Combined lock-in thermography and SQP algorithm for non-intrusive reconstruction of optical and thermal properties in semitransparent medium

A combined lock-in thermography (LIT) technique with sequential quadratic programming (SQP) algorithm is proposed for simultaneous reconstruction of the optical and thermal property distributions in a 2D semitransparent medium. The LIT technique is applied to identify the positions of inclusions in media, in which the discrete correlation method is employed to extract the amplitude and phase angle of thermal wave signals. The SQP algorithm is employed as the main optimization method to simultaneously reconstruct the absorption coefficient, scattering coefficient, and thermal conductivity distributions in the medium. The LIT-SQP technique combines the advantages of rapid identification of inclusion position of the LIT technique and accurate reconstruction of inclusion properties of the SQP algorithm. The inclusions of various positions, shapes, numbers, and sizes are investigated. All the inclusions can be accurately reconstructed by the proposed LIT-SQP technique, which is proved to be more efficient and accurate than the LIT technique and the SQP algorithm alone.