Two wavelengths from the IR range used were found to be respon-sive to the layer thickness of water–dilutable compounds. The IR range of the EM spectrum also offered reference wavelengths for data normalization. These discovered wavelengths were then used to develop a practical method for layer thickness measurement.
Even though the method was developed with image data, it can also be scaled down for a line or a spot.
The Lambert–Beer law was found to be limited to layer thickness measurement within the thickness recommendations of compound manufacturers. Thus, an empirical third–order polynomial model was developed to obtain the required dynamic range for the mea-surement. The Lambert–Beer law could probably also have been customized using external terms to obtain a greater thickness range, but this was not tested because the empirical model was already seen to be appropriate.
Discussion
The method was also successfully extended from adhesives to other compounds due to their common diluter, water. This in-creases the number of possible applications in the wood industry, from glued wood production to all–over surface finishing. Further-more, the method could be applied for other background materials than wood, if the determined requirements for a background mate-rial are fulfilled.
The main advantage of the method from a practical point of view is that it requires only two wavelengths to be observed. Hence, the sensors used could be simple and low–cost compared to hyper–
spectral ones and computation would be simple due to the small dimensional data, which is a solid basis for real–time processing.
These are important features when searching for a high–throughput production line implementation.
The ability to measure wet compounds is a great benefit com-pared to the techniques employed for dry ones. Wet measurement allows the repair of faulty parts or their rejection from the pro-cess, which could save production resources. The method was also found to be applicable to drying process monitoring because dry-ing is the loss of water in the compounds used and this affects to the absorption of near–infrared radiation. On the other hand, this sets a constant water concentration requirement for the compounds used.
8 Conclusion
In this work a wide spectral range imaging system with photolu-minescence imaging ability has been introduced. The developed setup can be widely customized for different measuring geometries and spatial resolutions. The system was benchmarked with dif-ferent tests in order to obtain knowledge about performance and the appropriate operational range. The most significant challenges were found in the UV region of the EM spectrum due to several reasons. However, the UV region could hold a great deal of in-formation about the samples under examination, which suggests directing more effort towards the challenges of UV range imaging in the future.
The wide spectral range imaging system was applied to two re-search cases in the context of the wood industry. The first rere-search case was the acquisition of a public spectral image database of sawn timber for research purposes. Sample handling and selection were to be performed so that the final database would correspond to data from real sawmill conditions and be as versatile as possible. As a result, the final database contains approximately 44 million spec-tra, which connect spectral information over the UV–VIS–IR range pixel–wise. According to the performed analysis it was confirmed that the database provides possibility to access the spatial distribu-tions of wood properties and compounds.
The second research case was the development of a practical method for adhesive layer thickness measurement. Key wavelengths associated with layer thickness were determined and used to derive a model which requires the monitoring of only two wavelengths.
Furthermore, the key wavelengths were the absorption peaks of water, which allowed the extension of the method to other water–
diluted compounds as well. This feature offers considerable possi-bilities for the method because nowadays water is a common diluter in industrial and surface finishing compounds.
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Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences
Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences
Tapani Hirvonen
A Wide Spectral Range Imaging System
- Applications in Wood Industry
This study introduces a new wide spectral range imaging system with photoluminescence imaging capabil-ity. The system is benchmarked and applied to two research cases in the context of the wood industry. The first research case is the acquisition of a public spectral image database of sawn timber which potential is demonstrated with an analysis examples. The second research case is the development of a non-de-structive method for layer thickness measurement of freshly applied water-dilutable compounds.
dissertations| No 173 | Tapani Hirvonen | A Wide Spectral Range Imaging System - Applications in Wood Industry