A variable resolution video machine vision system has been built which will determine the 3-D shape of a firing pin from the dent it has made in the primer of a shell. The system can also compare primer dents in different shells and determine if a 3-D match exists. A real time optical 3-D shape recognition feature allows the operator to align the shells, determine if there is a match, and quantify the quality of the 3-D match. The system uses a laser interferometer to project variable spatial frequency structural illumination on the shells which are viewed by video cameras. A computer determines the 3-D shape of the primer dent and predicts the shape of the firing pin. Optical shape recognition and alignment is achieved by mixing the video data with a computer generated mask and using this signal in a spatial light modulator to diffract a laser beam onto a detector. The magnitude and shape of the detector output indicates in real time the quality of the match or can be used to align the shells. A demonstration of the validity of the 3-D shape prediction has been carried out with large scale firing pin and shell-primer models, and the shape of a real firing pin has been determined from an actual fired shell. The result is a mixed video-optical processing system that could be used for real-time shell and firing pin matching in the forensic laboratory. Limitations and improvements in the current system are discussed.
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