## 2 Methods and Materials
### Non-Destructive Testing (NDT) Techniques
Three-dimensional laser scanners precisely capture the size and shape of real-world objects, generating point clouds of data to create detailed three-dimensional digital models. This method is especially accurate for inspecting and measuring curved surfaces, providing a more comprehensive description than conventional measurement techniques [57]. Three-dimensional LiDAR (Light Detection and Ranging) scanning uses pulsed laser beams to measure distances by calculating the time it takes for the light to bounce back from an object. This creates a detailed point cloud, producing a three-dimensional digital map of the environment [58]. Table 3 highlights the key distinc \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Method** & **Heat Source** & **Heating Duration** & **Depth of Penetration** & **Primary Application** & **Suitable Aircraft Defect** \\ \hline PT & High-energy flash/laser & Short, intense pulse & Shallow to medium & Surface and near-surface defects & Surface cracks, delamination, impact damage \\ \hline LIT & Modulated laser/halogen lamp & Modulated, periodic & Medium & Deeper subsurface defects & Fatigue cracks, corrosion, bonding defects \\ \hline SHT & Infrared lamps/heaters & Gradual, continuous heating & Medium to deep & Subsurface and deep defects & Delamination, deep cracks, corrosion \\ \hline LPT & Continuous lamp/laser & Extended duration & Deep & Deeper subsurface defects & Corrosion, internal cracks, delamination \\ \hline FMT & Frequency-modulated energy source & Variable frequency pulses & Medium to deep & Subsurface defect detection & Delamination, debonding, cracks \\ \hline LST & Focused laser spot & Focused, short pulse on a spot & Shallow & Localised defect detection & Localised cracks, impact damage \\ \hline LLT & Laser line & Focused, continuous line heating & Medium & Defted detection along a line & Cracks, corrosion along structural lines \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison table of most common methods used in optically stimulated active thermography in the aerospace industry [39]. Figure 6: The schematic diagram for the thermography test [50]. tions between laser scanning and LiDAR scanning, focusing on aspects such as purpose, method, applicability, range, accuracy, and data output. [PERSON] et al. [40] used the FARO Focus 3D X130 laser scanner (FARO Technologies, Inc., Lake Mary, FL, USA) in a research project aimed at developing 3D scanning technology capable of inspecting an entire Su-30 MKM aircraft (Irkut Corporation, Irkutsk, Russia) and its components. The key innovation of this research was the creation of relevant 3D models to assist the maintenance team in enhancing aircraft maintenance operations and supporting reverse engineering processes. Three-dimensional laser scanning could achieve high acquisition speeds, ranging from hundreds to tens of thousands of data points per second, making it an ideal approach for inspection tasks. As a result, a complete scan could be accomplished in a matter of hours or days, depending on the object's size. The maintenance engineers scanned the aircraft from six angles, merging and aligning the resulting point clouds to create a comprehensive 3D model. The study concluded that 3D scanning could be beneficial for assessing damages with sub-millimetre accuracy. Notably, the Royal Malaysian Air Force used this technology to develop an Illustrated Parts Catalogue (IPC) for the Su-30 MKM aircraft and its components. An alternative approach to 3D scanning, compared to laser or LiDAR technologies, is photogrammetry. This method involves capturing 2D images from multiple angles and using 3D computer vision software to reconstruct a 3D model of the object.