under conditions such as direct sunlight, effect coatings may also seem to glitter. Visual texture analysis may help characterizing effect coatings, by adding a new ...
Observation of Visual Texture of Metallic and Pearlescent Materials Eric Kirchner*, Geert-Jan van den Kieboom, Lan Njo, Rianne Supe`r, Roel Gottenbos Akzo Nobel Car Refinishes, Technology Center Colorimetry, Rijksstraatweg 31, 2171 AJ Sassenheim, The Netherlands
Received 9 May 2006; revised 17 August 2006; accepted 31 August 2006
Abstract: We have investigated the visual texture properties of effect coatings. The key question addressed in this work is how the visual texture properties of effect coatings can be visually assessed in a reproducible way. We show that it is possible to define two important visual texture parameters: Diffuse Coarseness and Glint Impression. These two parameters describe the visual texture under two different illumination conditions: diffuse and directional lighting, respectively. Strict definitions of these illumination conditions were found to be crucial when discussing visual texture of effect coatings. For each of these two visual texture parameters, experiments were set up, and observation procedures were designed that standardize the observation conditions. Visual tests were performed on both visual texture parameters. We found good results in terms of observer-repeatability and observer-accuracy. Our results show that if the visual texture of effect coatings needs to be visually characterized under different illumination conditions, such as different light booths or changing weather conditions, both diffuse and directional illumination should be included separately. Ó 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 256 – 266, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20328
Key words: appearance; metallics; pearlescent materials; effect coatings; color matching; perception INTRODUCTION
When a car needs to be repaired after an accident, the color of the repaired part should match closely with the color of the rest of the car. Color characterization techni-
Contract grant sponsor: Senter (Dutch Ministry of Economic Affairs). *Correspondence to: E. J. J. Kirchner (e-mail: eric.kirchner@akzonobel. com). C 2007 Wiley Periodicals, Inc. V
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ques have reached a high level for uniform (solid) colors. However, nowadays most cars carry effect colors, for which the level of instrumental and visual characterization lags behind, in spite of decades of research. Further improvements are necessary because of the growing percentage of cars with effect coatings,1 and because increasingly exotic special effect pigments are introduced in the market.2–9 Metallic coatings are the oldest type of effect coatings, and consist of aluminum-flake pigments in a transparent medium. In the ’eighties, pearlescent pigments were introduced with flakes containing interference layers. Flakes are approximately flat, thin and circular, with a diameter between 5 and 50 mm. They tend to lie parallel to the coating surface, thus increasing light reflection towards the specular direction. The popularity of effect coatings originates primarily from their change in color depending on viewing and illumination geometry. In contrast to conventional solid coatings, for metallic coatings, chromaticity and lightness strongly depend on viewing/illumination geometry. Therefore, a metallic coating accentuates the curved profile of an object, thus increasing its attractiveness. For pearlescent coatings, also the hue changes with viewing/illumination angle, which makes the color effect more spectacular when compared with metallic coatings.10–13 The large impact of effect pigments on the angular dependence of color is the reason why many publications address the questions how to measure this angular dependence,14–22 and how effect coatings can be characterized by it.8,23-29 However, effect pigments also affect another aspect of coating appearance, namely the visual texture. This term refers to the perceived small-scale nonuniformity of the color of an effect coating, when viewed at a distance of a meter or less.30,31 Depending on the type of effect pigment and the viewing conditions, the coating surface appears to be very smooth, or a fine or coarse irregular pattern may be visible in the coating. But COLOR research and application
under conditions such as direct sunlight, effect coatings may also seem to glitter. Visual texture analysis may help characterizing effect coatings, by adding a new dimension to the current color analysis. It is also important because visual texture affects the perception of the overall color.32–37 Visual texture has been mentioned as an important appearance attribute by Hunter38 and others,39–42 but the available literature on visual texture of effect coatings is relatively small and plagued by inconsistent terminology.30 This work seeks to propose a method to characterize visual texture of effect coatings and provide a useful terminology. We will discuss existing literature, and propose clear concepts and consistent terminology. Based on these definitions, we will discuss visual test procedures developed using the concepts thus developed, and discuss the accuracy thus obtained.
EXISTING LITERATURE
There is a large amount of literature on visual texture in general,43 but the available scientific and patent literature on visual texture specifically of effect coatings is very limited. The older literature on characterizing metallic coatings ignores visual texture aspects.44–46 Even recently, some descriptions47,48 of the visual appearance of effect coatings can be found, in which visual texture aspects are ignored. Besides color and gloss, older literature on characterizing metallics mentions brilliance.49 Illumination and viewing conditions for observing brilliance are not clearly specified, but brilliance seems to refer to a very large reflection value close to the gloss angle. At such viewing angles it is impossible to distinguish visually brilliance because of flake reflections from direct reflection at the air-paint interface (gloss), because both brilliance and gloss show as a highlight of an entire area. Therefore here we will not consider it as part of visual texture. In more recent literature, visual texture is generally appreciated as an important aspect of visual appearance of effect coatings. This was explicitly stated for example in studies and patents from the National Institute of Standards and Technology,41,42 DuPont,50 Renault,51 and BASF.52 In a series of patents and scientific publications,53–62 researchers from Kansai Paint and Murakami Research Laboratories study visual texture, or ‘‘microbrilliance’’ in their words. The aim of the Kansai/Murakami work is on image analysis parameters rather than on correlates of visual perception, as we choose here. Finally, researchers from Toyota have tried to couple visual texture parameters, not dissimilar to those described in this article, to the macroscopic appearance of effect coatings.63–67 In most literature on this subject, only one aspect of visual texture is considered (cf. Ref. 68). This parameter is mostly called sparkle,69–75 although sometimes the terms brilliance76,77 or glitter75,78 are used. These three Volume 32, Number 4, August 2007
terms are rarely defined, and even when the terms are defined,75 it remains unclear whether these terms refer exactly to the same phenomenon. The one aspect that all available descriptions seem to share is that these terms refer to flakes that show up very intense under directional illumination. Apart from sparkle (and/or brilliance and/or glitter) other visual texture aspects are rarely mentioned. In several publications (cf. Ref. 79) it is even stated that when the directional illumination is replaced by diffuse illumination, the visual texture of effect coatings nearly vanishes. This is surprising, because although it is true that sparkle vanishes under diffuse illumination, another visual texture aspect shows up. In the rare cases2,54,74,78,80 where this aspect is mentioned, it is mostly called coarseness or graininess. But again the terminology is not clear. Although seldom mentioned in literature on effect coatings, the importance of coarseness should not be overlooked. Outside the effect coating literature, coarseness has been shown to be the most important visual texture parameter for many materials.37,81,82 We note that suppliers of effect pigments do use the term coarse(ness). However, in that case the term relates only to coarse or fine grades of aluminum pigments, as an indication for the diameter of the flake pigments. However, this use of the word coarse(ness) should not be confused with the term coarseness as we use it here as a visual attribute of texture. It is not possible to find a oneto-one relation between the flake diameter and the visual coarseness of the paint containing that pigment. For example, the visual coarseness of the same effect pigment can change when different binders are used, or when the orientation distribution of the flakes in the dry paint is changed by additives. To date, two articles by McCamy are the only ones that seek to give a complete description for the appearance of effect coatings, including visual texture.30,31 In these articles many visual aspects of effect coatings are discussed, including sparkle. Also the importance is stressed of specifying illumination conditions for observing various aspects of appearance, and the terms diffuse versus directional lighting are introduced in this context. In our work the thoughts and ideas of McCamy have been extended in several ways. To that end we introduce a well-defined terminology, standardize visual assessments and develop equipment to carry them out, and quantify the visual texture parameters. Apart from this, McCamy limits himself explicitly to visual texture (or micro appearance, in his terms) observed under directional light, whereas we also discuss visual texture under diffuse lighting. We have further specified the term ‘‘diffuse lighting’’ with respect to McCamy’s use of the term. Finally we have assigned priority to two aspects of appearance while McCamy’s work stops after giving a long list of different appearance aspects. In our opinion, other parameters such as coherence glitter, binocular luster, binocular glitter and binocular mottle are more difficult to recognize visually, and therefore less important in practical visual tests. 257
In this article we will not discuss visual texture phenomena that are unwanted artifacts of effect coatings. An example of this is orange peel,75 which may be described as wavy patterns of light and dark color regions. GENERAL TERMINOLOGY
Literature overview has shown that when it comes to visual texture and the various aspects of it for effect coatings, many different terms are being used by different authors, and the same term may be used in different meanings. A clear cause for this confusion is that these terms are rarely well defined, and often it is unclear what aspect of appearance is exactly being referred to. In the few cases where an attempt is made to describe what a term refers to, often contradictions are found with other such descriptions. For example, the term glitter is explained as ‘‘a grainy finish’’ in one publication,78 or as the situation where the pinpoints of sparkles are widely separated from each other.83 Sparkle has been defined as the static situation of pinpoints of light,83 but also as the phenomenon in which nearby shining flakes quickly turn on and off.84 In order to make an end to this confusion, we will propose here definitions for various visual texture aspects of effect coatings. The definitions will be formulated as strict as possible, in order to avoid further confusion. For the same reason, we will use new terms that are easily understandable, thus avoiding possible confusion with earlier meanings of the terms. It has been said43 that ‘‘we recognize texture when we see it but it is very difficult to define.’’ This holds especially as we avoid trying to explain visual texture aspects in terms of physical flake properties such as diameter, flatness, or concentration. Instead, we will focus on the visual aspects, as shown in the following definitions. Visual texture is the nonuniformity of the color over the surface of an object depending on the size and organization of small constituent parts of its surface material. The nonuniformity may relate to variations in the three color dimensions but also in intensity. The variations may manifest itself as an irregular pattern on any length scale, or as isolated spots. The concept of visual texture includes many aspects, because we can change the nature of the variations and their length scale. For effect coatings, literature and intensive discussions with people from our own color laboratories have led us to select the following two dominant visual texture aspects for effect coatings: Glint Impression is the overall impression of several or many tiny light-spots (‘‘glints’’31) that are strikingly brighter than their surrounding. Instead of brightness, it may also be their color that distinguishes the glints from the background. The glints may be expected to switch on and off when the observation/illumination geometry is changed. Glint Impression is visible under intense unidirectional illumination conditions only. 258
Under intense unidirectional (abbreviated as: directional) illumination, glints are so striking that other visual texture aspects than Glint Impression have no practical importance. However, when the illumination conditions become less directional, the glints become less pronounced. In the ultimate case of fully diffuse light, the glints have totally disappeared30 (which is very striking when observed for the first time!). However, during the process of changing the illumination conditions from directional into purely diffuse, a type of visual texture shows up that is totally different from Glint Impression. This visual texture is more pattern-like but less intense than Glint Impression, and was not recognized as such by the most important previous investigators in this field. Diffuse Coarseness is the perceived contrast in the light/dark irregular pattern exhibited by effect coatings viewed under diffuse illumination conditions. The perceived value of the Diffuse Coarseness will depend on the lightness difference between the light and dark regions, and also depend on the length scale of the pattern. This definition includes a recent definition37 for textile fabrics, where it was stated that ‘‘coarseness is related to the spatial repetition period of the local structure’’. An important part in the definition of Diffuse Coarseness is the term ‘‘diffuse illumination conditions’’. This term will be described in more detail in the next section. By definition Glint Impression is the visual texture seen at directional illumination, whereas Diffuse Coarseness is the (totally different) visual texture seen at diffuse illumination. Roughly speaking, these situations correspond to outdoor conditions under direct sunlight versus overcast sky. Obviously, in uncontrolled situations like outdoors the actual situation will probably be neither perfectly diffuse nor perfectly directional, but at some point in between. Also in the case of light booths, which are often used for color matching, the illumination conditions are a mixture of diffuse and directional illumination. Obviously this does not mean that observations under the extreme illumination conditions have no relation to observations in the light booth. Instead, it means that we should find the relative contributions from diffuse coarseness and glint impression when observing visual texture in the light booth. By quantifying the visual texture under the extreme conditions, it will be possible to relate visual texture properties observed in one light booth with those from another light booth.
ANCHOR PANELS
Even with the definitions given in the previous section, observers may find it difficult to quantify an aspect of visual texture. For example, an observer may find it difficult to balance the number of visible glints and their apparent intensity into one parameter Glint Impression. We have solved such problems by introducing anchor panels. Eight neutral gray metallic coating panels were prepared with a gradual change in the number and intensity of glints. The COLOR research and application
FIG. 1. Anchor panel sets for (a) Diffuse Coarseness and (b) Glint Impression. Note that the large dynamic range of glints makes the reproduction on a picture far from adequate.
overall color of the panels was kept as constant as possible. Observers were told that the Glint Impression for these panels is exactly equal to one, two,... etc. up to eight. The anchor panels were used in a series of eight-alternative forced choice experiments where observers attempted to match Glint Impression of a sample panel to the Glint Impression value provided for the eight anchor panels. Obviously, in this way the observer is forced to balance the various factors that constitute Glint Impression. The required number of observers and anchor panels was found by power analysis and a step-by-step optimization of the procedure. It was found that we needed separate anchor panel sets for diffuse coarseness and glint impression (Fig. 1). The color formulas for these two sets are not exactly the same, but they are very similar. We note that the photographic reproduction of glint impression is poor, because of the high dynamic range of glints.
VISUAL TESTS ON DIFFUSE COARSENESS
ing as good as possible the situation of a car exposed to overcast weather conditions, in which the sky can be considered as a diffusely illuminating horizontal ceiling. We note here that a different interpretation of diffuse illumination would be isotropic illumination, i.e., illumination with equal brightness from all angles. In that way, diffuse illumination condition would be exactly the opposite of directional illumination conditions, in which all light comes from one single direction. However, perfectly isotropic illumination is impossible to achieve in practice. We have tried such an approach for example by using a commercial integrating sphere for observing effect coatings. In that case only one eye can be used for observation, no more than two panels can be viewed at the same time, and viewing conditions are very uncomfortable leading to eye strain.35 Our final implementation of diffuse illumination is shown schematically in Fig. 2(a). A dedicated ‘‘diffuse room’’ was made in our laboratory, in which an isotropic and luminous flux is emitted all over the ceiling. The walls are white to enhance the light reflection. Clearly this situation does not make the light intensity equal when measured in all directions, but it does correspond to the more practical case of diffuse illumination on a clouded, overcast sky without direct sunlight. Figure 2(b) shows a picture of the ‘‘diffuse room.’’ Light is emitted from the ceiling that consists of diffuser cloth, so as to make the light diffuse that comes from a large number of daylight simulating fluorescent lamps. Figure 2(a) shows that light incident on the panels comes from a large solid angle. The illumination level can be varied at will, but for standardized visual tests we have chosen a level of 2000 lux in order to be comparable with outdoor conditions and with typical light booths. The color temperature was measured to be 6150 K. Observation height, angle, and distance were fixed using a visor. We placed a regular table covered with grey felt to minimize disturbances of the illumination field. On that table we mounted a rotatable sample table to fix the panels. During several visual tests we determined the optimum angle of this table. We found that the best reproducible results were found when the rotation table is tilted by 158 and when the observer looks from 498 at the sample, where both angles are measured with respect to the horizontal [Fig. 2(a)]. This corresponds to an observation angle of 268 with respect to the sample normal. Observers are asked to determine which anchor panel best resembles the sample panel as far as Diffuse Coarseness is concerned. In case the Diffuse Coarseness of the sample panel would appear to be larger than for the anchor panel numbered eight, a larger value could be given.
Experimental Set-Up The ASTM specifies diffuse illumination as ‘‘an extended-area source.’’85 We have made a well-defined practical implementation of diffuse illumination by imitatVolume 32, Number 4, August 2007
Accuracy Test A set of 398 effect colors was used to test the accuracy of the visual test method. The set contains only car col259
FIG. 2. (a) Sketch of experimental set-up for visual tests of Diffuse Coarseness. (b) Pictures taken in diffuse room during visual test on Diffuse Coarseness.
ors, selected such as to give an even distribution over color space (Fig. 3) and a wide variation in Diffuse Coarseness values. In this set, 41% of the panels are purely metallic panels (i.e., containing aluminum flakes and noneffect toners), 21% are purely pearlescent (i.e., containing pearlescent flakes and noneffect toners), while the remaining 37% of the panels contain a mixture of metallic, pearlescent and noneffect toners. For 42 panels, which is about 10% of the total, the colors were sprayed out in duplicate, and these duplicates were included in the visual test to get an estimate of the observer repeatability. Seven observers of normal color vision participated in the test. A total of 3066 magnitude estimations were made and recorded. Each observer had experience in observing visual texture, and was trained to recognize visual texture attributes. During the main experiment, each observer was asked to assign a numerical value to the dif260
fuse coarseness of each sample panel presented. In order to improve observer accuracy and reproducibility further, observers were allowed to use not only the integer values for Diffuse Coarseness that are given by the anchor panels, but also noninteger values up to quarter precision. Therefore the allowed assessment values are 1, 1.25, 1.50, 1.75 up to and including 9.00. We found that allowing observers to give noninteger assessments significantly improved the intraobserver and interobserver accuracy. For example, the percentage of samples where an individual observer deviates by only one Diffuse Coarseness unit or less from the average observer, increases from 46% to 67% by allowing quarter precision judgments. The large number of sample panels involved made it necessary that each observer had to attend seven observation sessions with each 56 or 57 sample panels to be assessed. Each session was performed within 1 h. The samples for each COLOR research and application
FIG. 3. Distribution in color space of sample set, used for Diffuse Coarseness tests at 08 illumination and 458 detection angle.
session were presented in a unique random order for each observer, in order to minimize effects from learning, fatigue etc.
Results and Discussion A Grubbs test86 was performed to check the data for outliers. In this way we found that 1.5% of the observations are outliers, and these were removed from the data set. We also investigated whether there were observers significantly differing from all other observers. This was determined using the multiple range test of Bonferroni.87 For every sample the differences between each individual Volume 32, Number 4, August 2007
observer and all other observers are calculated. These differences are averaged with a 95% confidence interval for each observer. Since no observer significantly differed from all other observers, no observers had to be removed from data analysis. The correlation coefficient between the individual observer judgments and the average values of all observers varies from 0.91 to 0.96 for the various observers, with an average value of 0.95. Therefore the correlation between the visual assessments of the different observers was really good. The absolute difference in Diffuse Coarseness between an individual observer and the average observer varies from 0.42 to 0.72 unit for the various observers, with an average value of 0.55 unit. This is a first indication that the difference in Diffuse Coarseness between subsequent anchor panels is in reasonable correspondence with the discrimination ability of the observers. The observer repeatability (i.e., intraobserver agreement) gives a similar indication, as shown in Table I. We found that in 85% of all repeated visual assessments the two assessments agree up to a maximum of one Diffuse Coarseness unit. In 62% of the cases the agreement is even only 0.50 unit at most. This suggests that one Diffuse Coarseness unit is indeed the preferred distance between subsequent anchor panels, because the repeatability data suggest that a Gaussian fit would give a standard deviation of 0.50 units. An analysis of the observer reproducibility (i.e., interobserver agreement) leads to the same conclusion. In 84% of the cases, the assessment by an individual observer differs one Diffuse Coarse unit or less from the average value from all other observers. In 58% of all observations, the agreement is even better than 0.50 unit. The results on interobserver and intraobserver agreement are considered to be very well, especially taking into account that all observers agree that the difference in Diffuse Coarseness between subsequent anchor panels appears to be very small when the anchor set is seen for the first time. For five samples, which is 1% of the total, the average observer assigned a value larger than eight. This suggests that a ninth anchor panel on the large Diffuse Coarseness end of the scale does not seem useful. On the other end of the Diffuse Coarseness scale, a solid color would provide an obvious nontextured starting point of the scale. In the anchor panel set used here, the first panel was not a solid panel but an effect panel with barely perceivable diffuse coarseness. However, the sample set of 398 panels did not contain panels with a Diffuse Coarseness value smaller than for the first anchor panel, so no additional anchor panel was needed for this test. All Diffuse Coarseness classes (of one unit width) turned out to contain at least 20 samples, providing statistical justification for our results. We conclude that the intraobserver and interobserver agreement both suggest an acceptance criterion of one Diffuse Coarseness unit. 261
TABLE I. Observer repeatability and reproducibility for Diffuse Coarseness and Glint Impression at three geometries. Diffuse coarseness
Glint impression at 258
Glint impression at 458
Glint impression at 758
Maximum Repeatability Reproducibility Repeatability Reproducibility Repeatability Reproducibility Repeatability Reproducibility disagreement (%) (%) (%) (%) (%) (%) (%) (%) 0 0.25 0.5 1.0 2.0
30 43 62 85 97.6
9 36 58 84 98.1
38 38 74 74 99.4
4 29 53 81 98.6
VISUAL TESTS ON GLINT IMPRESSION
Experimental Set-Up For observations of glint impression, the discussion in the previous sections already showed that relatively intense, unidirectional light sources are required. After testing various illuminance settings, we chose to have just over 10000 lux falling on the anchor panels. Special attention was paid to make the illuminance over the full set of anchor panels as uniform as possible, and in the final arrangement we measured values ranging from 11750 to 13730 lux, i.e., a variation of 14%, which we found acceptable. The color temperature was measured to be 5600 K. Glint Impression depends strongly on the angles at which a panel is illuminated and observed. Therefore it is not possible to define a set of anchor panels that keeps the same Glint Impression for several illumination/observation geometries. Since we want to be able to investigate the dependence of Glint Impression values on angle, we could not have the anchor panels change angles together with the sample panels. Therefore we introduced two rotation tables, one containing the anchor panels, the other the sample panel. As a consequence, two identical light sources had to be used (Fig. 4). In order to facilitate the research on ideal viewing/illumination angles, we automated the rotation tables for easy operation. Similar to the set-up for Diffuse Coarseness observations, a visor was used to fix the observer’s eye. By making the illuminance at the sample panels slightly smaller than at the anchor panels, the difference in observation distance is compensated for, and a similar glint impression results for anchor and sample panels. At the sample panels, we measured an illuminance of 15430, 13220, and 8730 lux for the geometries with 358, 458, and 608 illumination angle, respectively. The effective illumination angles are shifted by 28 because the sample panels are partly illuminated by the light source at the backside. The illumination and observation geometry is illustrated in Fig. 4(a). The rotation angle for the anchor panels was fixed at 558 from horizontal, which corresponds to an illumination angle of 558 and an observation angle of 18 with respect to the panel normal. This corresponds to an aspecular angle of 558. Also here, the effective illumination angle is shifted by 28. 262
36 36 76 76 100.0
5 34 57 87 99.7
39 39 72 72 100.0
4 37 62 90 99.9
The rotation angle for the anchor panels is fixed for all observation sessions, in order to enable a comparison of the Glint Impression of a sample at various rotation angles. We tested sample panels at various rotation angles in order to find a small set of angles that covers the change in Glint Impression when rotating an effect coating. We chose three basic rotation angles (Table II). Moreover, the automatic rotation table of the sample panel was programmed such that it wiggled automatically around each basic rotation angle, with an amplitude of 38 and a rotational speed of 58 per second. This wiggling was introduced to make observers better aware of the glint phenomenon, and to emphasize the dynamic character of glints (which in everyday life is caused by motion of the car or observer). In the final set-up, the angular diameter of the light sources is 3.38 and 2.98 with respect to the anchor panels and sample panels, respectively. These small values ensure that the illumination is highly unidirectional. Additionally a baffle was introduced to prevent the observers from looking at their own reflections in the sample panels. For evaluating the accuracy of visual assessments of Glint Impression, we used a set of 216 effect colors. Almost all samples (94%) in the set were car colors. This set was not identical to the set used in the Diffuse Coarseness test, because we wanted to have an approximately even distribution of samples over the various Glint Impression values. A fast pretest showed that this was indeed the case, and that the samples covered color space quite well (similar to the test panels used for Diffuse Coarseness, Fig. 3). The pretest also showed that it made no sense to ask for judgments on a quarter scale. Because we now asked for only half scale values, the number of panels required for the Glint Impression test is smaller than in the case of the Diffuse Coarseness test. The sample set contained 39% metallic panels, 28% pearlescent panels and for 33% contained a mixture of metallic and pearlescent toners. In order to investigate the observer repeatability, 24 panels (i.e., 11%) were sprayed in duplicate, giving a total of 216 þ 24 ¼ 240 panels. Seven observers of normal color vision judged Glint Impression for each of the 240 panels under three observation/illumination geometries, giving a total of 7 240 3 ¼ 5040 magnitude estimations. The observers all COLOR research and application
FIG. 4. (a) Sketch of experimental set-up for visual tests of Glint Impression (not to scale). In the sketch, the sample panel is at the so-called 458 geometry. By rotating the sample, the two other geometries are obtained, at 258 and 758. The anchor panel is always at the 558 geometry. (b) Pictures taken for experimental set-up for visual assessments of Glint Impression.
had experience in observing visual texture. The way they were instructed and the values they could specify for Glint Impression were exactly copied from the procedure described for Diffuse Coarseness assessments in the previous section.
For assessing Glint Impression at the 258 geometry, the correlation between individual observers and the average values of all observers is 0.97, with a minimum value of 0.96 and a maximum value of 0.98. For the other geometries these numbers are comparable, with averages of 0.95
Results and Discussion
TABLE II. Optimum geometries for observing Glint Impression.a
For the Glint Impression test, a Grubbs test86 showed that the data contained some outliers, for which the tvalue was larger than 2.02 and the difference in largest and smallest judgment was at least 1.0. This gave a total of 33 outliers (i.e., 0.6% of all judgments) that were removed from the analysis. The multiple range test of Bonferroni87 showed that no observer significantly differed from all other observers. Therefore no observers had to be removed from data analysis. Volume 32, Number 4, August 2007
Parameter
Illumination angle (8)
Anchor panels First angle, samples Second angle, samples Third angle, samples
55 35 45 60
Observation angle (8) 7 13 3 12
Resulting aspecular angle (8) 48 22 42 72
a Illumination angle and observation angle are given with respect to the panel normal.
263
for the 458 geometry and 0.97 for the 758 geometry. These correlations are also very similar to those obtained for Diffuse Coarseness. At the 258 geometry, the absolute difference in Glint Impression between individual assessments and the average value of all observers is 0.54 units on average, (minimum and maximum values: 0.43 and 0.72 units). This result is very similar to what was obtained for Diffuse Coarseness. At the 458 and 758 geometry, however, the average value decreases to 0.49 and 0.43, respectively, which indicates that observers judge more accurately at these angles. Apparently for each geometry, the average observer can well distinguish the anchor panels, which are separated by 1 unit. Although the range of anchor panels stops at the value of 8, the observers were allowed to give values for Glint Impression up to and including 9. Similarly, values starting from zero could be given, although the range of anchor panels starts at the value of 1. Further, observers could give any integer value or half-integer value. Therefore the range of allowed judgments was 0, 0.5, 1.0,...,8.5, and 9.0. The results of observer repeatability and observer reproducibility are included in Table I. The observer repeatability for all three geometries is shown to be very similar to what was found for Diffuse Coarseness. The observer reproducibility was found to be best for the 458 and 758 geometries, with a value very similar to what we found for Diffuse Coarseness. For Glint Impression at the 258 geometry, the reproducibility seems to be slightly worse. The results also confirm our decision to use anchor panels that are separated by one unit, and to allow observers to give judgments with half unit precision. Although the distributions are not very even, each Glint Impression class contains at least 15 sample panels if we define classes with a spacing of one Glint Impression unit. The only exception to this is that at the 758 geometry the class of Glint Values rounded off to 8. Only six panels had this large glint value so far away from the gloss angle, which is logical since most flakes are aligned flat in the paint and reflect light to angles closer to the gloss angle. We found that generally the Glint Impression value decreases when the observation angle is further away from the gloss angle, as expected. The sample sets that we have used for assessing Diffuse Coarseness and Glint Impression are not identical, but they do have 163 panels in common. Figure 5 shows that for Glint Impression at the 258 geometry, to a large degree these two visual texture parameters are independent. For example, a medium value of Diffuse Coarseness can be realized by effect coatings with very small but also with relatively large values of Glint Impression. Although the physical origin of Diffuse Coarseness and Glint Impression are obviously both the reflection and transmission characteristics of flakes, the two parameters apparently probe different aspects. The mutual independence of these two visual texture parameters proves that for practical purposes such as repairing effect coatings of 264
FIG. 5. Comparison between Diffuse Coarseness value and Glint Impression value (at 258 geometry), for the 163 panels that were present in both visual tests. For Glint Impression at other geometries, very similar graphs result.
cars, it can be important to consider both diffuse and directional illumination conditions. The results for observer repeatability and reproducibility for Glint Impression are well comparable with those obtained for Diffuse Coarseness. Therefore there is no direct need to try to refine the Glint Impression parameter into constituent parameters, although sometimes observers find it hard to assign a Glint Impression value to a sample. For example, for a sample with many glints that all have small intensity it is unclear if a small or a large Glint Impression value should be assigned. Therefore it is tempting to let observers evaluate sub-parameters such as the Number of glints and the Intensity of glints separately. Also the apparent size of glints, which may exceed the actual size of the reflecting flakes because of glare effects,79 is such a sub-parameter. Exactly the measurement of these sub-parameters has been published in a patent by DuPont.50 However, here we decided not to make that distinction, because our results showed that this is not needed for obtaining reproducible results, and because the visual assessment of these sub-parameters are very hard to standardize. Another potential sub-parameter is the angular extend over which a particular glint remains visible. In the case of metallic pigments, for silver dollar type flakes (that are very flat) this angular extend may be small, whereas for corn flake types (that are much less flat) this angular extend may be much larger. Thus defined, the angular life-time of glints may be an important aspect for characterizing the vividness of an effect coating, and seems to coincide with the term ‘‘dynamic hot spot travel’’ in a patent from General Motors.80 We are planning to investigate this parameter more closely in the future. CONCLUSIONS
We have investigated the visual texture properties of effect coatings. Our results show that it is possible to define two important visual texture parameters for which observation procedures can well be standardized. Strict COLOR research and application
definitions of illumination conditions were found to be crucial when discussing visual texture of effect coatings. Diffuse Coarseness and Glint Impression are applicable under diffuse lighting and under directional lighting, respectively. For each of these two parameters, experimental set-ups were made and visual tests were performed. It was shown for both parameters that 8 to 10 different levels can be distinguished by observers that had 5 h of training in visual tests on visual texture. The correlation coefficient of individual observers with all other observers is large, 0.96 on average for Diffuse Coarseness and 0.95–0.97 for Glint Impression. Observer repeatability and accuracy were shown to be 0.5 units on a scale from 1 to 8, both for Diffuse Coarseness and for Glint Impression. Obviously, practical observation conditions for effect coatings, such as light booths or outdoors, are neither purely diffuse nor purely directional. This does not make the present investigation purely academic. Instead, it shows that both extreme lighting conditions should be considered, in order to ensure that the visual texture properties of an effect coating are characterized well also when a different light booth would be consulted, or when the weather would change. Therefore, when a lighting system needs to be chosen for inspection or quality control of effect coatings, new demands should be fulfilled. Diffuse illumination is one condition that should be considered, whereas assessments under directional light are to be included as well. These specifications are currently not part of the demands to light systems for assessing paint appearance.88 A full psychophysical characterization of visual texture will require measurements of physical quantities that correlate with it. We are currently carrying out such research, with the aim of developing an instrument for measuring glint impression and diffuse coarseness.89
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