat. No. 5,654,801 describes a fired cartridge illumination method and imaging apparatus which includes a light source and a microscope to image impressions on the surface of the cartridge. Images of the impressions are then used for comparative analysis, during which a first image from a test cartridge and a second image from a computer data bank are compared with each other and a maximum correlation value between the first and second images is obtained.
As is common among the current systems capturing data from bullets and cartridges, the device described in the '801 Patent captures strictly visual data which does not distinguish between shallow scratches or deep scratches on the surface of the examined cartridge or bullet. Therefore, important analysis parameters are not considered which lessens matching reliability and reduces the provability of consistent conclusions.
A fundamental problem of all computer aided ballistic analysis systems is that bullets fired from the same gun do not match exactly for a number of reasons, including the facts that the cartridge cases may have different amounts of powder, or that the gun barrel may have been at different temperatures when bullets are fired as compared to the test firing. Due to the fact that the impressions made by a gun on a bullet can differ from firing to firing, all comparison algorithm must necessarily be statistical and cannot look for an exact or even nearly exact match of all striations on the bullet's surface.
Currently, the algorithms which compare bullets have a high false positive match rate. Qualitatively, this means that automatic searching of a large data base of ballistic data which may have tens of thousands of entries is not viable. By using the large data base, there would be so many false matches requiring many comparisons, that essentially no useful information would be obtained.
The current poor false match rate using current algorithms is the result of fundamental problems, most of which are associated with the fact that the data used for the bullet comparisons is 2D data. 3D data is much more reliable and robust than 2D data. Let us consider the physical phenomenon involved in the 2D data capture. A source of light is directed at the bullet's surface, and a camera records the light as it is reflected by that surface. The data capture process is based on the fact that the light reflected by the bullet's surface is a function of the surface features. However, this is an indirect measurement, because it involves a transformation of the incident light into the light recorded by the camera. By comparison, the 3D acquisition process is simply the distance between the surface features and an imaginary plane, and is thus a direct measurement. The disadvantages associated with the indirectness of the 2D data capture are.
Robustness: A significant problem associated with 2D data capture lies in the fact that the transformation relating the light incident on the bullet's surface and the light reflected by it depends not only on the features of the bullet's surface, but also on a number of independent parameters such as the angle of incidence of the light, the angle of view of the camera, variations on the reflectivity of the bullet surface, light intensity, etc. This implies that the captured data (the data recorded by the camera) is dependent on these parameters too. To attempt to eliminate the effect of these parameters on the captured data would be next to impossible (except possibly for light intensity). As a consequence, the 2D captured data is vulnerable to considerable variability, or in other terms, it is non-robust.
Indeterminate conditions: A different kind of problem associated with 2D data capture is the presence of indeterminate conditions in the data. Given an incident light source angle, some of the smaller surface features can be “shadowed” by the larger features. This implies that there will be regions of the surface where the captured data will not accurately reflect the surface features. In mathematical terms, the transformation between the incident light and the reflected light is non-invertible. Furthermore, this is an example where the angle of incidence of the light source can have a critical effect on the captured data, because arbitrarily small changes in the angle of incidence may determine whether smaller features are detected or not. In mathematical terms, the transformation between the incident light and the reflected light is discontinuous with respect to the angle of incidence.
In summary, 2D data capture methodologies can be affected by extraneous variables that can be next to impossible to control. Moreover, because these variables are not measured, their effects on the captured data cannot be compensated for. As a consequence, the normalized data resulting from such capture processes is also vulnerable to significant variability, or in other words, lack of repeatability. The performance of even the most sophisticated correlation algorithms will be degraded in the presence of non-repeatable data. Taking in consideration that the bullet matching problem is quite demanding to begin with, it is not surprising that ballistic matching methodologies based on 2D captured data have had significant difficulties delivering satisfactory performance.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide reliable and highly accurate ballistic analysis on bullets based on 3D data acquisition, particularly
Bhat Aditya
Hilten John S.
Intelligent Automation, Inc.
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