Special effect coatings have been increasingly used in many industries (e.g. automotive, plastics industry) over
the past two decades. The measurement of perceived color differences on such coatings cannot be done by means
of traditional color-difference formulas (e.g. CMC(l:c), CIEDE2000, AUDI2000) as they lack to consider distinct
optical properties such as coarseness, glint and goniochromatism. However, there is a need to ensure quality
and colorimetric accuracy when designing and processing special effect coatings. In this paper, we present a
psychophysical experiment intended to serve as a basis for future work on a new generation of color-difference
formula(s) for multiple viewing geometries (viewing and illumination angle). We are especially interested in
assessing whether judging under a single geometry can lead to different results as judging under several (two)
geometries, i.e. whether the sum is more than its part.
To completely characterize the color of effect coatings, the spectral BRDF should be measured for a large number of
illumination/detection geometries. This is possible with the GEFE, the gonio-spectrophotometer developed at Instituto de
Óptica in CSIC (IO-CSIC). Twenty-four effect coating samples were prepared by AkzoNobel, containing metallic and/or
interference flake pigments. The spectral BRDF of all samples was measured by GEFE using a normalized procedure
under 448 different geometries. The results are presented in two-dimensional a*-b* diagrams and in three-dimensional
CIELAB diagrams to show the color travel. Four different descriptors were used to quantify the color travel of these
samples. We show that for many samples and geometries, the resulting colors fall outside the color gamut of a typical
display. It was found that the reflection data do not vary considerably at reciprocal geometries, allowing a reduction of
geometries to be done. Finally, we show that for 3D rendering applications, the reflection data from BRDF can be
strongly reduced. For example, we show that using the concept of flake-based parameters it is possible to only use inplane
geometries. The resulting rendered images were shown to be accurate enough for rendering on displays.
The grey scale method is commonly used for investigating differences in material appearance. Specifically, for testing
color difference equations, perceived color differences between sample pairs are obtained by visually comparing to
differences in a series of achromatic sample pairs. Two types of grey scales are known: linear and geometric. Their
instrumental color differences vary linearly or geometrically (i.e., exponentially), respectively. Geometric grey scales are
used in ISO standards and standard procedures of the textile industries.
We compared both types of grey scale in a psychophysical study. Color patches were shown on a color-calibrated
display. Ten observers assessed color differences in sample pairs, with color differences between ΔEab = 0.13 and 2.50.
Assessments were scored by comparison to either a linear or a geometric grey scale, both consisting of six achromatic
pairs. For the linear scale we used color differences ΔEab = 0.0, 0.6, 1.2,..., 3.0. For the geometric scale this was
ΔEab=0.0, 0.4, 0.8, 1.6, 3.2, 6.4. Our results show that for the geometric scale, data from visual scores clutter at the low
end of the scale and do not match the ΔEab range of the grey scale pairs. We explain why this happens, and why this is
mathematically inevitable when studying small color differences with geometric grey scales. Our analysis explains why
previous studies showed larger observer variability for geometric than for linear scales.
Conference Committee Involvement (1)
Measuring, Modeling, and Reproducing Material Appearance
3 February 2014 | San Francisco, California, United States