Potential of Lock-In Infrared Thermography for the Recovery of Defaced Serial Numbers

Infrared thermography is an evolving approach useful in nondestructive evaluation of materials for industrial and research purposes. A two stage method is proposed here to utilize infrared thermography to recover defaced serial numbers. The first stage involves employing the lock-in thermography technique to develop phase images which are less susceptible to environmental conditions and uneven heating than raw thermal images. The second stage involves the application of principal component analysis (PCA), a multivariate technique that transforms a dataset of possibly correlated variables into a set of new uncorrelated variables on the phase images to enhance subtle variations of temperature gradient across the surface under investigation. This two stage approach has the advantage of utilizing the benefits of lock-in thermography as well as the potential of PCA as an analytical tool to recover defaced serial numbers. The results are presented for a stainless steel plate stamped with several numbers and then defaced.