## 1 Introduction
### Wear Measurement Methods
#### 1.6.1 Close-Range Photogrammetry  
Photogrammetry, specifically close-range photogrammetry, is a non-contact measurement method that can be used to reconstruct and evaluate the three-dimensional structure and properties of various objects using close-range images. By examining high-resolution images taken from different angles, close-range photogrammetry enables the generation of dense point clouds that represent the spatial distribution of surface points on an object. These point clouds form a basis for the creation of precise 3D models and enable further analysis to extract the relevant parameters and surface features of the object [46]. One of the main advantages of photogrammetry is that it does not require physical contact with the disc cutter. This reduces the risk of the cutter being altered or damaged during the measurement process. In addition, photogrammetry is also known for its cost-effectiveness compared to other sophisticated methods for generating dense point clouds as it relies on standard digital cameras and software tools for data processing. This makes it a more accessible and economically viable option for wear analyses based on 3D models in tunneling. Until now, researchers had not used close-range photogrammetry to assess the wear of TBM disc cutters. However, this method has been used to assess the wear of other mining tools (e.g., conical picks) [47; 48] or to measure the volume of rock fragmentation during TBM tunneling in granite formations [7]. [PERSON] et al. [47] aimed to create a photogrammetric 3D model of commercially available conical picks before and after wear. They performed a three-factor experiment on two levels to determine an optimal setting for the scanner system, considering light intensity, the presence of a polarization filter, and the distance to the scanned object. Using an algorithm developed by the authors, they compared a 3D scan of worn samples with a master model and were able to accurately assess the wear rate and condition. The study demonstrates the effectiveness of photogrammetry in assessing the wear of mining  
Figure 3: Tribological system with key components in the case of TBM tunneling. equipment and highlights its potential for use in cost-effective regeneration processes. [PERSON] et al. [48] investigated the wear rate of conical picks using a camera with a 12.2 MP sensor, a photo tent with a built-in light source, and a stepper-motor-driven turntable. Images of the cutting tools were captured from different angles and then processed to create 3D scans before and after use, which enabled the calculation of the volume difference and an evaluation of wear. [PERSON] et al. [7] used photogrammetry to measure the volume of excavated rock after tests using a linear cutting machine (LCM). They used the ShapeMetrix3D system, which generated three-dimensional geometry and digital coordinate data for the cut rock face. This enabled an accurate determination of the cutting volume, which was crucial for calculating the specific energy (SE) and evaluating the cutting performance. The results showed that the photogrammetric determination of the volume of the mined rock enabled the accurate identification of the optimal cutter spacing based on the SE values. This method improved the accuracy of SE estimation and the selection of optimal cutting conditions, resulting in higher efficiency in LCM testing and TBM operation. Photogrammetry offers several advantages for industrial applications, including the ability to provide detailed information about surface changes in material texture and localized damage in cutting tools. It is a cost-effective method that does not require expensive equipment or extensive training. It also facilitates the rapid scanning and sending of 3D texture files for analysis by wear experts who may not be available on-site. However, photogrammetry also has its limitations.