## 1 Introduction
### Measurement Methods to Determine the Limiting Mechanisms of Laser Beam Cutting  
Fundamental studies investigate the reasons for the compromises in cutting speed and cut edge quality in LBFC. Therefore, correlations between the laser beam, the material, and the gas jet need to be known, which are experimentally difficult to access. Reasons include the complexity of parameter influences, missing analyzing parameters, and methods to characterize the process mechanism and the miniaturized dimensions of the process zone [21; 22]. Hence, most knowledge of the process mechanism is based mainly on theoretical considerations. The latest research activities concentrated on developing measurement approaches to obtain detailed insights into the process mechanisms. The energy deposition, respectively heat conduction, was found to be the most important influencing factor for LBFC [1; 23; 23].  
The heat conduction itself is not measurable, but high heat conduction achieves a fast cutting speed as well as a high-quality cut edge and requires a thin melt film with a high temperature difference [1; 3; 23]. Accessible indicators are the melt film thickness, the melt flow, or the temperature distribution at the cut front. The melt film thickness is addressable by cross-sections of the cut kerf [24; 25]. This method is only applicable after finishing the cutting process and requires a high preparation effort. In addition, the melt film thickness during one cut varies depending on the striations. Therefore, a high number of cross-sections per cut have to be performed to gain reliable results. Observing the melt flow is common to draw conclusions about the cutting result [21; 22; 26; 27; 28; 29; 30; 31]. Due to the limited accessibility of the process zone, artificial conditions have been created for process observation [21; 27; 28] or complex experimental setups have been designed [21; 22; 27; 28; 29; 30], which are not suitable as standard evaluation. Nevertheless, the investigations found that melt flow generates striations [21; 27] and influences burr attachment [29]. In addition, the melt flow contains information about the resulting intensity respectively temperature distribution of the process zone [30; 32; 33]. For this purpose, the angle-dependent absorption of the wavy melt flow has been calculated. These indirect temperature-measuring methods are based on assumptions and simulations. An important result is the strong correlation between the temperature distribution of the process zone and the cut edge quality [26]. Consequently, further measurement setups have been developed to directly address the temperature of the melt film [34; 35; 36; 37; 38; 39]. The direct temperature measurement differs from the indirect one by a calibration. The process zone to be imaged on the detecting sensor is quite small for laser beam cutting. A sufficient spatial resolution ensures a differentiation of various temperature regions in the process zone. All of the examined direct temperature measurement setups achieve spatial resolution of the complete processing zone just by repositioning of the sensor [34; 35; 36; 37; 38; 39]. Camera-based measurement setups performed particularly well, as they provide a sufficient temporal resolution in addition [26; 38]. The temporal resolution is especially relevant for observing the melt flow or during high-speed cutting. A higher temporal resolution makes the application of the temperature measurement setup more versatile.