cff-version: 1.2.0
abstract: "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."
authors:
  - family-names: Dingemans L. M. (Liesbeth)
    given-names: 
  - family-names: Papadakis
    given-names: V. M. (Vassilis)
    orcid: "https://orcid.org/0000-0003-3188-4577"
  - family-names: Liu
    given-names: P. (Ping)
    orcid: "https://orcid.org/0000-0002-9975-3998"
  - family-names: Adam
    given-names: A. J. L. (Aurèle)
    orcid: "https://orcid.org/0000-0002-6727-946X"
  - family-names: Groves
    given-names: Roger
    orcid: "https://orcid.org/0000-0001-9169-9256"
title: "Quantitative coating thickness determination"
keywords:
version: 1
identifiers:
  - type: doi
    value: 10.4121/uuid:5e253d43-6859-448e-820b-5ae2eb470cc7
license: 4TU General Terms of Use
date-released: 2017-12-04