###### Abstract
In addition, the heating energy can be applied in a harmonic form, pulsed form, or step-heated form [20; 21]. When a sample is heated or cooled, heat propagates by diffusion through the material, which directly affects the temporal behavior of the surface temperature. Then, the thermal response of the investigated object, recorded by the infrared camera, is observed to reveal the presence of the defect. When a test target is heated by incident thermal energy, heat propagates by diffusion through the material, which directly affects the temporal behavior of the surface temperature. Then, the presence of defects is indicated by investigating the thermal gradient response on the surface of the inspection target with an IR camera. Controlled test conditions, such as heat source type, form, and amount of thermal energy, enable quantitative analysis and identification of defects [17]. The two primary common methods of active thermography are pulsed thermography (PT) and lock-in thermography (LIT). In PT, short and high-power thermal energy is applied to the surface of the target surface being inspected. In contrast, in LIT, sine-wave-modulated thermal energy is incident on the surface of the target, and phase delay and amplitude response are evaluated. An essential advantage of LIT is that it can characterize not only the defect's shape and size but also the defect's depth [14]. The mechanical behavior of composite structures can be affected by the presence of defects that may occur in the manufacturing process or in service conditions. In this regard, non-destructive testing, such as thermography, may be necessary to detect and characterize defects. Lock-in thermography is one of the most used active thermography techniques for inspecting composite materials [22]. Investigations on glass-reinforced polypropylene using lock-in thermography can be found in previous studies [7; 23; 24; 25; 26]. IRT was mainly conducted for the evaluation of internal inclusions of PP composites, detection of defects on the subsurface [27], evaluation of damage [28; 29], detection of delamination [30; 31], and evaluation of manufacturing defects (porosity and misalignment) [32]. Previous studies have demonstrated the capability of lock-in thermography techniques to detect defects. However, only some defect conditions and limited heat source conditions were considered. Lock-in thermography results are obtained very differently depending on the excitation power of the external heating source, the excitation frequency band, the total thermal heating cycle, defect conditions, and the thermal properties of the test object. In other words, it is necessary to comprehensively consider the above major experimental factors, conduct experiments, and apply them to the field. Therefore, in this study, an experimental investigation on the detection and non-destructive evaluation of various subsurface defects of polypropylene/glass fiber composites using multiple lock-in frequency modulated thermography is presented. The main objective of this study was to conduct quantitative evaluation of lock-in imaging on reference samples under different conditions of 24 defects of various sizes and depth. Fast Fourier transform (FFT) and harmonic approximation (HA) are the most widely used lock-in algorithms with high processing speed. Detectability was compared by FFT and HA. The second purpose was to quantitatively analyze the detectability of defects in terms of phase contrast and signal-to-noise ratio (SNR). Finally, the relationship between the SNR and the thermal image and its effect are presented. The rest of the paper is structured as follows. Section 2 provides information on basic concepts and algorithms related to this study. Subsequently, Section 3 provides information on samples and experimental methods applied in this study. Sections 4 and 5 present the experimental results and conclusion, respectively.