Coarseness and glints - European Coatings Journal with diffraction gratings. However, all these new...
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Quelle/Publication: Ausgabe/Issue: Seite/Page: European Coatings Journal 11/2006 46 Coarseness and glints Texture parameters help to characterise appearance of effect coatings. New effect pigments are constantly being developed with increasingly complex colour shifts and appearance, which cannot be characterised adequately with conventional multi-angle spectrophotometers. Even more sophisticated measurement geometries do not guarantee full characterisation. However, two additional 'texture' parameters, namely the diffuse coarseness and the glint impression, can be used to effectively distinguish different effect coatings shades. An instrument to measure texture is under development. Eric Kirchner*, Lan Njo, Klaas de Haas, Michael Rösler. * Corresponding Author. Contact: Dr. Eric J.J. Kirchner, Technology Center Colorimetry, Akzo Nobel Car Refinishes, P.O. Box 3, 2170 BA Sassenheim, Netherlands, Tel: +31 71 3083168, Fax: +31 71 3083011, [email protected]. Over several decades, effect pigments have become a key ingredient in paints for the car industry and other coating markets. This started with the introduction of aluminium flake pigments in metallic coatings. In contrast to solid colour coatings, effect coatings show what is called a flop effect: the colour depends on illumination and viewing angle. This is caused by the predominantly flat orientation of the flakes in the paint, which increases reflection in the specular direction and suppresses reflections at other angles. For metallic coatings, the result is a light-dark flop effect, which accentuates curved contours of e.g. a car. Active research in this field showed around 1990 that three illumination/detection geometries are sufficient to accurately characterise the colour of metallic coatings, and these were adopted in the ASTM E-2194 standard [1, 2]. A popular multi-angle spectrophotometer such as the X-rite "MA68II" uses five illumination/detection geometries. Newer effect pigments show more complex behaviour Later, pearlescent pigments were introduced. Initially these consisted mainly of mica/metal oxide flakes, adding a relatively small change in colour hue to the colour shift of effect coatings. More recently, many different types of effect pigments have been put on the market. They are based on different physical mechanisms, such as combinations of selective interference, diffraction, scattering and absorption of light. These phenomena are realised by using single- or multi-layer structures with specific thicknesses and refractive index values, sometimes embossed with diffraction gratings. However, all these new effect pigments produce coatings with oftenlarge colour shifts when viewing conditions are changed. Thus, many modern effect coatings are no longer adequately characterised by using conventional multi-angle spectrophotometers. A long debate has been held on how many and which illumination and detection angles are to be chosen for characterising effect coatings [3, 4, 5, 6]. No firm conclusions have yet been reached, and several different working groups are addressing this problem [7, 8]. According to the standard practice for colour measurement of interference pigments currently being developed by ASTM, it is expected that adequate characterisation of effect coatings requires at least seven geometries [7]. However, there will always be a risk that a new effect pigment is introduced with a striking colour shift "in between" two geometries that are included in any new standard. Also, including many geometries in standard measurement protocols represents huge costs in instrumentation and data management for the thousands of colour formulations found in the car refinishing industry. Clearly a balance needs to be found between the benefits of using many measurement geometries for colour characterisation and the efforts involved in measuring and handling data. This balance may be favourably influenced if a property that may be termed 'texture' is incorporated as a different approach to characterising effect coatings. Why spectrophotometers cannot identify 'texture' In addition to colour, the visual texture of effect coatings can be used for appearance or pigment characterisation. Experience has shown that two effect coating samples could be measured as a match when using a multi-angle spectrophotometer, but not pass visual acceptability if they differ in sparkle or texture [9]. The reason is that a spectrophotometer averages the colour over a certain area of the coating. For example, with a standard multi-angle spectrophotometer the very coarse and very fine coatings shown in Figure 1 are measured as a very close match, because their average colour is the same. Therefore a multi-angle spectrophotometer totally fails to identify the respect in which these two coatings differ. Obviously texture is an important (but often overlooked) characteristic aspect of the visual appearance of effect coatings. For many effect coatings, texture determination is a more efficient means of characterisation than increasing the number of angles for colour measurement. A comprehensive study of the appearance of effect coatings, including several texture-related aspects, was published by McCamy [10]. The two most important texture aspects of effect coatings have now been identified [11]. These relate to two opposing illumination conditions, diffuse and directional light. Diffuse light conditions occur when the light intensity is equal from all directions. Examples are a clouded sky, a typical spray booth for a car, or a very diffuse light booth such as that from Verivide. Under diffuse lighting, a light/dark pattern is seen in effect coatings, which can appear to be coarse or fine depending on the coating. Directional light conditions imply that the greatest light intensity comes from essentially one direction. Examples are a sunny sky, or a coating inspected under an intense spotlight. Under directional lighting, tiny light spots can be seen in an effect coating, which are often strikingly brighter than their surroundings. This effect is known under a variety of names, such as brilliance, sparkle and glitter. In order to avoid confusion with these often ill-defined names, we have chosen to introduce a new term, glint impression, to correspond precisely to the phenomenon just described, as observed or measured under well-defined lighting conditions. A more detailed description of the texture parameters set out here can be found in reference [11]. Texture is evaluated under two lighting conditions Two visual testing setups were created to test this concept of visual texture parameters. The dedicated 'diffuse room' shown in Figure 2a has a ceiling filled with daylight fluorescent tubes and covered by a smooth diffuser cloth. Together with the white walls this setup gives very uniform lighting conditions. A series of eight grey metallic panels was prepared which have small colour differences, but vary in coarseness from very fine to very coarse (see Figure 2b). These so-called Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Transcript of Coarseness and glints - European Coatings Journal with diffraction gratings. However, all these new...
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Coarseness and glints
Texture parameters help to characterise appearance ofeffect coatings.New effect pigments are constantly being developed withincreasingly complex colour shifts and appearance, whichcannot be characterised adequately with conventionalmulti-angle spectrophotometers. Even more sophisticatedmeasurement geometries do not guarantee fullcharacterisation. However, two additional 'texture'parameters, namely the diffuse coarseness and the glintimpression, can be used to effectively distinguish differenteffect coatings shades. An instrument to measure texture isunder development.Eric Kirchner*, Lan Njo, Klaas de Haas, Michael Rösler.* Corresponding Author. Contact: Dr. Eric J.J. Kirchner,Technology Center Colorimetry, Akzo Nobel Car Refinishes,P.O. Box 3, 2170 BA Sassenheim, Netherlands, Tel: +31 713083168, Fax: +31 71 3083011,[email protected] several decades, effect pigments have become a keyingredient in paints for the car industry and other coatingmarkets. This started with the introduction of aluminiumflake pigments in metallic coatings. In contrast to solidcolour coatings, effect coatings show what is called a flopeffect: the colour depends on illumination and viewing angle.This is caused by the predominantly flat orientation of theflakes in the paint, which increases reflection in the speculardirection and suppresses reflections at other angles. Formetallic coatings, the result is a light-dark flop effect, whichaccentuates curved contours of e.g. a car.Active research in this field showed around 1990 that threeillumination/detection geometries are sufficient to accuratelycharacterise the colour of metallic coatings, and these wereadopted in the ASTM E-2194 standard [1, 2]. A popularmulti-angle spectrophotometer such as the X-rite "MA68II"uses five illumination/detection geometries.
Newer effect pigments show more complex behaviourLater, pearlescent pigments were introduced. Initially theseconsisted mainly of mica/metal oxide flakes, adding arelatively small change in colour hue to the colour shift ofeffect coatings. More recently, many different types of effectpigments have been put on the market.They are based on different physical mechanisms, such ascombinations of selective interference, diffraction, scatteringand absorption of light. These phenomena are realised byusing single- or multi-layer structures with specificthicknesses and refractive index values, sometimesembossed with diffraction gratings. However, all these neweffect pigments produce coatings with oftenlarge colourshifts when viewing conditions are changed.Thus, many modern effect coatings are no longeradequately characterised by using conventional multi-anglespectrophotometers. A long debate has been held on howmany and which illumination and detection angles are to bechosen for characterising effect coatings [3, 4, 5, 6]. No firmconclusions have yet been reached, and several differentworking groups are addressing this problem [7, 8].According to the standard practice for colour measurementof interference pigments currently being developed byASTM, it is expected that adequate characterisation of effectcoatings requires at least seven geometries [7]. However,there will always be a risk that a new effect pigment isintroduced with a striking colour shift "in between" twogeometries that are included in any new standard.Also, including many geometries in standard measurement
protocols represents huge costs in instrumentation and datamanagement for the thousands of colour formulations foundin the car refinishing industry. Clearly a balance needs to befound between the benefits of using many measurementgeometries for colour characterisation and the effortsinvolved in measuring and handling data. This balance maybe favourably influenced if a property that may be termed'texture' is incorporated as a different approach tocharacterising effect coatings.
Why spectrophotometers cannot identify 'texture'In addition to colour, the visual texture of effect coatings canbe used for appearance or pigment characterisation.Experience has shown that two effect coating samples couldbe measured as a match when using a multi-anglespectrophotometer, but not pass visual acceptability if theydiffer in sparkle or texture [9].The reason is that a spectrophotometer averages the colourover a certain area of the coating. For example, with astandard multi-angle spectrophotometer the very coarse andvery fine coatings shown in Figure 1 are measured as a veryclose match, because their average colour is the same.Therefore a multi-angle spectrophotometer totally fails toidentify the respect in which these two coatings differ.Obviously texture is an important (but often overlooked)characteristic aspect of the visual appearance of effectcoatings. For many effect coatings, texture determination isa more efficient means of characterisation than increasingthe number of angles for colour measurement.A comprehensive study of the appearance of effectcoatings, including several texture-related aspects, waspublished by McCamy [10]. The two most important textureaspects of effect coatings have now been identified [11].These relate to two opposing illumination conditions, diffuseand directional light.Diffuse light conditions occur when the light intensity isequal from all directions. Examples are a clouded sky, atypical spray booth for a car, or a very diffuse light boothsuch as that from Verivide. Under diffuse lighting, alight/dark pattern is seen in effect coatings, which canappear to be coarse or fine depending on the coating.Directional light conditions imply that the greatest lightintensity comes from essentially one direction. Examples area sunny sky, or a coating inspected under an intensespotlight. Under directional lighting, tiny light spots can beseen in an effect coating, which are often strikingly brighterthan their surroundings.This effect is known under a variety of names, such asbrilliance, sparkle and glitter. In order to avoid confusion withthese often ill-defined names, we have chosen to introducea new term, glint impression, to correspond precisely to thephenomenon just described, as observed or measuredunder well-defined lighting conditions.A more detailed description of the texture parameters setout here can be found in reference [11].
Texture is evaluated under two lighting conditionsTwo visual testing setups were created to test this conceptof visual texture parameters. The dedicated 'diffuse room'shown in Figure 2a has a ceiling filled with daylightfluorescent tubes and covered by a smooth diffuser cloth.Together with the white walls this setup gives very uniformlighting conditions.A series of eight grey metallic panels was prepared whichhave small colour differences, but vary in coarseness fromvery fine to very coarse (see Figure 2b). These so-called
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anchor panels were numbered from one to eight. In this waycolour experts can quantify the 'diffuse coarseness value' ofany original effect coating, simply by comparing it to each ofthe eight anchor panels and deciding which one gives theclosest match to the original.Similarly, a set-up was developed for assessing glintimpression, using two intense daylight spots as shown inFigure 3a. One illuminates a sample panel whose glintimpression needs to be determined. The other spotilluminates a series of eight anchor panels (Figure 3b), thatby definition vary in glint impression from one to eight.These anchor panels were developed in a very similarmanner to those used for diffuse coarseness.The anchor panels are observed under fixed illumination(55°) and viewing (1°) angles. In this situation, the anchorpanel labelled '1' shows very few, low-intensity glints,whereas anchor '8' shows many and intense glints. To allowan examination of how glint impression varies with viewingand illumination angle, the sample panels were mounted ona rotating table.In Figure 4, the resulting values for diffuse coarseness andglint impression are presented. The data is based on aseries of 163 effect coatings, all representing actual carcolours, including purely metallic colours, pearlescentcolours and mixtures of these. The figure shows that diffusecoarseness and glint impression can both vary from zero tonine. The accuracy of such visual judgments was found tobe remarkably good: typically more than 80% of thejudgments agree within one unit of the texture scale with an"average observer".Figure 4 also shows that these two texture parameters havea poor correlation, and are therefore quite independent fromeach other. For example, an effect coating with a mediumvalue for diffuse coarseness can have a very small or a verylarge value for glint impression, or any value in between.Clearly both parameters have added value in characterisingan effect coating.
Differences in coarseness are readily identifiedThe added value of determining texture is best illustrated bya number of practical examples. When trying to match aneffect coating, a well known problem is that colour matchingsoftware often does not find the right set of effect pigmentsfor the imitation.As an example, Figure 1a shows a silver metallic coatingthat has been used by Honda for American cars. Supposethat this colour needs to be matched, and that the softwarehas found a reasonably good colour match as shown inFigure 1b (actually a colour used by another manufacturer).According to colour measurements with standardmulti-angle spectrophotometers, the two panels match towithin ∆E ab < 1.1 for all aspecular angles between 25° and110°. Apparently the average colour in Figures 1a and 1b,as measured by a multi-angle spectrophotometer, is verysimilar.Colour matching software may then try to imitate the colourof the original fine metallic by optimising the concentration ofthe coarse metallic flakes that are within the second coating.Obviously this will never lead to a satisfactory match. Atervisual examination, a human colour matcher willimmediately stop this process. However, in more subtlecases this unfortunate situation can lead to a large increasein the costs of colour formula development.In colour matching laboratories, this well-known deficiencyof standard colour matching equipment is often anticipatedby introducing an initial step where a colour maker visuallyexamines the original coating and identifies the effectpigments using a microscope. However, this is a costlysolution, and leaves many cases where the right set of effect
pigments is only found after many attempts. By quantifyingthe texture (in this particular case the diffuse coarseness) ofan effect coating, expensive mistakes such as this arereduced.
Some pigments are designed to emphasise sparkleAnother practical example is that the visual characteristics ofsome of the new pigments, such as the popular "Xirallic"range, in effect coatings are hard to characterise in terms ofcolour only, but the texture is easy to recognise. Dependingon the illumination condition and on concentration, pigmentsof this type can not only colour the coating, but they alsohave a distinct property that is called 'living sparkle' by thesupplier (Merck KGaA). This immediately shows that thesepigments distinguish themselves clearly in terms of texture,rather than just by colour as measured by a conventionalmulti-angle spectrophotometer.Figure 5 shows two different but very similar blue metallicsamples used on Honda vehicles. According to multi-anglecolour measurements, the sample on the right has a slightlybluer shade than the sample on the left. The colourdifference is small but visible, with ∆E ab < 1.1. However,Figure 5 clearly shows that it is not the colour in which thesetwo effect colours differ most. Instead, the sparkling effect inone sample is much stronger than in the other. We foundthat the first has a glint impression that is three units largerthan the second sample.Therefore, by measuring the texture parameter glintimpression, it becomes obvious that the first colour variantcontains more sparkling effect pigment and moreanti-orientation agent than the shade on the left-hand side ofFigure 5.
Instrument to measure texture is under developmentThe data in this article is based on visual tests, usingspecially designed experimental set-ups. Seven observerswere used to visually determine the numerical value of thetexture parameters as accurately as possible. However, inpractice these set-ups and number of observers make thedetermination of texture values cumbersome andtime-consuming. Obviously, the same situation would betrue for quantifying colour coordinates such as lightness,chroma and hue if no colour measurement equipment wereavailable.For this reason the instrument supplier Byk Gardner iscollaborating with Akzo Nobel and Merck. Currently, aninstrument is being developed that allows the user to readilymeasure not only colour but also diffuse coarseness andglint impression.This new instrument will allow its users to add a newdimension to the characterisation of effect coatings, apartfrom measuring colour under six illumination/detectionangles (with a 45° illumination angle, the detector ispositioned at -15°, +15°, +25°, +45°, +75° and 110°aspecular angles). The instrument is fully compatible withthe X-Rite standard for multi-angle colour measurement,and uses a digital camera for texture measurement.Information about texture has been shown to be an efficientway of characterising effect coatings [12]. It is thereforeexpected that this instrument will open up new opportunitiesfor e.g. colour formulation and quality control laboratories inall coating sectors where effect pigments are used.
REFERENCES[1] H. J. A. Saris, R. J. B. Gottenbos, H. van Houwelingen,Color Res. Appl. 15 (1990) p. 200[2] A. B. J. Rodrigues, Die Farbe 37 (1990) p. 65[3] P. W. Gabel, in Ninth congress of the InternationalColour Association. Proceedings of SPIE Vol. 4421 (2002)
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p. 798[4] M. E. Nadal, E. E. Early, Color Res. Appl. 29 (2004) p. 38[5] F. Leloup, P. Hanselaer, M. Pointer, J. Versluys, AICProceedings 2005, Granada, p.515[6] G. Pfaff, P. W. Gabel, Europ. Coat. Jnl., (2005) (06), p.30[7] ASTM group E12.12 WK 1164, Standard practice forMultiangle color measurement, Identification andCharacterization of Interference pigments. Concept April2006[8] DIN 6175 part 2 Tolerances for automotive paints - Part2: Goniochromatic paints (Farbtoleranzen fürAutomobillackierungen - teil 2: Effektlackierungen) (2001)[9] A. B. J. Rodrigues, AIC Color and Paints, Interim meetingproceeding (2004) p. 103[10] C. S. McCamy, Color Res. Appl. 23 (1998) p. 362[11] E. J. J. Kirchner, G. J. van den Kieboom, S. L. Njo, R.Supèr, R. J. B. Gottenbos, Color Res. Appl. (accepted)[12] K. de Haas, S. L. Njo, E. J. J. Kirchner, R. Gottenbos,Method for matching paint. International patentWO2006/030028A1, filing date 16.09.2005
Results at a glance- Effect coatings with increasingly complex colour shifts arebeing introduced in the coating industry. Theircharacterisation by conventional multi-anglespectrophotometers requires an increasing number ofmeasurement geometries.- Here, we propose a different strategy. In addition to colourmeasurement, we also focus on texture.- The visual texture aspects of effect coatings have beencharacterised by specifying two separate textureparameters. Diffuse coarseness and glint impressionrepresent the texture seen under diffuse and directional lightconditions, respectively.- Practical examples demonstrate that the measurement oftexture is often an efficient way to characterise effectcoatings and identify the type of effect pigments which havebeen used in a coating.- A combined texture and colour measurement instrument isbeing developed in a collaboration between Byk Gardner,Akzo Nobel Car Refinishes and Merck.
The authors:-> Eric Kirchner obtained his doctorate in chemistry andphysics at the Free University of Amsterdam. He joinedAkzo Nobel in 1996 and is now project leader forcolorimetric modelling at the Technology Center Colorimetrydepartment, Akzo Nobel Car Refinishes, Sassenheim.-> Lan Njo obtained her doctorate in chemical technology atthe Technical University of Delft. She has been a researcherat the Technology Center Colorimetry department, AkzoNobel Car Refinishes, Sassenheim since 2000 and workson instruments for colour and texture characterisation.-> Klaas de Haas obtained his PhD degree in physics(rheology) in 1996 at the University of Twente. He is projectleader for colour measurement equipment at the TechnologyCenter Colorimetry department, Akzo Nobel Car Refinishes,Sassenheim.-> Michael Rösler obtained his doctorate in physics at theUniversity of Halle. He joined Merck in 1998 and is nowgroup leader in the Application Technology Department forCoatings at Merck KGaA.
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Figure 1: Images of two effect coatings under diffuse lighting conditions, both having avery similar colour as measured by standard multi-angle spectrophotometers.
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Figure 2: (a) Diffuse lighting room, and (b) anchor panels used to quantify diffusecoarseness.
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Figure 3: (a) Experimental set-up for visual assessments of glint impression, with (b)anchor panels used to quantify glint impression.
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Figure 4: Comparison between diffuse coarseness value and glint impression value at25° geometry.
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Figure 5: Images of two effect coatings, taken under intense spot lighting, bothsamples having a very similar colour as measured by standard multi-angle
spectrophotometers.
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