## 2 Optical Inspection Technologies in Railways
### Overview  
Conventional measurement systems are still used in many areas of railway infrastructure inspection. Systems based on tactile methods or visual inspection still make up the majority of railway measurement techniques. The recording and post-processing of data is often still carried out manually or semi-automatically. However, the use of optical sensor technology is being driven forward, primarily to achieve independence from human observers and to further establish automation.  
Visual inspection using cameras relies on computer vision techniques. Image processing is used, including edge detection or segmentation. The images obtained are automatically analyzed using customer-specific image processing software. Typical applications are crack or screw detection [9]. The advantage here is the high speeds that can be achieved at relatively low cost. It is considered the most attractive technique for detecting surface defects [10]. A disadvantage is the dependence on ambient lighting. Camera systems only work in daylight or under artificial lighting. Adequate focusing must also be ensured, which is not trivial with recording rates that are often very high and with changing distances.  
The use of laser scanners is becoming increasingly important in many areas of application. They provide impressive results by capturing precise data in a short amount of time and are often used for routine measurement of clearance. A big challenge for the use of laser-based measuring devices is the different reflectivity of objects, and also harsh environmental conditions, e.g., fog [11]. In contrast to camera-based methods, laser scanners can also be used at night without artificial lighting. They deliver high point densities of up to several million points per second with a precision of a few millimeters. This, in turn, generates large amounts of data that need to be processed. However, the post-processing time can be reduced by analyzing and interpreting the measured values in real time, e.g., by comparing measured geometries with a digital model. Another important issue is absolute eye safety, which must be guaranteed. This can be considered by using special wavelengths and fast-rotating deflection mirrors. An overview of laser scanning technologies and their applications for road and railway infrastructure monitoring can be found in [12].  
Fiber optic sensors are characterized by their high sensitivity, resistance to electromagnetic interference, and robustness in harsh environments [13]. The technology is based on changes in the physical properties of light waves that propagate in the optical fibers due to external excitation. They are used in civil structures to measure strain, temperature, and vibration. They enable autonomous long-term monitoring over long distances and can detect changes to infrastructures at an early stage. Installation poses a major challenge. Broken fibers or bending loss can occur, which can lead to signal loss [13]. Compensatingfor varying ambient temperatures is another problem. Maintenance must also be carried out on site. In the following section, the focus is more on scanners and camera systems and less on fiber optics.