Significance Infrared thermography is a typical representative of non-destructive testing technologies. According to the external excitation source, infrared thermography can be roughly divided into passive thermography and active thermography two categories. Among them, passive thermography uses the difference between the temperature of the measured target and the surrounding environment to realize infrared detection of the target in the heat exchange process between the measured target and the environment, and can detect the health status of the equipment and components in operation without the need to impose an external excitation source. Unlike passive thermography, active thermography requires external excitation source to stimulate the object to be measured and generate thermal contrast, which is more suitable for defects detection. Compared to other external excitation, active thermography with laser as excitation source has been widely used in recent years because of its advantages of stable output power, uniform energy distribution and strong controllability.

Progress The basic principle of laser thermography non-destructive testing and the composition of laser thermography non-destructive testing system are introduced at first. The laser thermography non-destructive testing system is generally composed of laser driver, an infrared thermal imager, an image acquisition unit, an image processing unit, and a motion control device to improve the detection efficiency. Then the laser thermography is classified from two perspectives of excitation mode and heating mode. There are three excitation modes of laser thermography, which include laser point, line and surface. While laser thermography can be divided into laser pulsed thermography, laser lock-in thermography and laser pulsed phase thermography three categories according to the heating mode. Different excitation and heating modes have their applicable application scenarios. Since laser thermography was first proposed in the 1960s by Kubiak, the main research focus on improving the performance of laser thermography non-destructive testing. To overcome insufficient precision, low efficiency and limited detection objects, researchers from different countries have continuously optimized and innovated the laser thermography non-destructive testing technology from various aspects such as excitation and heating mode, detection conditions, image processing, and put forward a series of improved methods and strategies. Because of the advantages of laser thermography, countries have successfully carried out a number of studies to detect defects. The application scenarios have been gradually expanded, no longer limited to the defect detection of metal material and composite materials. It has been applied to the defect detection of chips, inductors, ceramic materials, as well as the measurement of thermal properties of materials and even artworks. The end of this paper surveys on prospects of laser thermography non-destructive testing, which intends to provide reference to the development and research of laser-based thermography technology.

Conclusions and Prospects  At present, there are still few studies focusing on the improvement of laser thermography heating uniformity and the reduction of the noise influence from the perspective of improving the excitation mode. Most studies still need complex image post-processing methods to improve the image quality. At the same time, the deeper applications such as three-dimensional defect measurement are less considered in defect detection. The level of intelligence in the quantification and automatic identification of defects also needs to be improved. Limited by the detection system, laser thermography still has a lot of room for improvement in large components and field applications. With the development of a new generation of information technology and the improvement of detection technology accuracy requirements, laser thermography non-destructive testing has expanded from surface defects such as cracks to internal defects including delamination and debonding, from qualitative defect analysis to quantitative defect calculation, and from two-dimensional defect identification to three-dimensional defect reconstruction. Based on the detailed analysis of the existing research, it is considered that 3D quantitative defect detection combined with intelligent algorithm is a future development direction of laser thermal imaging. And the development of high performance small portable laser can also expand the application field of laser thermography non-destructive testing.