# Detection of Pest Feeding Traces on Industrial Wood Surfaces with 3D Imaging
## 4 Discussion  
This work proposes a three-dimensional surface imaging system using laser triangulation. A sensor with a resolution of 1536 \(\times\) 512 has been used. For a field of view (FOVx) of 175 mm, a resolution of 0.125 mm is obtained in the X- and Y-axes. The ability to detect wormholes larger than 1 mm in diameter has been confirmed at this resolution. Increasing the imaging resolution makes it possible to image holes with smaller diameters. To do this, sensors with a higher resolution can be used, i.e., 4096 \(\times\) 3072 (Automation Technology), which allows imaging with a resolution of \(\Delta\)X = 175 mm/4096 pixels = 43 \(\upmu\)m/pixel. These are theoretical values. Such a solution requires much more computing power and a higher registration frame rate. It would have to be tested in the laboratory and then on an industrial scale in the presence of noise generated by the production line. The analysis showed that images recorded at a resolution of 0.125 mm/pixel allow a valid assessment of both the detection of wormholes and a complementary analysis of the quality of the surface finish. Images recorded at resolutions of 0.2 and 0.25 mm/pixel can be used to assess the presence of wormholes. However, images recorded at 0.5 mm/pixel resolution should not be used for this inspection task due to insufficient points per 1 mm\({}^{2}\) area. Laser triangulation imaging allows the determination of geometric parameters describing the surface at the chosen imaging resolution. This type of imaging is unaffected by changes in surface colour caused by changes in wood colour or moisture content. The effect is achieved by using a focused laser beam to illuminate the imaged surface. The 3D image is reproducible regardless of changes in surface colour. A 2D image taken under visible light would reflect colour changes in the wood surface, which in extreme cases would result in the inability to distinguish defects from the surface. If the inspection task is defined such that any wormholes detected on the surface disqualify the inspected material from further processing, the task is completed when at least one hole is detected. Such material is marked as defective. If the material is sorted into quality categories, it is possible to analyse the distribution of holes when more than one hole is found. This makes it possible to decide whether part of the material should be removed and the remaining part reused in other technological processes. The surface inspection task can be extended to include measurement of hole diameter and area to evaluate further the suitability of the material for use in another product range. Example measurement results are shown in Figure 5.  
By recording a three-dimensional image of the wood surface, it is possible to determine a wide range of parameters describing the quality of the wood surface. This work focuses on detecting specific defects, i.e., wormholes larger than 1 mm in diameter. The system has been designed and manufactured for use at the initial sorting and quality control stage when materials enter production. It removes material with visible damage and sorts it into quality classes. Other wood surface damage classes are also detectable using the proposed configuration. Previous studies have focused on defects related to, among others, fungi, wane, cracks, and dimensional defects [55; 56]. In the case of natural cracks, it should be noted that the surface area of such a flaw will be significantly larger than the hole made by the pests. These cracks are also considered surface defects. However, defects can be sorted using either defect area information or defect distribution information. Healthy knots will not be considered a defect because their surface is correct and flat. In the case of knots with a visible perimeter resulting from wood decay processes, this perimeter can also be detected using point analysis and classified as a defect. The discolouration does not affect the evaluation of geometrical defects visible on the wood surface.