This paper presents an automated hidden void detection and quantification technique for inspecting triplex bonding layers in liquefied natural gas (LNG) carriers using active lock-in thermography. Hidden voids are first detected and visualized by an amplitude image and a series of binary image processing. Then, the sizes of the detected voids are quantified using an empirical mapping function, relating the detected void sizes to the void sizes obtained by an independent X-ray testing. The performance of the proposed technique is blind tested using two triplex specimens. The experimental results reveal that the hidden voids can be successfully detected and quantified.