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Photos of two different opal glass reflection standards under UV irradiation at 365 nm, (a) is a poor-quality standard showing an intense inhomogeneous orange fluorescence, (b) is a high-quality standard appearing with a black surface indicating the absence of fluorescence
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The European Metrology Research Program (EMRP) is a metrology-focused program of coordinated Research and Development (RD) funded by the European Commission and participating countries within the European Association of National Metrology Institutes (EURAMET). It supports and ensures research collaboration between them by launching and managing dif...
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... The quantification of appearance of translucent materials is important in many fields of industry, research, and entertainment. For industries involving cosmetics [1], car paint [2], and food [3], BTDF values could play a pivotal role in quality control and the strategic development of products. Earthobserving satellites [4,5] rely on calibrated detectors that have traceability to reference materials calibrated for their BTDF to study Earth and space. ...
... The error bars show the expanded uncertainty of measurements (k = 2). from equation (9) scaled by incident irradiance and the factor Ω cos θ t from equation (2). Figure 5 shows that the angle dependent deviations in the relative difference of the instruments are minimized. ...
We delve into theoretical and experimental considerations for determining the spectral bidirectional transmittance distribution function (BTDF) of thick samples across a broad viewing zenith angle range. Nominally, BTDF is defined as the ratio of transmitted radiance to incident irradiance measured from the same plane. However, when employing thick samples for BTDF measurements, the viewing plane of the transmitted beam may shift from the front to the rear surface of the sample, altering the measurement geometry compared to using the sample front surface as the reference plane. Consequently, the viewing zenith angle from the sample rear surface increases relative to the sample front surface, and the sample-to-detector-aperture distance decreases by an amount corresponding to the sample thickness. We introduce a method for determining the BTDF of thick samples, considering the transformation of practical measurement results to a scenario where the measurements are conducted at a very large distance from the sample. To validate the method, we utilize a BTDF facility equipped with two instruments that significantly differ in their sample-to-detector-aperture distances. We evaluate the impact of a 2 mm sample thickness on the BTDF by assessing the ratio of transmitted and incident radiant fluxes as a function of viewing zenith angle relative to the sample rear surface. The evaluation is conducted in the wavelength range from 550 nm to 1450 nm in 300 nm steps, and in the viewing zenith angle range from −70° to 70° in 5° steps. Measurements are performed in-plane at an incident zenith angle of 0°. It is concluded that consistent determination of BTDF of a thick sample is possible by converting the experimental parameters of the real measurements at relatively short distances from the sample to correspond to those that would be obtained from measurements at very large distances from the sample.
... The scattering of light from the surfaces of different objects tells us about the colour, gloss, texture, and transparency of the objects around us. As new materials and advanced measurement techniques become available, the need for traceable measurements of 'appearance' is growing in a range of industries including computer graphics, remote sensing, lighting design, cosmetics, and automotives [1][2][3][4][5][6]. There is also demand for calibrated reference standards of directional reflectance for various applications in remote sensing and Earth observation from space [7][8][9][10][11]. ...
Measurements of both hemispherical reflectance and the bidirectional reflectance distribution function (BRDF) are of interest to a wide range of industries including computer graphics, remote sensing, lighting design, and cosmetics. The scale of directional reflectance is usually realised at national metrology institutes using goniospectrophotometers, and by integrating BRDF measurements made over the hemisphere, the scale of hemispherical reflectance can also be realised. This paper describes the measurement model for BRDF and hemispherical reflectance using the MSL goniospectrophotometer, which uses rotation stages to adjust the angle of the sample. The measurement model is applied to measurements of a white Spectralon sample and a white ceramic tile to demonstrate the performance of the instrument. The relative standard uncertainty in the BRDF of a white Spectralon sample at 550 nm is less than 0.1% for in-plane measurements, while the relative standard uncertainty in the hemispherical reflectance of a white Spectralon sample at 560 nm is 0.27%.
... Since the 2000s, improvements have been made in these directions, combined with significant gains in acquisition times. European metrological research projects have been run to improve the references and measurement techniques [8]. More compact devices have begun to be marketed. ...
The scattering of light by a surface is described by the bidirectional reflectance distribution function (BRDF). Unfortunately, this function cannot be straightforwardly acquired or modeled. French researchers have proposed interesting contributions to the field, with several models and accurate experimental systems. For instance, the National Metrological Institute (LNE-CNAM) has implemented the best angular resolution goniospectrophotometer (0.015°). Modeling the BRDF has also been deeply studied in France, especially with the microfacet theory in recent years, a better understanding of the shadowing–masking function, new general distribution functions, visible normals, interfaced Lambertian microfacets, and analysis concerning light multiple reflections. This paper presents the state of the art regarding some significant French contributions in these fields.
... Therefore, industries are striving for novel, more reliable but still efficient approaches to coatings characterization (Höpe et al., 2014). Majority of the methods rely mostly on color and reflectance characteristics. ...
... One reason is an increased demand in R&D applications, such as material characterization of satellite components [2] or calibration of reflectance standards for space-based earth observation [3,4]. The industrial interest arises, for example, from the necessity of characterizing goniochromatic coatings, whose visual appearance changes significantly depending on the combination of irradiation and viewing directions [5], typically due to the use of interference [6] or diffraction pigments. ...
The field of spectral radiance factor (SRF) measurements has seen growing interest in recent years. Scale conformity has so far only been established between the national metrology institutes (NMIs) of Germany and the USA. This study aims at a bigger, multilateral scale comparison. For this purpose, a total of six NMIs participated in a scale comparison of goniospectrophotometers based on neutral and colored diffusely reflecting ceramics samples. In addition, two universities, providing a home-built gonioreflectometer and two widely used commercially available color measurement instruments, respectively, were involved. The wavelength range of the scale comparison covers the visible wavelength range from 380 nm to 780 nm. Results indicate systematic issues and that the uncertainty evaluation of the NMIs requires further work; although for the greatest part of the covered spectral range the agreement is good.
... The authors gratefully acknowledge Sean Hillman for providing the LUC sample. This research was performed under the European Metrology Research Programme (EMRP) projects xDReflect (Multidimensional reflectometry for industry) [23], Solar UV (Traceability for surface spectral solar ultraviolet radiation) [24] and MetEOC 2 (Metrology for Earth Observation and Climate 2) [25]. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. ...
Fluorescent brightening agents are widely used in various industries to enhance the appearance of materials. The angular profiles of emission and reflectance of fluorescent surfaces have been shown to deviate from Lambertian behaviour, however, in industry and calibration facilities single geometry measurements are often used, which requires assumptions to be made on the angular distributions. In addition, the angular distribution of reflectance has been shown to deviate from that of fluorescence. In this work, it is shown that the angular distribution of reflectance is dependent on the excitation wavelength and the effect is explained by qualitative and quantitative models. These angular and spectral effects may cause measurement errors when single geometry bidirectional measurements are carried out. The angular distributions can be taken into account by using goniometrical measurements, which however, result in increased calibration time and cost. Alternatively, a reference material could be used where the angular dependencies are minimised. In this work, a novel material is presented which demonstrates more Lambertian emission and reflectance profiles than conventional polytetrafluoroethylene (PTFE) based materials and a smaller dependence of angular reflectance on the absorbance of the sample.
... The driving forces behind this trend are the stringent requirements from industry and research and development (R&D). The industrial demands are due in large part to a variety of new materials, so-called gonioapparent or goniocromatic materials, whose visual appearance changes significantly depending on the angular pair of irradiation and viewing [1][2][3]. Such materials include metallic paint finishes, luster pigments, and interferometric coatings [4][5][6]. ...
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.
... The agreement between the NMIs, for "cooperative" samples (pressed PTFE powder, Spectralon), and classical angular geometries (in plane, 0°:45°) is better than 1% (k=2), which is, for such a complicated measurand, very good [12], [13]. At the present time, a new comparison, involving 6 NMIs and 2 universities, is carried out in the framework of the European metrological project xDReflect, with the purpose to provide new elements on the mastering of BRDF measurements at the highest level [14]. ...
Among the complete bidirectional reflectance distribution function (BRDF), visual gloss is principally related to physical reflection characteristics located around the specular reflection direction. This particular part of the BRDF is usually referred to as the specular peak. A good starting point for the physical description of gloss could be to measure the reflection properties around this specular peak. Unfortunately, such a characterization is not trivial, since for glossy surfaces the width of the specular peak can become very narrow (typically a full width at half maximum inferior to 0.5° is encountered). In result, new BRDF measurement devices with a very small solid angle of detection are being introduced. Yet, differences in the optical design of BRDF measurement instruments engender different measurement results for the same specimen, complicating direct comparison of the measurement results. This issue is addressed in this paper. By way of example, BRDF measurement results of two samples, one being matte and the other one glossy, obtained by use of two high level goniospectrophotometers with a different optical design, are described. Important discrepancies in the results of the glossy sample are discussed. Finally, luminance maps obtained from renderings with the acquired BRDF data are presented, exemplifying the large visual differences that might be obtained. This stresses the metrological aspects that must be known for using BRDF data. Indeed, the comprehension of parameters affecting the measurement results is an inevitable step towards progress in the metrology of surface gloss, and thus towards a better metrology of appearance in general.
... Another difference with virtual reality models is that in computer graphics measurement uncertainties are essentially never present. This is not the case in metrology, reflectometry and in any real-world based industry [8]. Since measurement errors can greatly influence shape and properties of BRDF manifolds, there is a clear need to develop new methods for handling BRDFs with measurement uncertainties. ...
Characterizing the appearance of real-world surfaces is a fundamental problem in multidimensional reflec- tometry, computer vision and computer graphics. For many applications, appearance is sufficiently well characterized by the bidirectional reflectance distribution function. BRDF is one of the fundamental concepts in such diverse fields as multidimensional reflectometry, computer graphics and computer vision. In this paper, we treat BRDF measurements as samples of points from high-dimensional non-linear non-convex manifold. We argue that statistical data analysis of BRDF measurements has to account both for nonlinear structure of the data as well as for ill-behaved noise. Standard statistical methods can not be safely directly applied to BRDF data. Our study clarifies certain pitfalls in analysis of BRDF data, and helps to develop more refined estimates and unsupervised learning procedures for BRDF models. We also apply the notion of Pitman closeness to compare different estimators and learning procedures for BRDF models. This criterion for comparison is loss function- free and seems to be especially appropriate for applications in metrology and in comparing different types of learning methods. Additionally, we outline a multiple testing procedure for testing a hypothesis that a material has diffuse reflection in a generalized sense.
... The agreement between the NMIs, for "cooperative" samples (pressed PTFE powder, Spectralon), and classical angular geometries (in plane, 0°:45°) is better than 1% (k=2), which is, for such a complicated measurand, very good [12], [13]. At the present time, a new comparison, involving 6 NMIs and 2 universities, is carried out in the framework of the European metrological project xDReflect, with the purpose to provide new elements on the mastering of BRDF measurements at the highest level [14]. Nevertheless, if we can be confident of the BRDF measurement results on so-called "diffuse samples", measurements made on samples presenting strong geometrical variations may reveal important issues. ...
Gloss is the second most relevant visual attribute of a surface beside its colour. While the colour originates from the wavelength repartition of the reflected light, gloss originates from its angular distribution. When an observer is asked to evaluate the gloss of a surface, he always first orientate his eyes along the specular direction before lightly tilting the examined sample. This means that gloss is located in and around the specular direction, in a peak that is called the specular peak. On the one hand, this peak is flat and broad on matte surfaces on the other hand, it is narrow and sharp on high gloss surfaces. For the late ones, the FWHM of the specular peak is less than 2° which can be quite difficult to measure. We developed a dedicated facility capable of measuring specular peak with a FWHM up to 0,1 °. We measured the evolution of the peak according to the angle of illumination and the specular gloss of the sample in the restricted field of very glossy surface. The facility and peaks measured are presented in the paper. The next step will be to identify the correlations between the peak and the roughness of the sample.
