## 2 Material and Methods
### Experimental Setup Details--IRT  
A state-of-the-art IRT module was developed and presented previously to account for extreme compactness and lightweight required for automated inspection on a VR platform [20]. In brief, the module consisted of two main parts: a microbolometer shutterless longwave infrared (LWIR) sensor, the spectral response of 7-14 \(\upmu\)m, made of resistive amorphous silicon in aluminum housing that was equipped with a lens with a ratio of F/1.25, a focal length of 16.7 mm, wide field of view of 37.5\({}^{\circ}\), and manual focus end piece, and a 300 W halogen lamp excitation source housed in a bell-shaped reflector with IR-filtering eyepiece to block IR reflections from hitting back to the sensor. The sensor offered advanced technical specifications, including thermal sensitivity (noise equivalent temperature difference, NETD) of less than 50 mK, resolution of 640 \(\times\) 480 pixel, time-to-image of less than 1 s, and a frame capture rate adjustable between 9 Hz and 120 Hz.  
The module was assembled by mounting the sensor and source on an autoclave-cured carbon fiber reinforced polymer (CFRP) plate. Figure 2a presents the module face, while Figure 2b demonstrates it mounted on the VR platform. The lens protruded from the plate by 20 mm while the lamp position in the reflector was recessed by 30 mm from the plate. The dimensions of the sensor and the complete module were 30 \(\times\) 30 \(\times\) 45 mm\({}^{3}\) and 200 \(\times\) 150 \(\times\) 100 mm\({}^{3}\), respectively. The optimization of the module height (distance of plate from the surface under examination) was performed under the condition of IFOV maximization while maintaining full thermographic information from the surface. The IFOV should be maximum for the corresponding scanned area to be as wide as possible, which minimizes the acquisition timescale requirement for the whole aircraft structure. Based on this, a module height of 400 mm from the surface provided a optimal field of view of 320 \(\times\) 240 mm\({}^{2}\). Indicatively, a much smaller field-of-view of 200 \(\times\) 150 mm\({}^{2}\) was measured at a module eight of 250 mm. Most importantly, the respective weights of sensor and complete module, 70 g and 407 g, are indicative of the compactness and maneuverability offered, which makes them ideal for the targeted field application and mounting method. Prior to the start of IRT inspection, the lens end piece was focused on the surface under examination and a sensor calibration procedure, where the following involved exposure of the sensor to uniformly hot and cold surfaces, in sequence. During the initial phase of PPI-LT, the lock-in frequency was selected after the identification of the thermogram associated with the maximum contrast among those that were collected at various frequencies. In the second step, the surface within the field of view was analyzed in detail by locking thermography at the selected frequency.  
Figure 2: IRT module (**a**) dimensions and arrangement and (**b**) mounted on the prototype Vortex Robot Platform developed by Luleå University of Technology [31; 32] for automated aircraft skin inspection of BVID by Pulsed Phase-informed Lock-in Thermography (PPI-LT).  
A dedicated Graphical User Interface (GUI) was developed to allow for sensor control, thermogram acquisition, analysis, and communication with the VR platforms. The software offered functions for adjusting capture frame rate, calibrating, commencing, and ending PPI-LT, displaying live thermographic information, recording, analyzing, and post-processing thermograms, automatically detecting defects by pattern recognition, performing non-uniformity corrections of the IR sensor, thermogram analysis and post-processing, and many others. All of the thermograms were analyzed in the phase domain.