Visibility of sparkle in metallic paints

For suitable illumination and observation conditions, sparkles may be observed in metallic coatings. The visibility of these sparkles depends critically on their intensity, and on the paint medium surrounding the metallic flakes. Based on previous perception studies from other disciplines, we derive equations for the threshold for sparkles to be visible. The resulting equations show how the visibility of sparkles varies with the luminosity and distance of the light source, the diameter of the metallic flakes, and the reflection properties of the paint medium. The predictions are confirmed by common observations on metallic sparkle. For example, under appropriate conditions even metallic flakes as small as 1 μm diameter may be visible as sparkle, whereas under intense spot light the finer grades of metallic coatings do not show sparkle. We show that in direct sunlight, dark coarse metallic coatings show sparkles that are brighter than the brightest stars and planets in the night sky. Finally, we give equations to predict the number of visually distinguishable flake intensities, depending on local conditions. These equations are confirmed by previous results. Several practical examples for applying the equations derived in this article are provided.European Metrology Research Programme (EMRP) Spanish Ministry of Economy and Competitiveness(DPI2011-30090-C02) Comunidad de Madrid (CM: S2013/MIT-2790). The EMRP is jointly funded by the EMRP participating countries within the European Association of National Meteorlogy (EURAMET) and the European Union

Texture Evaluation of Automotive Coatings by Means of a Gonio-Hyperspectral Imaging System Based on Light-Emitting Diodes

Sparkle and graininess are textural effects that appear as intrinsic spatial features of coatings containing goniochromatic pigments, whereas others such as mottling are undesired outcomes. In this study, we present new methods for the evaluation of sparkle, graininess and mottling of automotive coatings through images acquired with a novel gonio-hyperspectral imaging system based on light-emitting diodes with extended spectral sensitivity beyond the visible range (368–1309 nm). A novel analysis of sparkle that considered the amount of sparkling spots was used. Graininess was quantified through a novel statistical descriptor based on the energy descriptor computed from the histogram. A new index was proposed for an enhanced detection of mottling based on the analysis of striping, which was better observed and quantified in the infrared range. Spectral assessments showed a strong relationship with color and spectral reflectance for graininess and mottling. In conclusion, the results showed that these new spatial and spectral methods are a suitable, alternative and potential approach for the assessment of these textural features.Ministerio de Ciencia e Innovación (MICINN, DPI2011-30090-C02-01) and Ministerio de Economía y Competitividad (MINECO, DPI2014-56850-R); and the European Union

Measurement and rendering of complex non-diffuse and goniochromatic packaging materials

Realistic renderings of materials with complex optical properties, such as goniochromatism and non-diffuse reflection, are difficult to achieve. In the context of the print and packaging industries, accurate visualisation of the complex appearance of such materials is a challenge, both for communication and quality control. In this paper, we characterise the bidirectional reflectance of two homogeneous print samples displaying complex optical properties. We demonstrate that in-plane retro-reflective measurements from a single input photograph, along with genetic algorithm-based BRDF fitting, allow to estimate an optimal set of parameters for reflectance models, to use for rendering. While such a minimal set of measurements enables visually satisfactory renderings of the measured materials, we show that a few additional photographs lead to more accurate results, in particular, for samples with goniochromatic appearance

Review of instrumental inter-agreement study of spectral and colorimetric data of commercial multi-angle spectrophotometers

"This is the peer reviewed version of the following article: Perales, E., Micó-Vicent, B., Huraibat, K., Martínez-Verdú, F. M., Dietz, C., & Yamanoi, Y. (2019). Review of instrumental inter-agreement study of spectral and colorimetric data of commercial multi-angle spectrophotometersspectrophotometers. Color Research & Application, 44(2), 168-175., which has been published in final form at https://doi.org/10.1002/col.22320. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving."[EN] 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.EMRP, Grant/Award Numbers: Bidirectional reflectance definitions (16NRM08), 16NRM08; Spanish Ministry of Economy and Competitiveness, Grant/Award Number: DPI2015-65814-R; Ministry of Economy and Competitiveness, Grant/Award Number: DPI2015-65814-R; European Union; EURAMETPerales Romero, E.; Micó-Vicent, B.; Huraibat, K.; Martínez-Verdú, FM.; Dietz, C.; Yamanoi, Y. (2018). Review of instrumental inter-agreement study of spectral and colorimetric data of commercial multi-angle spectrophotometers. Color Research & Application. 1-8. https://doi.org/10.1002/col.22320S18Streitberger, H., & Dössel, K. (Eds.). (2008). Automotive Paints and Coatings. doi:10.1002/9783527622375Akafuah, N., Poozesh, S., Salaimeh, A., Patrick, G., Lawler, K., & Saito, K. (2016). Evolution of the Automotive Body Coating Process—A Review. Coatings, 6(2), 24. doi:10.3390/coatings6020024Lee, Y.-K., Zhao, G.-F., Yu, B., Lim, H.-N., & Lu, H. (2011). Goniochromatic properties of human tooth dentin. Color Research & Application, 37(3), 206-211. doi:10.1002/col.20663Debeljak, M., Hladnik, A., Černe, L., & Gregor-Svetec, D. (2012). Use of effect pigments for quality enhancement of offset printed specialty papers. Color Research & Application, 38(3), 168-176. doi:10.1002/col.20753Kandi, S. G. (2012). The effect of paper appearance on printed color of inkjet printer. Color Research & Application, 38(4), 284-291. doi:10.1002/col.21724Rich, D. C., Marcus, R., Lovell, V., & Kreutz, T. (2016). Modeling the appearance of metal-like packaging printing. Color Research & Application, 42(1), 38-49. doi:10.1002/col.22035Topp, K., Haase, H., Degen, C., Illing, G., & Mahltig, B. (2014). Coatings with metallic effect pigments for antimicrobial and conductive coating of textiles with electromagnetic shielding properties. Journal of Coatings Technology and Research, 11(6), 943-957. doi:10.1007/s11998-014-9605-8Leloup, F. B., Audenaert, J., Obein, G., Ged, G., & Hanselaer, P. (2016). Repeatability and reproducibility of specular gloss meters in theory and practice. Journal of Coatings Technology and Research, 13(6), 941-951. doi:10.1007/s11998-016-9813-5Gómez, O., Perales, E., Chorro, E., Viqueira, V., & Martínez-Verdú, F. M. (2016). Visual and instrumental correlation of sparkle by the magnitude estimation method. Applied Optics, 55(23), 6458. doi:10.1364/ao.55.006458Winston Wang, Z., & Ronnier Luo, M. (2016). Looking into special surface effects: diffuse coarseness and glint impression. Coloration Technology, 132(2), 153-161. doi:10.1111/cote.12203Baek, Y. S., Kwak, Y., & Yang, S. (2015). Visual appearance measurement of surfaces containing pearl flakes. Journal of the Optical Society of America A, 32(5), 934. doi:10.1364/josaa.32.000934Kirchner, E. J. J., & Ravi, J. (2012). Setting tolerances on color and texture for automotive coatings. Color Research & Application, 39(1), 88-98. doi:10.1002/col.21767Gómez, O., Perales, E., Chorro, E., Burgos, F. J., Viqueira, V., Vilaseca, M., … Pujol, J. (2015). Visual and instrumental assessments of color differences in automotive coatings. Color Research & Application, 41(4), 384-391. doi:10.1002/col.21964Nadal ME Wyble D Zarobila CJ Color and appearance Germer TA Zwinkels JC Tsai BJ Spectrophotometry: Accurate Measurement of Optical Properties of Materials Oxford Elsevier 2014Burgos-Fernández, F. J., Vilaseca, M., Perales, E., Chorro, E., Martínez-Verdú, F. M., Fernández-Dorado, J., & Pujol, J. (2017). Validation of a gonio-hyperspectral imaging system based on light-emitting diodes for the spectral and colorimetric analysis of automotive coatings. Applied Optics, 56(25), 7194. doi:10.1364/ao.56.007194Simonot, L., Hébert, M., & Dupraz, D. (2011). Goniocolorimetry: From measurement to representation in the CIELAB color space. Color Research & Application, 36(3), 169-178. doi:10.1002/col.20605Ferrero, A., Perales, E., Rabal, A. M., Campos, J., Martínez-Verdú, F. M., Chorro, E., & Pons, A. (2014). Color representation and interpretation of special effect coatings. Journal of the Optical Society of America A, 31(2), 436. doi:10.1364/josaa.31.000436Ďurikovič, R., & Mihálik, A. (2014). Modeling the BRDF from spectral reflectance measurements of metallic surfaces. Applied Surface Science, 312, 87-90. doi:10.1016/j.apsusc.2014.05.099Haindl, M., & Filip, J. (2013). Visual Texture. doi:10.1007/978-1-4471-4902-6Dong, Y., Lin, S., & Guo, B. (2013). Material Appearance Modeling: A Data-Coherent Approach. doi:10.1007/978-3-642-35777-0Ferrero, A., Bernad, B., Campos, J., Perales, E., Velázquez, J. L., & Martínez-Verdú, F. M. (2016). Color characterization of coatings with diffraction pigments. Journal of the Optical Society of America A, 33(10), 1978. doi:10.1364/josaa.33.001978Ferrero, A., Rabal, A., Campos, J., Martínez-Verdú, F., Chorro, E., Perales, E., … Hernanz, M. L. (2013). Spectral BRDF-based determination of proper measurement geometries to characterize color shift of special effect coatings. Journal of the Optical Society of America A, 30(2), 206. doi:10.1364/josaa.30.000206Strothkämper, C., Hauer, K.-O., & Höpe, A. (2015). How to efficiently characterize special effect coatings. Journal of the Optical Society of America A, 33(1), 1. doi:10.1364/josaa.33.000001Kirchner, E., & Cramer, W. (2011). Making sense of measurement geometries for multi-angle spectrophotometers. Color Research & Application, 37(3), 186-198. doi:10.1002/col.20679Seubert, C. M., Nichols, M. E., Frey, J., Shtein, M., & Thouless, M. D. (2015). The characterization and effects of microstructure on the appearance of platelet–polymer composite coatings. Journal of Materials Science, 51(5), 2259-2273. doi:10.1007/s10853-015-9528-5Ferrero, A., Campos, J., Bernad, B., Pons, A., Hernanz, M. L., Martínez-Verdú, F. M., & Höpe, A. (2016). Consistency analysis of multidimensional gonio-spectrophotometric measurements in interlaboratory comparisons. Metrologia, 53(4), 1024-1030. doi:10.1088/0026-1394/53/4/1024Strothkämper, C., Ferrero, A., Koo, A., Jaanson, P., Ged, G., Obein, G., … Campos, J. (2017). Multilateral spectral radiance factor scale comparison. Applied Optics, 56(7), 1996. doi:10.1364/ao.56.001996Chorro, E., Perales, E., Burgos, F. J., Gómez, O., Vilaseca, M., Viqueira, V., … Martínez-Verdú, F. M. (2015). The minimum number of measurements for colour, sparkle, and graininess characterisation in gonio-apparent panels. Coloration Technology, 131(4), 303-309. doi:10.1111/cote.12157Ferrero, A., Campos, J., Perales, E., Martínez-Verdú, F. M., van der Lans, I., & Kirchner, E. (2014). Global color estimation of special-effect coatings from measurements by commercially available portable multiangle spectrophotometers. Journal of the Optical Society of America A, 32(1), 1. doi:10.1364/josaa.32.000001Imura, K., & Teraoka, Y. (2015). Model-based corrections of geometric errors in multiangle measurements of gonio-apparent coatings. Color Research & Application, 41(4), 372-383. doi:10.1002/col.21962Sharma, A., Leung, E., & Adams, R. (2016). Evaluation of intermodel agreement using ISO 13655 M0, M1, and M2 measurement modes in commercial spectrophotometers. Color Research & Application, 42(1), 27-37. doi:10.1002/col.22045Perales, E., Chorro, E., Viqueira, V., & Martínez-Verdú, F. M. (2012). Reproducibility comparison among multiangle spectrophotometers. Color Research & Application, 38(3), 160-167. doi:10.1002/col.2171

Evaluating the Graininess Attribute by Visual Scaling for Coatings with Special-Effect Pigments

[EN] In our society, objects' visual appearance is an essential factor because it allows us to recognize and differentiate one object from another. In different industrial sectors like cosmetics, textiles and automotive, special-effect pigments are largely used to achieve attractive visual features. These pigments provide a color change with viewing and illumination direction, and visually provide texture. Depending on a finish's properties, and also on the viewing and illumination conditions, coatings exhibit sparkle or a graininess-like texture. Currently, not many scientific works on the visual perception of these texture effects can be found in the literature. In addition, choice of experimental method can influence the measurement scale obtained from visual data. For this reason, the purpose of this work was to analyze graininess visual scaling constructed by two different psychophysical methods. The experimental design was based on the rank-order and paired-comparison methods. The data analysis was conducted by following the law of comparative judgments to obtain a visual scale of the graininess attribute to compare it to instrumental data. A good correlation appeared between both magnitudes with a correlation coefficient close to 0.9. Both methods provided useful results with a reasonable correspondence between them, which ensures that data can be considered reliable, while the visual obtained scale can act as a good graininess scale perceived by the human visual system.This research was funded by EMRP, grant number 16NRM08, and by the Ministerio de Ciencia, Innovacion y Universidades, grant number RTI2018-096000-B-I00.Perales, E.; Micó-Vicent, B.; Huraibat, K.; Viqueira, V. (2020). Evaluating the Graininess Attribute by Visual Scaling for Coatings with Special-Effect Pigments. Coatings. 10(4):1-10. https://doi.org/10.3390/coatings10040316S11010

Spectral BRDF-based determination of proper measurement geometries to characterize color shift of special effect coatings

A reduced set of measurement geometries allows the spectral reflectance of special effect coatings to be predicted for any other geometry. A physical model based on flake-related parameters has been used to determine nonredundant measurement geometries for the complete description of the spectral bidirectional reflectance distribution function (BRDF). The analysis of experimental spectral BRDF was carried out by means of principal component analysis. From this analysis, a set of nine measurement geometries was proposed to characterize special effect coatings. It was shown that, for two different special effect coatings, these geometries provide a good prediction of their complete color shift.The authors are grateful to “Plan Nacional de Física” for funding this work (FIS2010-19756-E), to CSIC’s JAE Program, and the “European Social Fund” for awarding us a research trainee. This study was also supported by the Spanish Ministry of Economy and Competitiveness under the grant DPI2011-30090-C02-02 and the European Union

Color characterization of coatings with diffraction pigments

Coatings with diffraction pigments present high iridescence, which needs to be characterized in order to describe their appearance. The spectral bidirectional reflectance distribution functions (BRDFs) of six coatings with SpectraFlair diffraction pigments were measured using the robot-arm-based goniospectrophotometer GEFE, designed and developed at CSIC. Principal component analysis has been applied to study the coatings of BRDF data. From data evaluation and based on theoretical considerations, we propose a relevant geometric factor to study the spectral reflectance and color gamut variation of coatings with diffraction pigments. At fixed values of this geometric factor, the spectral BRDF component due to diffraction is almost constant. Commercially available portable goniospectrophotometers, extensively used in several industries (automotive and others), should be provided with more aspecular measurement angles to characterize the complex reflectance of goniochromatic coatings based on diffraction pigments, but they would not require either more than one irradiation angle or additional out-of-plane geometries.European Metrology Research Programme (EMRP) (EMRP IND52); Comunidad de Madrid (SINFOTON-CM: S2013/MIT-2790); Ministerio de Economía y Competitividad (MINECO) (DIP2015-65814)

Measurement, modeling and perception of painted surfaces : A Multi-scale analysis of the touch-up problem

Real-world surfaces typically have geometric features at a range of spatial scales. At the microscale, opaque surfaces are often characterized by bidirectional reflectance distribution functions (BRDF), which describes how a surface scatters incident light. At the mesoscale, surfaces often exhibit visible texture - stochastic or patterned arrangements of geometric features that provide visual information about surface properties such as roughness, smoothness, softness, etc. These textures also affect how light is scattered by the surface, but the effects are at a different spatial scale than those captured by the BRDF. Through this research, we investigate how microscale and mesoscale surface properties interact to contribute to overall surface appearance. This behavior is also the cause of the well-known touch-up problem in the paint industry, where two regions coated with exactly the same paint, look different in color, gloss and/or texture because of differences in application methods. At first, samples were created by applying latex paint to standard wallboard surfaces. Two application methods- spraying and rolling were used. The BRDF and texture properties of the samples were measured, which revealed differences at both the microscale and mesoscale. This data was then used as input for a physically-based image synthesis algorithm, to generate realistic images of the surfaces under different viewing conditions. In order to understand the factors that govern touch-up visibility, psychophysical tests were conducted using calibrated, digital photographs of the samples as stimuli. Images were presented in pairs and a two alternative forced choice design was used for the experiments. These judgments were then used as data for a Thurstonian scaling analysis to produce psychophysical scales of visibility, which helped determine the effect of paint formulation, application methods, and viewing and illumination conditions on the touch-up problem. The results can be used as base data towards development of a psychophysical model that relates physical differences in paint formulation and application methods to visual differences in surface appearance

Machine-readable universal data format for bidirectional reflectance distribution function and BiRDview—An open-source web-based application for viewing and comparing bidirectional reflectance data

Modern studies of bidirectional reflectance distribution function (BRDF) and its applications using data and machine-driven science require formatting of BRDF data according to Findable, accessible, interoperable and reusable (FAIR) data principles. As a solution a FAIR universal BRDF file-format based on Java Script Object Notation (JSON) is proposed. JSON principles as well as file structure are explained and examples are given. Automatic validation of universal BRDF file format is realized with the help of JSON schema. Furthermore, the source code and accompanying documentation are presented in dedicated supporting material files. It is expected that after its wide adoption, the proposed BRDF file format will enhance collaboration between different research groups and benefit machine-driven science. The uptake is facilitated by introducing a BiRDview—a modern open-source web-based application for BRDF visualization.This work has been done in the frame of the projects 16NRM08 BiRD and 18SIB03 BxDiff, that have received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. The work has also been supported by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167

Computational Light Transport for Forward and Inverse Problems.

El transporte de luz computacional comprende todas las técnicas usadas para calcular el flujo de luz en una escena virtual. Su uso es ubicuo en distintas aplicaciones, desde entretenimiento y publicidad, hasta diseño de producto, ingeniería y arquitectura, incluyendo el generar datos validados para técnicas basadas en imagen por ordenador. Sin embargo, simular el transporte de luz de manera precisa es un proceso costoso. Como consecuencia, hay que establecer un balance entre la fidelidad de la simulación física y su coste computacional. Por ejemplo, es común asumir óptica geométrica o una velocidad de propagación de la luz infinita, o simplificar los modelos de reflectancia ignorando ciertos fenómenos. En esta tesis introducimos varias contribuciones a la simulación del transporte de luz, dirigidas tanto a mejorar la eficiencia del cálculo de la misma, como a expandir el rango de sus aplicaciones prácticas. Prestamos especial atención a remover la asunción de una velocidad de propagación infinita, generalizando el transporte de luz a su estado transitorio. Respecto a la mejora de eficiencia, presentamos un método para calcular el flujo de luz que incide directamente desde luminarias en un sistema de generación de imágenes por Monte Carlo, reduciendo significativamente la variancia de las imágenes resultantes usando el mismo tiempo de ejecución. Asimismo, introducimos una técnica basada en estimación de densidad en el estado transitorio, que permite reusar mejor las muestras temporales en un medio parcipativo. En el dominio de las aplicaciones, también introducimos dos nuevos usos del transporte de luz: Un modelo para simular un tipo especial de pigmentos gonicromáticos que exhiben apariencia perlescente, con el objetivo de proveer una forma de edición intuitiva para manufactura, y una técnica de imagen sin línea de visión directa usando información del tiempo de vuelo de la luz, construida sobre un modelo de propagación de la luz basado en ondas.<br /