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.
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.
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 different types of project calls. Within the EMRP Call 2012 "Metrology for Industry", the joint
research project (JRP) entitled "Multidimensional Reflectometry for Industry" (xD-Reflect) was submitted by a
consortium of 8 National Metrology Institutes (NMIs) and 2 universities and was subsequently funded. The general
objective of xD-Reflect is to meet the demands from industry to describe the overall macroscopic appearance of modern
surfaces by developing and improving methods for optical measurements which correlate with the visual sensation being
evoked. In particular, the project deals with the "Goniochromatism", "Gloss" and "Fluorescence" properties of dedicated
artifacts, which will be investigated in three main work packages (WP). Two additional transversal WP reinforce the
structure: "Modelling and Data Analysis" with the objective to give an irreducible set of calibration schemes and
handling methods and "Visual Perception", which will produce perception scales for the different visual attributes.
Multidimensional reflectometry involves the enhancement of spectral and spatial resolution of reference
gonioreflectometers for BRDF measurements using modern detectors, conoscopic optical designs, CCD cameras, line
scan cameras, and modern light sources in order to describe new effects like sparkle and graininess/coarseness. More
information and updated news concerning the project can be found on the xD-Reflect website http://www.xdreflect.eu/.
Coatings can be classified by either their appearance, such as glitter, or by their function, such as corrosion protection. However, pigments are currently being manufactured with new and unique appearance attributes that can not be characterized by traditional methods. These coatings may exhibit differences in their perceived color with changes in the illumination or viewing angle, or both. Properties such as these have become rudimentary in the production of currency, cosmetics, and retroreflective materials. The primary impetus of goniospectrometry at NIST is to develop accurate measurement protocols for reproduction and quality control of appearance attributes, such as color matching, by determining the minimum set of illumination and viewing geometries needed to accurately characterize the perceived color. Here, we present a new goniospectrometer developed at NIST that allows the measurement of the complete bi-directional reflectance distribution function (BRDF) for colored surfaces with the objective of differentiating between the scattering mechanisms in the coating. The illumination is provided by a monochromator with a spectral resolution of 0.05 nm between 360 nm and 780 nm. The sample can be moved about 3 different axes, allowing illumination and viewing for any direction within the hemisphere about the sample, including grazing angles, with accuracy better than 0.01° for each axis. This equipment will become the future provider of standard BRDF measurements at NIST, for the characterization of complex surfaces like gonioapparent coatings or retroflective surfaces.
Conference Committee Involvement (1)
Measuring, Modeling, and Reproducing Material Appearance 2015
9 February 2015 | San Francisco, California, United States