## 2 Optical Inspection Technologies in Railways
### Sensor Systems
#### 2.3.7 Tunnel  
Permanent monitoring and evaluation of the structural characteristics are of crucial importance for the service life of the tunnel and for safety during operation. Tunnel closures must be avoided and at the same time the data must be recorded without gaps and with the necessary accuracy. Optical sensors can be used here to enable non-destructive inspections. The current trend is toward comprehensive 3D recording of tunnel surfaces with specific measurement and mapping of prominent damage such as cracks. Here, too, there are different realization approaches. Laser measurements are used most frequently, but combinations with photogrammetric measurements are also used. Dibit Measuring Technique offers a photogrammetric measurement system that allows optional combination with a line-scan laser [38]. During continuous movement, the system records the tunnel surface at 360 degrees. The high-speed cameras provide the texture of the tunnel structures at a range of up to 30 m. An absolute accuracy of about 5 mm and a geometric resolution of 1 mm is achieved. This allows the possibility of detecting cracks up to 0.3 mm in width [64]. With its T-Sight 200, MERMEC offers a system with a rotating laser mirror that scans 360\({}^{\circ}\) transverse profiles. The measuring range is specified as 0.5 to 25 m [39]. The Laser Tunnel  
Figure 7: CIS with open lid showing the individual sensors: CRS (middle bottom), two LPS (left and right bottom), and LED panels for illumination. The cameras for wire-wear measurement are located between the LED panels (not visible). The metal rails are used for the semi-automatic cleaning system.  
Scanning System (LTSS) from Pavemetrics uses six laser scanners to acquire 2D images and high-resolution 3D profiles of tunnel linings at a range of up to 12 m, with 1 mm longitudinal scanning interval at speeds of up to 110 km/h. The manufacturer specifies a vertical accuracy of 0.25 mm [40]. TVEMA also combines up to six high-speed laser scanners to achieve minimum distances (0.05 m) between the profiles even at speeds of up to 320 km/h [41]. The Tunnel Inspection System (TIS) from Fraunhofer IPM uses two laser beams with different wavelengths to provide moisture content in addition to 3D information [65]. The system works with a relative measuring accuracy of 1 to 5 mm at a measuring rate of 2 MHz. Approximately 350\({}^{\circ}\) of the surroundings are detected with a range of up to 10 m [42].  
Fraunhofer IPM is currently developing a system for the contactless detection of cavities behind tunnel surfaces using laser technology. This innovative system leverages advanced laser scanning techniques to identify and map hidden cavities and irregularities without the need for physical contact with the tunnel structure. The use of laser technology allows for high precision and accuracy in detecting these anomalies, which is crucial for maintaining the structural integrity and safety of tunnels. This system represents a significant advancement in non-destructive testing methods and it is expected to greatly enhance the ability to conduct thorough and reliable tunnel inspections [65; 66].