Visual and instrumental assessments of color differences in automotive coatings

The interest in gonioapparent pigments (metallic, pearlescent, interference, or diffractive) has increased in the last few years, especially for applications in the automotive industry. To assure a proper characterization of colors with gonioapparent pigments, commercial devices have appeared to characterize the color in different geometries, which are called multiangle spectrophotometers. As the gonioapparent pigments and multiangle instruments are relatively new, no studies exist regarding the instrumental‐based procedure followed in the industry, and if the results provided are in agreement with the observer perception. Consequently, the main objective of this study was to examine the correlation of the instrumental color differences with visual assessments. The instrumental color difference was calculated with the color difference formula AUDI2000 (specific for this sector) between the pairs of similar samples of three types of coated panels (solid, metallic, and pearlescent). The values measured by a telespectroradiometer in a directional lighting booth and the colorimetric values obtained by means of a multiangle spectrophotometer BYK‐mac were considered for this purpose. Additionally, a visual experiment was conducted to quantify the color difference by using the gray‐scale method. The results revealed that an acceptable instrumental correlation existed despite the visual and the instrumental correlation being worse. In particular, it was checked that observers accepted a larger number of color pairs, that is, the visual color difference was smaller than the tolerance demanded by the industry (derived from AUDI2000). © 2015 Wiley Periodicals, Inc. Col Res Appl, 41, 384–391, 2016     

The minimum number of measurements for colour,sparkle, and graininess characterisation in gonio‐apparent panels

Materials with new visual appearances have emerged over the last few years. In the automotive industry in particular there is a growing interest in materials with new effect finishes, such as metallic, pearlescent, sparkle, and graininess effects. Typically, for solid colours the mean of three measurements with repetitions is sufficient to obtain a representative measurement for colour characterisation. However, gonio‐apparent panels have non‐homogeneous colours, and there are no studies that recommend the minimum number of repetitions for colour, sparkle, and graininess characterisation of this type of panel. We assume that colour panels incorporating special‐effect pigments in their colour recipes will require a higher minimum number of measurements than solid colour panels. Therefore, the purpose of this study is to verify this assumption by using a multiangle BYK‐mac spectrophotometer, given that it is currently the only commercial device that can measure colour, sparkle, and graininess values simultaneously. In addition, a possible methodology is given for establishing the minimum number of measurements when characterising gonio‐apparent materials using a specific instrument, able to be implemented in future instruments when determining multiple appearance attributes (colour, gloss, sparkle, etc.) for many coloration technologies. Thus, we studied the minimum number of measurements needed to characterise the colour, sparkle, and graininess of three types of sample with solid, metallic, and pearlescent coatings respectively. Twenty measurements were made at twenty random positions (different target areas) of 90 samples. The minimum number of measurements for all these variables was determined on the basis of the point at which the cumulative mean value became constant. Thus, applying new statistical tools, it is clearly shown that metallic and pearlescent panels require more colour measurements than solid panels, in particular when geometries are being measured in a specular direction. As regards texture (sparkle and graininess), more measurements are needed for graininess than for sparkle, and more for metallic panels than for pearlescent panels.     

Review of instrumental inter‐agreement study of spectral and colorimetric data of commercial multiangle spectrophotometers

Several different instruments have been introduced into the market to obtain good color characterization for different measurement configurations. These commercial instruments have different optical and working configurations, however, the measurement data provided by each instrument should be similar to have good consistency when comparing them to each other measuring the same sample material. Therefore, the purpose of this work was to apply an inter‐model agreement study of spectral and colorimetric data of three instruments (CM‐M6, BYK‐mac‐I, and MA98). Two different statistical tests were applied following ASTM recommendations. In general, the measurement geometries close to the specular direction and the flop direction showed greater deviations. In addition, the partial color differences calculated for the comparison of MA98 versus CM‐M6 were larger than the BYK‐mac‐i versus CM‐M6 comparison. Finally, it can be concluded that most of the measurement geometries were statistically significantly different from each other which means that these differences were due to systematic or bias errors and not exclusively to random errors.     

Color measurements for pearlescent coatings

This article describes recent developments at the National Institution of Standards and Technology in the colorimetric characterization of pearlescent coatings. The goal of this research is to develop a measurement protocol for the accurate color characterization of these coatings using an understanding of their scattering mechanism as a guide. A large ensemble of bi‐directional reflectance measurements on a series of pearl interference pigmented coatings show general trends in the color variations with illumination and viewing angles. These measurements were used to define a set of geometries (illumination angles of 15°, 45°, and 65° and aspecular angles of 15°, 35°, 45°, 70°, and 85°) to characterize the angle dependent color travel observed in these coatings. © 2003 Wiley Periodicals, Inc. Col Res Appl, 29, 38–42, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10210     

Model‐based corrections of geometric errors in multiangle measurements of gonio‐apparent coatings

Numerical corrections of colorimetric errors caused by positioning error and irregular geometry in multiangle measurements of gonio‐apparent coatings are presented. These corrections use a model function approximating the gonio‐reflection characteristics of the coating to convert the angular displacement due to those geometric errors to reflectance factor errors to be corrected as well as to estimate the angular displacement conversely from the reflectance factors. The performance of these corrections was evaluated for 11 different gonio‐apparent coatings using a multiangle geometry conforming to ASTM E2539 consisting of eight subgeometries to which artificial displacements were applied so as to present typical positioning errors or irregular geometries. The results show that both corrections significantly reduce the colorimetric errors for all specimens in all subgeometries sensitive to geometric errors. © 2015 Wiley Periodicals, Inc. Col Res Appl, 2015     

Colour-difference measurement: The sensitivity of various instruments compared

Because of the progress in electronic technology and the increasing importance of colour quality control in various branches of industry, over the last decades several colour-measuring instruments have been developed, each one with its own design features; the lack of standardization among them might be an obstacle to the spread of colour quality control as a means for product qualification. Colour-difference measurements taken with 11 different models of spectrophotometers [four with 45 (circumferential)/0 geometry and the others with diffuse geometry] were compared with the aim of analyzing their correlation and dispersion. In order to cover different colour-space regions, measurements were taken on test panels painted with four saturated basic hue (red, yellow, green, and blue) acrylic products gradually modified by adding white or black paint. Basic statistical analyses made on colour-difference values (ΔE*_ab, ΔL*, Δa*, and Δb*) resulted in a high correlation among the examined instruments, but a certain variability, particularly as far as certain hues are concerned.     

Evaluation of the performance of metameric indices

Forty three metameric pairs were obtained by comparison, shade-adjustment, and redyeing of 660 self shades dyed with direct, azoic, vat, and reactive dyes. The pairs were regrouped shadewise and ranked visually in increasing order of metamerism. Spearman's rank correlation coefficients were calculated between 11 measures of metamerism and the visual metameric ranking under three illuminants A, D65, and TL84 in pairs. The illuminant-independent general indices included the indices based on reflectance differences (Bridgeman), weighted reflectance differences (Nimeroff and Yurow), Cohen-Kappauf's residual differences (proposed) obtained from the spectral decompositions of the reflectance spectra of the metameric pair. Illuminant-specific special indices included color-differences under test illuminant, addition, subtraction, division, and multiplication of color-differences under test and reference illuminants, indices based on ΔL*, Δa*, Δb* differences under two illuminants, based on chromatic adapted ΔL*, Δa*, Δb* differences with multiplicative corrections for tristimulus differences under reference illuminant (Berns-Billmeyer), differences of the color constancy indices. The present work showed that indices based on ΔL*, Δa*, Δb* differences under two illuminants, both unmodified and modified by Berns-Billmeyer performed best among the existing indices. The differences of color constancy indices showed good correlation with the degree of metamerism in some cases, and this may be utilized for developing newer indices of metamerism. © 1996 John Wiley & Sons, Inc.     

Artificial neural networks for optimising camera‐based colour measurements of prints enhanced with pearlescent pigments

Pearlescent pigments are widely used in printing due to their optical, chemical and physical properties. To analyse the effects of goniochromism they produce, the colorimetric characterisation of materials printed with pearlescent pigments requires multi‐angular measurements. In this study, the colours of prints enhanced with pearlescent pigments were measured by means of a digital camera, relying on the empirical camera characterisation method. Since this method is time‐consuming, it was altered to enable estimates of colorimetric values for different geometries to be measured on the basis of images captured at one viewing angle. This approach was based on the use of artificial neural networks which were shown to provide sufficient flexibility for the given task. The results indicate that the images obtained at the viewing angle of 45° aspecular (measuring geometry 45°/asp 45°) accurately estimate CIELab values for all of the tested measuring geometries. The proposed method is therefore not only time‐efficient but also reduces the associated errors due to the camera's movement, and enables the estimation of colorimetric values for those viewing angles inaccessible by camera.     

Evaluation of methods for verifying the performance of color‐measuring instruments. Part II: Inter‐instrument reproducibility

Part I described an experiment in which the repeatability of a number of common, commercially available spectrocolorimeters was compared using ASTM procedures. ASTM E2214‐02 Standard Practice for Specifying and Verifying the Performance of Color‐Measuring Instruments is intended to standardize the terminology and procedures used to evaluate color measuring instruments. In this article, we develop reproducibility results from a medium‐term study of 10 commercial spectrocolorimeters. The comparisons are presented so as to contrast between the traditional color difference based specifications found in the historical literature and in manufacturer's literature and the more complex multidimensional methods specified in E2214. Instrument to instrument contrasts are reported as well as tests of agreement across a set of instruments as a whole. The results confirm common understandings. Hemispherical diffuse instruments exhibit a higher level of inter‐instrument agreement than do bidirectional (45:0) instruments. The results also provide support for a surprising conclusion about the statistical significance of the minor differences in both inter‐instrument agreement and inter‐model agreement for a single manufacturer. Some speculations on the impact of these conclusions to the development of future comparisons of spectrocolorimeters are given. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 176–294, 2007     

Correlation and modeling between color variation and quality of the surface between accelerated and natural tropical weathering in Acacia mangium,Cedrela odorata and Tectona grandis wood with two coating

To prevent the photodegradation process, a wide range of coatings have been developed. This study evaluated wood color change (ΔE*) and the surface quality index (SQI) generated by ASTM D‐660 standards in wood protected with polyurethane and varnish finishes of three tropical species (Acacia mangium, Cedrela odorata and Tectona grandis) in two weathering conditions (natural and accelerated condition). In addition, a model was developed to predict ΔE* and SQI in natural weathering based on ΔE* and SQI from accelerated weathering. The results showed that the application of these finishes generated variations in the color parameters of wood before being exposed to the elements. Additionally, it was found that finishes generated increases in ΔE*, especially the varnish type finish, and decreasing of SQI. Finally, models were found to predict color changes or surface quality in natural weathering based on color changes or surface quality in accelerated weathering. The model algorithm was expressed by ΔE* or SQI measured (X) and time exposure (t) in accelerated weathering by equation . With these models, it is possible to predict the equivalence of exposure times in natural weathering in tropical conditions for different kinds of finishes in three tropical species. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 519–529, 2014     

Evaluation of methods for verifying the performance of color‐measuring instruments. Part I: Repeatability

A recent ASTM specification (ASTM E2214‐02) was created to standardize the terminology and procedures used to evaluate color measuring instruments. This specification addresses the need for uniformity among the manufacturers of such instruments when quantifying the performance of their products. The scope of E2214 is necessarily large, covering the procedures required to compare instruments for a variety of metrics, all of which are important to their overall performance. In this work, we will focus on one aspect of the instrument evaluation: repeatability performance. We will present repeatability results from a long‐term study of twelve commercial spectrophotometers. Comparisons will be made between traditional color difference metrics and the more complex multidimensional methods specified in E2214. © 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 166–175, 2007     

Canonical biplot statistical analysis to detect the magnitude of the effects of phosphates crystallization aging on the color in siliceous conglomerates

The Canonical Biplot method is used to determine the magnitude of the effects on the Δ chromatic coordinates (ΔL*, Δa*, Δb*) and the parameter ΔE*, where (ΔL*, Δa*, Δb*) are the difference in the values of the sample after each aging cycle and the value of the untreated sample and ΔE* = [(ΔL*)² + (Δa*)² + (Δb*)²]^1/2. We performed a study of the changes in color produced by two types of artificial aging procedures on four varieties of siliceous conglomerates from Zamora (Spain) that have traditionally been used in construction and later renovations in historical buildings in the zone. To accomplish this, 25 cycles of the following types of accelerated artificial aging were carried out: (a) freezing/thawing and cooling/heating (T1) and (b) combined freezing/thawing and cooling/heating + salt (phosphates) crystallization (T2). The results of the statistical study applied (Canonical Biplot) allowed us to differentiate the magnitude effect on the color of the surface brought about by T1 as compared with T2. These effects (p < 0.05) were observed in all but one (ochre conglomerate) of the varieties, but mainly in the variable governing red hue (Δa*) and yellow hue (Δb*). © 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 82–87, 2014     

Three‐dimensional color prediction modeling of single‐ and double‐layered woven fabrics

In this research, the three‐dimensional structural and colorimetric modeling of three‐dimensional woven fabrics was conducted for accurate color predictions. One‐hundred forty single‐ and double‐layered woven samples in a wide range of colors were produced. With the consideration of their three‐dimensional structural parameters, three‐dimensional color prediction models, K/S‐, R‐, and L*a*b*‐based models, were developed through the optimization of previous two‐dimensional models which have been reported to be the three most accurate models for single‐layered woven structures. The accuracy of the new three‐dimensional models was evaluated by calculating the color differences ΔL*, ΔC*, Δh°, and ΔE_CMC(2:1) between the measured and the predicted colors of the samples, and then the error values were compared to those of the two‐dimensional models. As a result, there has been an overall improvement in color predictions of all models with a decrease in ΔE_CMC(2:1) from 10.30 to 5.25 units on average after the three‐dimensional modeling.     

Color Change Kinetics of American Ginseng (Panax quinquefolium) Slices During Air Impingement Drying

The color change kinetics of American ginseng (Panax quinquefolium) slices were investigated in an air impingement dryer under different drying temperatures (35, 45, 55, and 65°C) using the CIE Lab color parameters (L*, a*, b*) as the assessment indicators. Results illustrated that all three color parameters (L*, a*, b*) increased with drying time. The L* value decreased with increasing drying temperature. However, a* and b* values increased with the increase in drying temperature. Furthermore, at the initial drying stage the change rate of L* increased significantly, while towards the end of drying it reduced significantly. As regards a*, it slowly changed at the initial and final drying stages rather than in the intermediate drying stage. In the case of b*, it increased with increasing drying time and drying temperature during the whole process. The zero-order, first-order, and fractional conversion models were fitted to the experimental data, and the model's parameters were determined using linear regression analysis. By comparing the fitting of kinetic models to the experimental data, the most suitable model was selected to describe the color change kinetics. An Arrhenius equation was used to calculate the activation energy for color change kinetics and it was found that the values were 33.87–38.55, 56.48, and 74.03 kJ/mol for L*, a*, and b*, respectively. The findings of this work contribute to a better understanding of ginseng color changes kinetics during drying, and the established change kinetics models are a good tool for predicting, evaluating, and controlling of color change of American ginseng during its drying process.     

Detailed analysis of the UV‐adjustment techniques used in paper and graphic industries

Many commercial materials (papers and boards) contain optical brightening agents also known as fluorescent whitening agents. Adequate adjustment of the UV content of a measurement device (e.g., spectrophotometers) is essential for accurate color measurement. As specified in the ISO standards, the UV content is adjusted against an assigned value of an international reference transfer standard, for example, CIE whiteness (D₆₅/10°) for the CIE illuminant D₆₅ or ISO brightness for the C illuminant. Because of the simplicity, these approaches have gained great popularity in papermaking industry. Yet, there has been little evidence indicating how accurate the total spectral radiance factor corresponding to the single assigned value is reproduced. Hence, we present a method that quantitatively evaluates the accuracy of the UV‐adjustment technique, through comparing the total spectral radiance factors obtained from UV adjustment with the assigned ones. This method has been applied to three second‐level international reference transfer standard illuminated by three standard illuminants, D₆₅, C, and D₅₀. We found that the major differences between the assigned spectra and those obtained from the UV adjustments occur in the blue band where fluorescence is strong. At a few wavelengths, the differences may be up to 4–5%. Nevertheless, their color differences corresponding to the assigned spectra and those obtained from the UV adjustments are still smaller than unity (1 ΔE*) for all of the illumination conditions. Two instruments using the representative UV adjusting techniques, for example, the conventional UV‐adjusting with an adjustable (GG395) UV filter and the numerical UV‐filtering, have been studied. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 19–26, 2017     

Evaluation of intermodel agreement using ISO 13655 M0, M1, and M2 measurement modes in commercial spectrophotometers

Spectrophotometers are routinely used for color measurement and color management in many commercial printing and proofing workflows. In the case of media containing optical brightening agents, ultraviolet (UV)‐induced fluorescence has led to poor levels of agreement between models from different manufacturers, and different models from the same manufacturer. A relevant standard, ISO 13655, has been revised and now clearly defines measurement modes and conditions for the UV component in spectrophotometers. ISO 13655:2009—Graphic Technology—Spectral Measurement and Colorimetric Computation for Graphic Arts Images now defines four measurement modes: M0, M1, M2, and M3. The intermodel difference between 10 commercially available spectrophotometers is evaluated for different substrate types in measurement modes (M0, M1, and M2) as allowed by each instrument. In particular, the authors compare devices using M0 legacy mode versus newer instruments that are compliant with the new M1 and M2 (UV‐included and UV‐excluded) measurement modes. A finding with significant practical applications is that there is greatly improved intermodel agreement between the new generation of ISO 13655‐compliant instruments in M1 (D50) mode when compared with the previous generation of hand‐held spectrophotometers. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 27–37, 2017     

On the relations between color,gloss, and surface texture in injection‐molded plastics

The relation between color and gloss of injection‐molded plastic specimens was evaluated by means of a 45°/0° geometry spectrophotometer and a glossmeter. The specimens were plaques having one smooth, glossy field and one rougher, textured region made of two different polymeric materials (acrylonitrile‐butadiene‐styrene and polypropylene) in a range of colors. A significant influence of the lightness L* of the specimens on the measured gloss of the textured field, with a low gloss appearance, was found. This can probably be attributed to a contribution from bulk scattering, which is linked to the reflectance from within the specimen. The influence of texture on the measured color of the plaques was assessed by evaluating the color difference between the smooth field and the more textured area on the specimens. When the surface was textured, the color changed; in general it became lighter and less saturated. The magnitude of the change depended on the color of the material. The darker the material was, the larger was the increase in lightness L* as the surface became rougher. The shift in chroma C* was determined not only by the saturation but also by the lightness of the specimens. Darker specimens exhibited a larger decrease in chroma when the surface was textured. The change in the measured color caused by increasing the surface roughness could be predicted in a satisfactory manner using a model developed for xerographic printing paper. © 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 291–298, 2009     

Color prediction of metallic coatings from measurements at common geometries in portable multiangle spectrophotometers

Illuminating and viewing geometries may strongly affect the color appearance of metallic coatings, which can be characterized accurately by bidirectional reflectance distribution function (BRDF) measurement devices. However, such devices with hundreds or even thousands of different geometries are usually expensive and complex. Accordingly, two modified models were developed in this study, based on the distribution of aluminum-flake pigments in the coatings, to, respectively, estimate the spectral radiance factors and the CIE tristimulus values of metallic coatings utilizing the measurements at 6 common geometries of portable multiangle spectrophotometers. Their performance was examined with 65 achromatic and 20 chromatic metallic coating samples under D65, A, and F11 illuminants. The average CIEDE2000 color differences over all 19 geometries were found to be less than 1.8 for both models, while the average CAM02-SCD and CIELAB color differences can, respectively, reach 1.7 and 2.0, indicating the effectiveness of our methods.     

Time‐temperature superposition principle application to the hygrothermal discoloration of colored high‐density polypropylene/wood composites

Discoloration kinetics of wood plastic composites (WPCs) made of recycled high density polyethylene (HDPE) during accelerated hygrothermal aging test at 45–65°C was conducted by means of time‐temperature superposition principle (TTSP). Color parameters measured with a spectrophotometer by CIELAB color system showed that L* and ΔE* increased while a* and b* decreased with aging time and temperature. A single horizontal shift was adequate for the superposition of color parameter master curves, indicating temperature can be used as accelerating factor to the discoloration of the studied WPCs. The regression line for Arrhenius plot showed a linearity for color parameters (R² = 0.9996), suggesting TTSP concept can be used to analyze and predict the discoloration kinetics of WPCs at low aging temperature. The prediction of color parameters at ambient aging temperature (21°C) can be extended to 7.2 years. The activation energy calculated from color parameters using TTSP method was 85.5 kJ/mol and similar to the reported values of HDPE thermal relaxation reactions calculated from other methods in the open literature. POLYM. COMPOS. 37:1016–1020, 2016. © 2014 Society of Plastics Engineers     

Spectrophotometric measurement of human skin colour

This article focuses on human skin colour. Three different colour measuring instruments were used: a tele‐spectroradiometer, a de:8° and a 45°:0° spectrophotometers. They were used to measure 47 subjects who were divided into four skin groups: Chinese, Caucasian, South‐Asian, and Dark. Eight locations for each subject were measured. The spectral reflectance results were first compared. They all showed the “W” shape between the 550 nm and 580 nm as found by the other studies. Those from the tele‐spectroradiometer had an increase from 600 nm while the others were flatter. The colorimetric data calculated from the spectral measurements revealed similar patterns to describe the colour distribution of each skin group. It was found that two scales: whiteness‐depth, and blackness‐vividness could well describe these distributions. The results also showed systematic differences between the four ethnic groups, between eight body locations, between two genders, and between the measurements from the three instruments.