This report presents the design of a 3D imaging device that captures the details of crime-scene impression evidence, such as tire track or shoe impressions, and the report also includes a calibration method for obtaining the 3D image with the proper metric information.
Currently, such evidence is captured with a two-dimensional (2D) color photo or making a physical cast of the impression. The 2D photographs taken under favorable illumination conditions can highlight details of the impression but cannot provide metric depth measurement information. Although a 3D physical cast of the impression provides depth information, the impression may be destroyed in the process. The use of a 3D imaging device can remedy the limitations of current techniques for capturing impression evidence. The report first presents a literature review of previous work on structured lighting methods for extracting 3D surface shape, noting the limitations of such efforts in forensic applications. The hardware setup of the current 3D imaging device for impression evidence is a motorized rail (actuator) with a HD video camera and two line laser lights, each with a different color. A description of the image processing algorithms addresses laser stripe detection and color texture image extraction. The calibration of triangulation configuration uses an L-shaped calibration object with known dimensions to calibrate the geometric configuration of the laser beams and the camera in computing the height map image. The procedure for calculating the height map is described. Experimental results from testing the device are reported. Two laser modules eliminated occlusions and improved performance. The project concludes that it has developed an inexpensive high-resolution 3D impression device for digitizing shoe and tire impressions at crime scenes. 16 figures and 13 references