dfo: Precisely determining the color shades of effect coatings

Effect coatings look great, but usually place higher demands on the coater when it comes to coating. They also require more know-how when it comes to color measurement - something that even appraisers sometimes lack.

Design samples, master samples, standardized color values or comparative coatings are often used to determine the desired color shade. Since the comparison of the color tone can only be made subjectively by eye, colorimetry is an objective tool for verifying the visual assessment of color tone differences.

In colorimetry, hue has been specified for many years using the CIELAB color space. In this three-dimensional color space, each hue can be assigned with only three measured values (L- (lightness), a- (red-green) and b- (blue-yellow) values). If you want to compare color tones among each other, the total color distance within the CIELAB color space is calculated according to the following formula.

Color measuring instruments are used to measure the color tone. As with all measuring and testing devices, the appropriate expertise of the user is required when using them in order to avoid misapplication and unnecessary measuring errors. The following consulting case of DFO Service GmbH shows that the incorrect use of color measuring instruments can lead to considerable problems.

In the case of decorative panels in facade construction, there were strong visually perceptible differences in the color of the individual panels (see Figure 1). Before DFO was commissioned, two experts had already been consulted, who had come to different conclusions on the basis of their measurement results for the respective color measurements. The first expert found only a minimal color difference, which resulted in a still tolerable ΔE. The second expert found a considerable color difference with a no longer acceptable ΔE.

Choosing the right measurement method

Now it had to be clarified which of the two experts had arrived at the correct result. For this purpose, Ernst-Hermann Timmermann, Managing Director of DFO and publicly appointed expert for paint and powder coatings as well as paint shops, was called in as a further expert. It quickly became clear that both previous experts had made a typical but serious error in their measurements.

The expert, who had not detected any significant color difference on the basis of his measured values, had not taken into account that the coating to be assessed was an effect coating and had carried out the measurement with a so-called single-angle colorimeter. When measuring with a single-angle colorimeter, the color tone is usually measured at an angle of 45°. This is sufficient for solid colors, since the incident light is diffusely scattered, i.e. with relatively uniform intensity regardless of the angle - the color tone is therefore always almost identical, regardless of the angle from which the surface is viewed. Accordingly, for solid colors, a colorimeter with only one measuring angle is completely sufficient to measure the color tone. The color tone of a special-effect coating, for example metallic or pearlescent colors, depends on the viewing angle. For this reason, a multi-angle colorimeter must be used, in which the color tone is usually measured over the measuring angles 15° / 25° / 45° / 75° and 110°, sometimes also -15° (see Figure 2).

Depending on the intensity of the color or brightness change, the measured values of the respective measuring angles can differ from each other to a greater or lesser extent. Since this dispute involved an effect coating, the measurements of the DFO were also carried out with a multi-angle colorimeter and, in the end, strongly differing measured values were measured at the viewing angles of 15° and 25° between the different plates.

There were no serious differences in the values also measured for the 45° viewing angle, which is also mostly used with single-angle meters. The use of a 45° single-angle measuring device was confirmed by the first expert, which explains why he came to the conclusion that no shade differences could be detected.

The second expert used a spherical spectrophotometer with a so-called d/8° measuring geometry, in which the sample is diffusely illuminated. This is also an unsuitable measuring method for effect coatings. As a result, hue values were measured rather randomly, resulting in an unacceptable ΔE.