Infrared Thermal Imaging for Use in Restoration of Defaced Serial Numbers

As submitted by the applicant: The problem under investigation is the recovery of serial numbers from metallic objects such as firearms or automobile engines using a non-destructive technique known as thermal infrared imaging or infrared thermography. The construction and implementation of three types of instruments that utilize an infrared camera to image the residual traces of defaced serial numbers is proposed. Transient infrared thermography (TIT), pulsed infrared thermography (PIT), and lock-in infrared thermography (LIT) all are sensitive to localized changes in the thermal conductivity of a substance. TIT, PIT, and LIT techniques have been successfully used to image defects in materials like airplane wings, circuit boards, and solar cells. With regard to serial number recovery, defaced areas exist as plastic strain regions or melted regions below serial numbers that are stamped or laser engraved into materials. The infrared thermographic instruments should be sensitive to these areas and nondestructive recovery of the numbers should be possible. It is expected that sophisticated multivariate image analysis (MIA) processes will enhance the recovery of the serial numbers. These studies will be used to determine the best practices to use when implementing these infrared themographic techniques for recovering defaced serial numbers. Both stamped and laser engraved materials made from carbon steel as well as aluminum alloys will be used in these investigations. Test samples would be obtained from Precision Forensic Testing in Dayton, OH and provided by our collaborators at the Idaho State Police Crime Lab or in some cases produced on site. The basis of all of these instruments is a sensitive camera that has a rapid response time. The infrared camera will detect differences in the temperature of material in the plastic strain region or melted region which exist below the filed away serial numbers and differences in the phase of a thermal wave as it passes through these regions. Differences in the thermal conductivity will result in differences in the ability of the material to conduct heat, so both the temperatures of the materials as well as the phase of a thermal waves traveling through the materials will be affected. These differences may not be readily apparent, but with MIA processes, the differences and hence, serial numbers that have been filed away, will be imaged and resolved. The data will be collected by the infrared camera as a function of time. The information will be used in two ways. In one process, the temperatures as a function of time will be used with powerful MIA software to reconstitute images of the original serial numbers. In a second process signals from the individual pixels will be passed into a lock-in amplifier. The lock-in signals will then provide phase information and an overall phase image which should also provide an image of the original serial numbers. If the phase images are noisy, they can be further resolved with MIA processes. Included in the quarterly milestones are construction and testing of the infrared thermographic techniques as well as their implementation for the recovery of stamped and laser engraved serial numbers from carbon steel and aluminum alloys that are characteristic of materials seen in forensic investigations. A final technical report will be prepared that includes results of the studies performed and recommendations for best practices based on the scientific data collected during the investigation period. This report will be suitable for public dissemination to forensic practitioners. ca/ncf