# Review on Rail Damage Detection Technologies for High-Speed Trains
## 3 Non-Destructive Testing Methods for High-Speed Train Rails
### Laser Vision Inspection Methods  
The railway track structure is not a large-area planar structure, so laser vision technology is often used to detect surface defects on the railway track based on visual technology. A laser vision system combines structural light sources with image acquisition devices to capture 3D images of railway track surface features. In order to efficiently and effectively capture railway track surface feature images, multiple angle acquisition devices are used to coordinate with each other (Figure 13). For example, one acquisition device is responsible for the inner feature image of the railway track, and another acquisition device is responsible for the outer feature image of the railway track.  
Through this method, real-time, efficient, and high-quality image data of rail surface features can be obtained, providing data for the detection of rail surface wear and rolling contact fatigue [70]. The laser vision system can also be combined with an inertial motion unit (IMU) to collect and measure a series of geometric parameters of the railway tracks in [71]. For example, the detection process of rail corrugation includes extracting effective laser stripe areas, suppressing stripe highlight textures, extracting stripe centerlines, defining rail boundaries, and measuring the position of and deviation in laser measurement points, as shown in Figure 14 [72]. Of course, gratings of different wavelengths can also be used to capture surface feature image data of railway tracks through illumination.  
In 2002, [PERSON], [PERSON], [PERSON], and others from Hebei University applied the light cutting method combined with visual inspection technology in the detection of rail wear, greatly reducing the time required for rail image processing [73]. In 2014, [PERSON], [PERSON], and others from Southwest Jiaotong University proposed a detection method for  
Figure 13: Schematic diagram of laser vision inspection.  
rail wear cross-sectional area based on existing rail inspection vehicles. They discretized the analytical formula for the standard rail profile and calculated the step integration based on profile registration to obtain the calculation formula for rail wear cross-sectional area [74]. [PERSON], [PERSON], [PERSON], and others from Southwest Jiaotong University, in 2015, proposed to reconstruct and restore the three-dimensional spatial contour of steel rails based on phase measurement contour technology. The research results showed that the online measurement of the three-dimensional contour of steel rails can be achieved. This method has good application prospects in measuring rail wear [75].  
[PERSON], [PERSON], [PERSON], and others from Beijing Jiaotong University conducted in-depth research and analysis on the calibration method and accuracy impact of cameras in measurement systems. They proposed a spatial coordinate conversion algorithm for laser contour points and applied it to develop a non-contact dynamic measurement device for rail wear, with an accuracy of 0.012 mm and a relative error of 0.28% [76]. In 2015, [PERSON] et al. combined laser vision measurement technology to reconstruct the full contour of the rail using the condition that the left and right rail head treads have the same characteristics, combined with the iterative nearest-point algorithm for accurate matching. They also achieved the dynamic measurement of rail wear within a certain detection distance. The performed tests on a manually operated track inspection vehicle at a track site. The results show that the calibration accuracy of the measurement system is 0.0049 mm, and the wear measurement accuracy can reach 0.1 mm [77].