## 1 Introduction  
Non-destructive testing (NDT) is the performance of an inspection without destroying an object [1]. As a representative example, when using steel materials, such as bridges, railways, structures, and pipes, to join metals together, most are welded, and various inspection techniques are used to inspect the soundness of the welded area without physically affecting these metal joints [2]. In other words, NDT measures changes in the properties and characteristics of applied energy due to the presence of defects by excitation with physical energy (sunlight, heat, radiation, ultrasound, and electricity) without changing the originality and function of the material [3; 4; 5]. It means an inspection to determine the presence or absence of defects by measuring through equipment. There are numerous metal materials that are widely used in industrial fields, and stainless steel (STS) is typically used [6; 7]. STS contains more than 12% chromium and is not easily rusted, so it is a material used in various fields such as industrial parts, automobile parts, medical devices, and aerospace fields. However, due to various causes (fatigue damage, manufacturing defects, corrosion, etc. ), defects occur on the surface or inside, which can lead to serious problems due to weak durability. Therefore, many NDT techniques are used to perform on-site inspections. An inspection for on-site maintenance requires promptness, safety, and reliability. Recently, infrared thermography (IRT) that meets these conditions is emerging as an advanced technology [8]. IRT is a technology that has developed rapidly in recent years and utilizes IR camera microsystem technology that integrates an IR detector, electronic technology, and computer technology [9; 10; 11]. IRT analysis is currently being applied to industry and R&D in various fields such as NDT inspection, condition monitoring and diagnosis, predictive maintenance, energy saving in the process and construction fields, and the detection of gas components. Among NDT inspection techniques, IRT is mainly used to detect defects, discontinuities, delaminations, and welding defects inside objects [2]. IRT is a technology that analyzes and acquires a thermal response signal using a non-contact-method-based thermal imager. After providing a heat source to the object to be inspected with an external thermal excitation source, the IR camera detects radiant energy belonging to the IR range of the electromagnetic spectrum. Based on this principle, it is possible to detect internal abnormalities [12; 13; 14]. NDT techniques for discontinuous points existing inside an inspection object are very diverse. Table 1 presents various NDT techniques for examining the discontinuous points of an inspection object. For all NDT techniques, an appropriate technique should be selected according to the type and defect of the inspection object. The STS304 plate applied in this study has circular defects of various aspect ratios on the back side, so an inspection technique that can inspect non-contact or large areas and acquire high-level quantitative data is required. The capacitive imaging (CI) technique is advantageous for detecting local defects present in thin layers or surfaces, so it is advantageous for detecting composite materials such as CFRP or GFRP. Ultrasonic testing (UT) can acquire quantitative data of defects but is performed with local detection and contact. Vibrothermography (VT) is suitable for tensile specimens in which cracks are present and there is a potential for defect growth due to frictional vibrations. Various image processing techniques are used to improve the detectability of the 2D thermal images acquired with an infrared camera or to detect defects automatically. Recently, machine learning is used, but a lot of data accumulation is required. In addition, noise may be generated by non-uniform heat sources when optical or electromagnetic