Comparison of Visual and Instrumental Assessment of Colour Differences on Finished Wooden Surfaces

Matjaž Pavlič,Marko Petrič

doi:10.5552/drvind.2020.1954

Abstract

Staining of wood with various substances and processes is an important part of surface finishing of wood. Colour differences as a result of staining and of exposure of coloured wood during its utilisation are usually evaluated by instrumental measurements. However, the measurement results can show something else compared to what our naked eye can see. Due to inhomogeneity of wood, this discrepancy can be even greater in the case of finished surfaces. The aim of our research was to evaluate distinctions between visual perception and numerically determined colour differences on differently finished wooden surfaces, to get information at which starting point the colour difference becomes visible, and to establish whether it is related to the nature of the surface. We established that the visual assessment is influenced by many factors and that there is a correlation between visual and instrumental assessments. The colour difference ΔE* of 0.5 should be considered as a value when it starts to become visible, and at the value of 2.0, observers already considered the colour difference as a different colour. It was stated that we have some tolerance in perceiving the colour change. This tolerance is more expressed in the case of transparent coating systems.

Highlights

We must have in mind that different colour of the substrate underneath the semi-transparent coating film somehow contributes to different colour of the whole surface system and induces a colour change
We did not use the CIEDE2000 (EN ISO 116646, 2016) or AUDI2000 formula (Gómez et al, 2016), but we found a good correlation between CIELAB colour difference formula and visual perception
It is dependant on the real colour differences, as they can be determined by measurements, and on the kind of the finished wooden surface system

Summary

Introduction

The decorative appearance of timbers is due to the texture, or to the figure, or to the colour of the material and, in many instances, due to combinations of these (Dinwoodie, 2000). Such variability in wood appearance gives endless possibilities for its use. Sometimes it is hard to achieve the same appearance between different surfaces or different pieces of a wooden product, especially if it is not produced in a small series. The appearance of the finished wooden surfaces than depends on the wood used as a substrate, on the materials applied on it and on various other parameters, such as, for instance, an application rate

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