## 3 Development of a Laboratory Setup
### Offline Laboratory Setup and Characterisation  
The integration process of an LCI measuring system into a laser machine is not trivial, in order to be able to make adjustments at an early stage, if there are deviations in the desired parameters, it is advisable to first build a laboratory system. A setup based on a Thorlabs LSM03-BB lens was constructed. Although the optical path is significantly shorter than later in the system, this setup can be used to characterize the light source used and the spectrometer. These two components will later be operated unchanged with the target components. The final system uses the same spectrometer and the same light source, but a lens developed for laser material processing is used. In order to perform a system characterization, it must be ensured that the intensity of light arriving at the line camera in the spectrometer does not lead to an overexposure of this sensor, see Figure 3. This overexposure would make the obtained results unsuitable. At the same time, throttling the output power of the light source leads to a changed spectrum and to a change in the optical properties of the system. To avoid this, a Thorlabs V800A fiber optic attenuator (FOA) was installed directly after the light source, which allows the output power to be attenuated without changing the spectrum of the light. An overload of the sensor in the spectrometer, as shown in the left picture in Figure 3, leads to the fact that the Fourier transformation cannot determine the frequency spectrum correctly, ghost frequencies are present, shown in Figure 3 on the right side. The result is not usable, because no utilizable information is available. By throttling the FOA, the amount of light is reduced so that the full dynamic range of the spectrometer can be used and thus the spectrum is correctly resolved by the Fourier transform. The degree of this throttling depends on the material to be measured. Since this provides a usable result in the Fourier transformation, the peak in the spectrum generated by the surface can now be identified and later translated into a valid distance information. This  
Figure 2: Final system model of the spectrometer beam shaping. The installed components are as follows: transmission grating NIR-1500-903 from Ibsen Photonics (1); lens package from Thorlabs consisting of AC254-125-B, LE1104, LC1120, LC1120, LE1156 and AC254-100-B (2 to 7); line scan camera AViVA M2 CL 2010 from e2v (8). ensures that the peak produced by the surface can be detected clearly and correctly later on, see Figure 4.  
A plane mirror is used to determine the measuring range of the system. This mirror is located in the focal point of the objective and on a piezo stepper, which can move the mirror in vertical direction. The piezo stepper (Physik Instrumente (PI) N-381.3A) used here has a maximum travel of 30 mm, with a resolution of 20 nm. This means that both the travel and the resolution are at least one order of magnitude better than the system can theoretically perform. The mirror was moved by the piezo stepper with a step size of 0.1 mm and measurements were taken at this position. This process was repeated until the measured signal could no longer be distinguished from the background noise of the sensor due to the distance. The result is that the measuring system has an axial measuring range of 2.3 mm and is therefore large enough to measure the defined structures in the process. In a next step of the calibration the axial resolution of the measuring system was determined and a correlation between a measured height and the position of a peak was derived. According to DIN 32 877 a random position was approached and the position of the resulting peak of the surface was recorded. A total 50 measuring points within the measuring range were considered. By changing the height of the piezo stepper, the position of the peak in the evaluation domain is changed so that a calibration factor can be determined. By dividing  
Figure 4: Measurement of a plane mirror with adjusted attenuation of the light intensity.