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Infrared Thermal Imaging for Use in Restoration of Defaced Serial Numbers

NCJ Number
Date Published
October 2017
86 pages
This project examined whether defaced serial numbers can be recovered from metallic objects by using non-destructive techniques based on the localized changes in the thermal conductivity of a substance known as thermal infrared imaging or infrared thermography, coupled with sophisticated multivariate image analysis (MIA).

The project constructed the following three types of instruments that use an infrared camera to image the residual traces of defaced serial numbers: 1) transient infrared thermography (TIT), 2) pulsed infrared thermography (PIT), and 3) lock-in infrared thermography (LIT). These techniques rely on the fact that plastic strain regions and/or melted regions exist below stamped or laser engraved serial numbers. Best practices for using infrared thermography coupled with sophisticated MIA processing were investigated using a FLIR Inc. SC6700 infrared camera to detect differences in the temperature and thermal save phase due to heat flow of material in the plastic strain region below the serial number that had been filed. The project required the production of a reproducible temperature change induced about some starting temperature, the suppression of background thermal noise, and the ability to collect infrared camera images for a particular amount of time and after a given amount of delay. In order to induce reproducible starting temperatures and cyclical temperature changes in the stamped and defaced metal surrogates, the materials had to be preheated to some initial temperature and then absorb a cyclical heating pulse. How pulsed heating cycles were achieved is described. A special variable timing circuit was constructed to provide for evaluation of the effect of the timing of the heating pulse relative to the camera shutter opening. The MIA procedures proved to be an essential part of recovering the defaced serial numbers.

Date Published: October 1, 2017