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8 págs.; 7 figs., The spectral bidirectional reflectance distribution function (BRDF) is the key quantity to specify the spectral reflectance of materials for any condition of irradiation and detection, and its characterization is quite important for surfaces with a high dependence on these conditions, such as iridescent coatings. In order to evaluate the calibration and measurement capabilities (CMC) of National Metrology Institutes with the ability to measure the spectral BRDF, a case study interlaboratory comparison is in progress. Spectral BRDF has both spectral and geometric dependence, and this multidimensionality must be treated in the comparison to provide useful information to the participants about their CMCs. A data analysis method for the comparison is presented in this work, which was tested by simulations for different scenarios. The proposed method assesses whether the experimental data from each participant are consistent with those from the others. Finally, one-dimensional and multidimensional degrees of equivalence are defined, which should allow systematic deviations of spectral and geometric nature to be identified., This report was compiled within the EMRP IND52 Project xD-Reflect ‘Multidimensional reflectometry for industry’. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Part of the authors (Instituto de Óptica ‘Daza de Valdés’, Agencia Estatal CSIC) are also grateful to Comunidad de Madrid for funding the project SINFOTON-CM: S2013/MIT-2790.

Coatings and plastics that contain special effect pigments show angular-dependent reflection characteristics. Interference pigments, for instance, exhibit a color impression (“appearance”) that changes with the directions of illumination and observation. It is currently an open question how the appearance of such coatings can be best characterized without extensive measurements of the spectral radiance factor. In this paper, a simple and efficient approach is presented on how to characterize the appearance of surfaces composed of metallic and interference pigments. Based on the rotational symmetry of the pigment distribution around the surface normal, it is demonstrated how the appearance at any configuration of illumination and observation can be estimated from a set of in-plane measurements. For practical applications, it is shown that a rather small number of geometries (e.g., 10) are sufficient. The presented methodology is an alternative to a recent principal components analysis-based procedure and offers the advantage of being based directly on an extensible physical model while having at least the same prediction accuracy.

The effects of goniochromatism and sparkle are gaining more and more interest for surface refinement applications driven by demanding requirements from such different branches as automotive, cosmetics, printing and packaging industry. The common background and intention in all of these implementations is improvement of the visual appearance of the related commercial products. Goniochromatic materials show strong angular-dependent reflection characteristics and hence a color impression depending on the effective spatial arrangement of illumination and observation relative to the surface of the artifact. Sparkle is a texture related effect giving a surface which is irradiated directionally, like direct sun light, a bright glittering effect, similar to twinkling stars at the night sky. The prototype for this new effect is the Xirallic® pigment of MERCK KGaA, Germany. The same pigment shows in diffuse irradiation, like on a cloudy day, a different visual effect called graininess (coarseness) which appears as a granular structure of the surface. Both effects were studied on especially manufactured samples of a dilution series in pigment concentration and a tonality series with carbon black. The experiments were carried out with the robot-based gonioreflectometer and integrating sphere facilities at Physikalisch-Technische Bundesanstalt (PTB) in multidimensional configurations of directional and diffuse irradiation. The research is part of the European Metrology Research Program (EMRP), which is a metrology-focused program of coordinated Research & Development (R&D) funded by the European Commission and participating countries within the European Association of National Metrology Institutes (EURAMET). More information and updated news concerning the project can be found on the xD-Reflect website http://www.xdreflect.eu/.

Interlaboratory comparisons, referred to as key comparisons, are completed for many metrological units within the framework of the mutual recognition arrangement of the Bureau International des Poids et Mesures. These comparisons are the responsibility of consultative committees of the different metrological areas. In the case of the Consultative Committee for Photometry and Radiometry, there are currently about 20 key comparisons for various measurands. While interest in the field of bidirectional reflectance has been growing in recent years among users in industry and research and development, there is currently no dedicated key comparison to demonstrate scale conformity. This is the basis of the comparison of the bidirectional reflectance scales between the National Institute of Standards and Technology (NIST) and the Physikalisch-Technische Bundesanstalt (PTB). Measurements of two distinct sets of white diffuse reflectance standards, two sintered polytetrafluoroethylene samples and two matte ceramic samples, were performed using the common and widely used 0∶45 geometry. The wavelength range of the comparison spans the ultraviolet (λ≥330 nm) to the near infrared (λ≤1150 nm), a technically important region. In total, five different facilities participated in this bilateral investigation. The results of the comparison show good agreement within the combined uncertainties.