Methods for identifying marks in a defaced metal surface by use of computer-implemented processing of images obtained according to a thermal lock-in imaging technique are described. Methods include processing phase images and/or amplitude images according to principal component analysis of a concatenated input matrix and development of a score image for each principal component determined by the analysis. Score images or extracted features of score images (e.g., Zernike moments) are compared to images/features in a reference data library and based upon the comparison, the defaced mark can be identified.