## 2 Materials and Methods  
In this research, a carbon fiber reinforced polymer (CFRP) coupon with flat bottom holes (FBHs), a CFRP coupon with barely visible impact damage (BVID), and a CFRP aircraft panel with backside stiffeners and a complex cluster of production defects were investigated. The samples are indicated as CFRP\({}_{\text{FBH}}\), CFRP\({}_{\text{BVID}}\), and CFRP\({}_{\text{PROD}}\), respectively.  
The CFRP\({}_{\text{FBH}}\) coupon measured 140 \(\times\) 90 \(\times\) 5.5 mm\({}^{3}\) and had a quasi-isotropic layup of [(\(-\)45/0/45/90)\({}_{3}\)]\({}_{8}\). Five circular FBHs with a diameter of 20 mm were milled from the back side, with remaining thicknesses of 0.85 mm, 1.64 mm, 2.47 mm, 3.68 mm, and 4.51 mm. A schematic illustration of this sample is provided in Figure 1a.  
The CFRP\({}_{\text{BVID}}\) sample measured 140 \(\times\) 90 \(\times\) 5.5 mm\({}^{3}\) and had a quasi-isotropic layup of [(\(+\)45/0/\(-\)45/90)\({}_{3}\)]\({}_{8}\). The BVID was introduced by dropping a 7.72 kg impactor from a drop height of 0.3 m, which delivered a measured impact energy of 18.5 J. A calibrated drop tower was used for the low-velocity impact [32]. An image of the impacted sample, on which the impact location is marked, is presented in Figure 1b. The amplitude maps obtained through an ultrasonic C-scan inspection (5 MHz focused ultrasound) in the reflection mode using dynamic time gating clearly revealed the complex damage cone through the depth of the sample (see Figure 1b).  
The CFRP\({}_{\text{PROD}}\) aircraft panel consists of a skin plate of \(500\times 1000\times 2.55\) mm\({}^{3}\) with several horizontal backside stiffeners (see Figure 1c). The part was originally intended to serve as a lower flap for an Airbus A400M. However, the component was scrapped because of a complex cluster of production defects. Through the use of a vibrometric NDT method, i.e., broadband mode-removed guided wave inspection, which was recently proposed by the current authors [33], a damage map was obtained, which revealed the presence of the backside stiffeners and the complex defect cluster (see Figure 1c).  
All samples were inspected through flash thermography in the reflection mode, in which a Hensel linear flash lamp was used to provide an optical flash with a flash duration of 5 ms, in which 6 kJ of energy was consumed. The cooling down regime of the CFRP\({}_{\text{FBH}}\) coupon was recorded for 100 s. The impacted CFRP\({}_{\text{BVID}}\) sample (both the impacted side and the back side) was recorded for 50 s, while a 40 s recording duration was used for the aircraft CFRP\({}_{\text{PROD}}\) panel. A cooled FLIR A6750 sc  
Figure 1: Photographs of (a) carbon fiber reinforced polymer (CFRP)\({}_{\text{FBH}}\) coupon (and schematic illustration of flat bottom hole FBHC); (b) CFRP\({}_{\text{BVID}}\) coupon, and the corresponding ultrasonic C-scan results; and (c) CFRP\({}_{\text{PROD}}\) panel with backside stiffeners, and the damage map obtained by a broadband vibrometric NDT technique [33].  
infrared camera with 640 pixels \(\times\) 512 pixels, a noise-equivalent differential temperature NEDT of \(\leq\) 20 mK and a bit depth of 14 bits was used. The IR camera, with an additional filter, was sensitive within the 3-5 \(\upmu\)m wavelength range. The distance between the flash lamp and each inspected sample was 300 mm, while the distance between the IR camera and the CFRP\({}_{\text{FBH}}\) and CFRP\({}_{\text{BVID}}\) coupons was 500 mm and 1000 mm for the aircraft panel. Hardware and software from Edevis GmbH guaranteed a proper synchronization between the flash excitation and the data acquisition. The recorded thermographic sequences were loaded into an in-house developed MATLAB toolbox to perform the post-processing.