Quantitative coating thickness determination

posted on 04.12.2017 by Dingemans L. M. (Liesbeth), V. M. (Vassilis) Papadakis, P. (Ping) Liu, A. J. L. (Aurèle) Adam, Roger Groves
The coating selected in this research was a film-forming low-gloss wood lacquer for outdoors (Transparant zijdeglanslak voor buiten, Wijzonol Bouwverven B.V.). This coating was selected because it is semi-transparent commonly used wood coating, making it a suitable coating to visualize with OCT and hyperspectral imaging. This spruce-colored coating is based on organic solvent with an alkyd binder and applied using a brush. The samples that were prepared are: 1) Coating in a very flat and reflective silicon wafer to be able to determine the refractive index of the coating. 2) Different thickness coatings applied to a thin cover glasses in order to be able to determine the K and S coefficients from the KM-model and the extinction coefficient from the LB model. In order to measure the coefficients, these cover glasses were placed on a black-and-white checkerboard, as described below. 3) One to four layers of the coating were applied to a Medium-Density Fibreboard (MDF) plate covered with acrylic gesso. This reflective non-absorbing background serves as a reference for assessing the performance of the models. The hysperspectral imaging setup used in this study consisted of an IMSPECTOR V10E (Specim©) spectral camera, operating in the 400-1000 nm range. The visible range was selected due to the main absorption characteristics of the studied coating layers which is within the range of 400-1000nm.Optical Coherence Tomography (OCT) is a suitable technique for imaging the interfaces in a semi-transparent material and is therefore a logical choice for measuring coating thickness. This technique is based on low-coherence interferometry to measure light reflections from refractive index interfaces. As shown in Fig. 5, a customized OCT system was built by using a superluminescent diode (FESL-1550-20-BTF, Frankfurt Laser Company) centered at 1550 nm with a full width at half maximum of 60 nm, resulting in a 20 µm spot size and an 11 µm theoretical axial resolution inside the coating layer (considering a refractive index of 1.5). Depth-scanning for OCT was realized by the means of an optical delay line (ODL-650,MC, OZ Optics, Ltd). Lateral scanning of a sample with an x-y translation stage (T-LS28M, Zaber Inc., Canada) allowed for a 28 mm scanning range in two directions. Obtained data were bandpass filtered and an envelope detector was used to recover the depth dependent signal.



TU Delft, Faculty of Aerospace Engineering, Non-Destructive Testing Laboratory; TU Delft, Faculty of Applied Sciences, Department of Imaging Physics


TU Delft


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