# Combined Use of 3D and HSI for the Classification of Printed Circuit Board Components
## 4 Discussion
However, the iterative strategy going from more dominant to fewer dominant rules has shown, that even smallest elements can be separated with acceptable accuracy. This is already a reliable base for further extensions. Nevertheless, it is necessary to pay attention to correct modelling, and thus it is important to study the spatial and spectral properties of the objects beforehand and to investigate to what extent the acquired data can make these properties visible. Generally, when setting up rules, it is important to avoid overfitting. Thresholds that are overly sensitive can result in incorrect classifications when applied to new datasets and should therefore be chosen carefully. In addition, the underlying 3D data quality must be considered when forming rules based on 3D geometry. As example, in this article the parameters and thresholds for rule formation were chosen with regard to the used sensor system and the resulting point cloud. In the case of a different 3D sensor with another resolution, accuracy and noise behavior, it would be necessary to adjust the parameters. Another aspect to consider is the performance of the system and the assessment of use in industrial practice. We are aware that the use in industrial practice poses certain challenges such as high-speed processing, an appropriate integration of the sensor technology into the sorting process and also the handling of dust and dirt in an industrial context is a factor that should not be underestimated. This means that the sensor technology used and the acquisition and processing strategy must meet these requirements. The approach proposed in this article has the potential to fulfil these demands, but certain modifications with regard to the 3D sensor technology are necessary. The strategy of the approach (sensors for acquisition, registration approach and data processing) is general and is mainly affected by the specifications and the capacity of the senor technology used. The structured light scanner used in this article is not advisable in the context of a sorting process due to the needed fast processing speed. However, it has the benefit to produce high quality 3D data, which in turn is advantageous in the context of small objects. The use of such a sensor system would therefore be recommended in the context of randomized quality checks where speed is not important. However, there are alternatives that are suitable for use in production lines [36; 37]. The study of [38] investigates the applicability of 3D sensor technologies in the context of production lines. As a result, four techniques for capturing 3D data in the context of industrial applications were identified, namely laser triangulation, time-of-flight (ToF), shape-from-focus (SFF) and stereovision [38]. Mainly laser scanners based on triangulation are widely used in production lines. Such systems are able to achieve depth resolutions down to the range of a few micrometers and are a robust and standardized technology that could also be used in the context of the proposed approach. The use of hyperspectral sensors in industrial applications is also conceivable and is particularly widespread in the food industry [39; 40; 41]. The Specim FX17 sensor used was able to detect 670 FPS at full resolution. It is not always necessary to use the full resolution. By analyzing the data beforehand, spectral ranges or even only a small number of bands can be used, which ultimately leads to a further increase in the detection rate. Important in the context of hyperspectral sensor technology is the illumination unit. This must be installed in the measurement setup in such a way that the 3D technology used is not influenced. At this point we would also like to briefly discuss the influence of object soiling on the measurements of the hyperspectral sensors, since in practice it is precisely such objects that are involved. Dirt on objects influences the reflectance on the surface of the objects.