## 4 Discussion  
In the following, the findings of this study will be discussed from different perspectives, specifically focusing on method engineering and its implications for gas leak detection or TLS in general. It is worth mentioning that the absence of uniform methodologies for the quantitative assessment of gas leaks makes it challenging to directly compare our results with the results presented in the literature. It is important to highlight that all three methods introduced in this study (DAS-F, DAS-f and DAS-2f) have demonstrated sufficient capability to accurately locate a methane leak with a leak rate as low as 0.25 mL/min within a single methane concentration image at a frequency of 10 Hz. This exceptional outcome very likely extends the capabilities of other state-of-the-art technologies or concepts, including conventional or passive OGI (optical gas imaging), extractive sniffing techniques for gas detection and established non-camera-based and camera-based remote detection concepts. The implementation of the DAS method for OGI (e.g., DAS-OGI) by an ICL and a fast MIR camera effectively combines sensitivity and spatiotemporal resolution. These features result in a superior performance that is essential for the detection of small gas leaks (e.g., 0.25 mL/min). In practice, this means that methane concentrations at the leak and around the leak are immediately and strongly diluted by ambient air, and only concentrations below 50 ppm\({}^{\text{t}}\)m can be detected in the vicinity of a few millimeters around the leak origin. A drawback of this active DAS-OGI approach is that a sufficient amount of light needs to be backscattered. In this study, the use of a rough aluminum plate offers almost optimal conditions in the sense of a non-cooperative target, which makes this study a kind of \"best-case scenario\". The concentration image sequences show an additional information potential for leak detection and further quantification, such as a mass flow quantification approach that has been recently demonstrated [16]. These concentration images motivate a more sophisticated analysis using the spatiotemporal concentration information for more sensitive and reliable leak detection, localization and further quantification. The active OGI concept in this work shows a remarkable potential for standoff gas leak detection for very small leaks. It can be transferred to other gases with IR activity in the wavelength range between 3 and 6 \(\upmu\)m. This can be realized by changing the gas-specific laser without changing the camera. In this way, the focus tailored to the application can  
Figure 7: The noise equivalent concentration (NEC) image shows the pixelwise standard deviation for an average time span of 1 s (or bandwidth 1 Hz) for each method (DAS-F, -f and -2f). The mode values of the three NEC images are documented for the different methods. be transferred to other gases where small leak rates may be of greater importance than for methane, such as stronger greenhouse gases or toxic gases. Another application could be a leak test scenario during production to ensure the functionality and/or legal requirements of products. To the best of the authors' knowledge, an absorption line scan has never been investigated or demonstrated in the context of camera-based TLS. The pixel-by-pixel application of conventional DAS signal processing based on a BLC of the laser in combination with a non-linear line fit of the absorption line is not practically feasible or appropriate for the spectroscopic imaging in this work. For instance, the amount of data (e.g., 200 \(\times\) 200 pixels per image times approx. 30 spectral sampling points within 10 Hz), as well as the signal-to-noise ratio for methane leak rates around 0.25 mL/min, required modified DAS signal processing for this TLS-OGI concept. Consequently, a BLC based on a single or global BLC for the image sequence was derived by means of reference image and normalization to avoid about 200 \(\times\) 200 BLCs.