Photo-scanning colorimetry of wood and transparent wood coatings

Abstract

Although conventional spectrophotometers are well-known to give very accurate and reproducible measuring results of the wood surface colour, the areal resolution of this method is limited due to the fixed aperture size. In this study, the applicability of a colour calibrated flatbed scanner for areal-resolved colorimetry was investigated. For this purpose, the colorimetric values of several wood species calculated from images of a colour calibrated photo-scanner were correlated with those obtained by a conventional spectrophotometer. Further, this method was used to evaluate the discoloration of earlywood and latewood of Siberian larch (Larix sibirica) during Xenon arc exposure as a demonstration of the potential application of this method. In addition, an investigation concerning the possibility of image analysis for the evaluation of the colour change as a consequence of the application of transparent coatings was done. Good correlation among the measured methods could be found by applying relative colour values ΔL* Δa*, Δb* and ΔE*, whereas absolute colour values L*, a* and b* obtained rather weak correlations. The investigation of the relative discoloration of earlywood and latewood of Siberian larch showed a higher amount of discoloration in the earlywood. The investigation concerning the colour change induced by the application of transparent coatings showed that it is possible to evaluate the discoloration by the coating for earlywood and latewood separately. This could be the basis for a more quantitative comparison of quality parameters of coatings like the visual contrast enhancement effect, known as wet-look or “Anfeuerung” of transparent coatings. It was concluded that when evaluating colour changes on wood the use of a calibrated photo-scanner is a valuable addition to conventional spectrophotometry.