Infrared Thermal Imaging for Use in Restoration of Defaced Serial Numbers
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The author(s) shown below used Federal funding provided by the U.S. Department of Justice to prepare the following resource: Document Title: Infrared Thermal Imaging for Use in Restoration of Defaced Serial Numbers Author(s): Rene Rodriguez, Lisa Lau, Ikwulono Unobe, John Kalivas, and Andrew Sorensen Document Number: 251208 Date Received: October 2017 Award Number: 2013-R2-CX-K012 This resource has not been published by the U.S. Department of Justice. This resource is being made publically available through the Office of Justice Programs’ National Criminal Justice Reference Service. Opinions or points of view expressed are those of the author(s) and do not necessarily reflect the official position or policies of the U.S. Department of Justice. Final Report on the Project "Infrared Thermal Imaging for Use in Restoration of Defaced Serial Numbers" Award Number: 2013-R2-CX-K012 Author(s): Rene Rodriguez, Lisa Lau, Ikwulono Unobe, John Kalivas, Andrew Sorensen Abstract The problem under investigation was the recovery of serial numbers from metallic objects such as firearms or automobile engines 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) techniques. The construction and implementation of three types of instruments that utilize an infrared camera to image the residual traces of defaced serial numbers was accomplished for this purpose, transient infrared thermography (TIT), pulsed infrared thermography (PIT), and lock-in infrared thermography (LIT). TIT, PIT, and LIT techniques have been successfully used to image defects in materials like airplane wings, circuit boards, and solar cells, and with regard to serial number recovery, 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 use of infrared thermography coupled with sophisticated MIA processing were investigated using a FLIR Inc. SC6700 infrared camera to detect differences in the temperature and thermal wave phase due to heat flow of material in the plastic strain region below the filed away serial numbers compared with the surrounding largely undisturbed areas. Our studies required the production of a reproducible temperature change induced about some starting temperature, the suppression of background thermal noise, and the ability to collect IR camera images for a particular amount of time and after a given amount of delay. 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. Both a Peltier cooler and a digital hot plate proved useful for our investigations of the effect of the initial temperature, and pulsed heating cycles were achieved by one of the following types of heating/cooling methods: a pulsed heat gun, a chopped high-wattage stage lamp, a chopped laser beam, and a Peltier cooler. The emissivity of the defaced surface due to the absorption and a reduction in the reflectivity of the sample also played a significant role in the ability of the camera to resolve sample emissions from reflected background noise, and black paint, graphite, and colored dye were used for this purpose. Several trials with each coating were part of the investigations. 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. This was essential for implementation and our investigations using the PIT and LIT techniques. Furthermore in the case of the LIT, the timing circuit allowed the camera to take images several times at set intervals over the course of one heating pulse. The MIA procedures proved to be an essential part of recovering the defaced serial numbers. Processing of the IR camera images using Principal Component Analysis (PCA) and Zernicke Moment Analysis proved to provide processed plots that reconstructed the defaced numbers even when there was no number present in the raw image or phase image. Employment of multiple similarity merit measurements to the processed plots, along with combination fusion rules, shows great promise in accurately determining the identities of defaced serial numbers. 1 This resource was prepared by the author(s) using Federal funds provided by the U.S. Department of Justice. Opinions or points of view expressed are those of the author(s) and do not necessarily reflect the official position or policies of the U.S. Department of Justice Table of Contents Abstract ........................................................................................................................................... 1 List of Figures ................................................................................................................................. 3 List of Tables .................................................................................................................................. 6 Executive Summary……………………………………………………………………………….7 I. INTRODUCTION ..................................................................................................................... 10 II. METHODS............................................................................................................................... 12 Modeling with Finite Element Analysis ................................................................................ 12 Multivariate Imaging Techniques ......................................................................................... 15 Infrared Thermometric Methods, TIT, PIT, and LIT ............................................................ 18 Sample Considerations...................................................................................................... 18 Transient Infrared Thermometry (TIT) ............................................................................. 21 Pulsed Infrared Thermometry (PIT) ................................................................................. 22 Lock-In Thermography (LIT) ........................................................................................... 25 III. RESULTS ............................................................................................................................... 28 TIT Results ............................................................................................................................ 28 PIT Results ............................................................................................................................ 29 LIT Results ............................................................................................................................ 31 Image Filtering Studies ..................................................................................................... 45 Thermal Imaging from Other Perspectives ....................................................................... 72 IV. CONCLUSIONS .................................................................................................................... 79 V. REFERENCES......................................................................................................................... 81 VI. DISSEMINATION OF RESEARCH FINDINGS ................................................................. 82 Appendix 1 Similarity Merits ..................................................................................................... 83 2 This resource was prepared by the author(s) using Federal funds provided by the U.S. Department of Justice. Opinions or points of view expressed are those of the author(s) and do not necessarily reflect the official position or policies of the U.S. Department of Justice List of Figures Figure 1 Meshed model of Aluminum plate 13 Figure 2 Model loaded with force used to engrave number 13 Figure 3 Model of plate engraved with a number 9 14 Figure 4 Isometric view of Elastic thermal strain showing plastic deformation on engraved surface before and after applying thermal load. 14 Figure 5 Photograph of the serial number area on stainless steel test sample before defacing it with the successive milling method. 19 Figure 6 Photograph of the serial number area on the same stainless sample after it was defaced with the successive milling method. 19 Figure 7 - Infrared image of test sample without any tilting of the sample relative to the camera. Temperatures range from 21.7C( dark purple) to 23.3C (light yellow). 19 Figure 8 Image of the same test sample with some small tilting relative to the camera. Temperatures range from 21.7C(dark purple) to 23.3C (light yellow). 20 Figure 9 Surfaces prepared with various films; bottom is an infrared image of a coated sample. 20 Figure 10 Picture of Insulated Sample holder below the infrared camera. 22 Figure 11 Overall pulse system schematic and pulsing circuit interface box. 24 Figure 12 Schematic diagram of the laser heating experiments. 25 Figure 13 Pulse scheme for one pulse/sec with 16 frames/pulse. 26 Figure 14 Pulse scheme for one pulse/second with 16 frames/pulse at a new start time delay. 26 Figure 15 Pulse scheme for two pulse/sec. with 8 frames/pulse at original start time delay. 26 Figure 16 Photographic image of a defaced unfiltered aluminum test sample of the defaced serial number 22789043. 28 Figure 17