## 2 Basic Concepts of Optical Thermography NDT
### Active or Dynamic Thermography  
In active thermography, a certain amount of energy is introduced to the object, which is initially in thermal equilibrium, and thermal contrast is produced to highlight the features of interest. The energy transfer to the object can be pulses or either in a modulated continuous form. A halogen lamp, optical flash lamp, and cold or hot air guns can be used as an energy source. Figure 1 shows the experimental setup of typical active thermography [23; 24].  
The controlled experimentation condition, such as the amount and form of stimulation, allows not only identification of defect but also quantitative analysis of anomalies [6; 25]. In NDT application, active thermography can be used to det  
Figure 1: General schematic diagram of active thermography testing.  
such as delamination and inserts in the automotive and aerospace industry, investigate the interior structures such as lightweight honeycomb structures, and recognize deeper material deficiencies [26].  
The inspection can be conducted either in transmission or reflection mode. In transmission mode, the heating source and thermal imaging device are positioned on opposite sides of the object being inspected, whereas in reflection mode, both the heating source and the thermal imaging device are positioned on the same side of the object.  
The two primary classical or conventional practices are pulsed thermography (PT) and lock-in thermography (LIT). In both techniques, thermal energy is delivered into the object in which the heat propagates by diffusion through the material. Then, the thermal response recorded by the infrared imaging device is observed to reveal the presence of the defect. In PT, a short- and high-power thermal energy is applied to the object's surface being inspected, and the thermal response of the stimulated surface is observed in a transient state. It is a fast and popular thermal imaging technique in NDT&E, whereas in the LIT, the object's surface being inspected is stimulated by a periodically modulated thermal wave, and the thermal response of the surface is observed in steady state.  
To access the defect information, efficient signal processing and filtering is very important. With the help of adapted image and signal processing algorithms, it is possible to detect small discontinuities inside structures or extract material characteristic information. By reducing the amount of acquired data, the process is accelerated, but a certain decrease in accuracy has to be kept in mind [27].  
Results obtained in active thermography are often damaged by different noise sources such as moisture, external reflections, variations in the specimen's emissivity, and non-uniform heating. In thermal images, these noise effects create unusual thermal patterns and complicate damage detection. To reduce noise in thermal images, several signal processing techniques have been developed to improve the defect detection and quantification evaluation of each active thermography method.  
Table 1 compares representative optics utilized in active thermography methods, including flash lamps used in pulsed thermography for defect detection [28; 29]. The primary differences between stimulation mechanisms of these devices determine two essential methods of optical infrared thermography [28]. We can submit a high-energy heat pulse in short (few milliseconds) time intervals using photographic flash. In contrast, halogen lamps provide a lower amount of energy submitted to the object in a relatively longtime frame (few seconds), usually in either continuous or modulated form.