Caractérisation optique et visuelle du brillant

Le brillant est un attribut visuel qui tire son origine de la distribution dans l'espace de la lumière réfléchie par une surface. Quantitativement, l'intégralité de l'information sur la réflexion lumineuse est contenue dans la fonction de distribution bidirectionnelle de la réflectance (BRDF). Des mesures réelles de BRDF ont été réalisées. Nous proposons un modèle de BRDF basé sur une théorie de micro facettes. L'ajustement de 2 paramètres et d'une fonction, à partir d'un nombre limité de mesures dans le plan d'incidence, suffit pour reconstruire l'intégralité de la BRDF. Parallèlement, une expérience visuelle contrôlée a permis de construire une échelle perceptive de brillant. Il s'avère que la sensation de brillant évolue de manière non- linéaire avec les valeurs de brillant spéculaire. Les résultats révèlent aussi la propriété de constance de brillant dans les jugements, et le fait que la vision binoculaire accentue la discrimination sur les surfaces les plus brillantes.Gloss is a visual perceptual attribute, which originates from the geometrical distribution of the light reflected from a surface. The relevant physical quantity to quantify the whole of the luminous reflection is the Bi-directional Reflectance Distribution Function (BRDF) Real measurements of BRDF have been carried out. We propose a model of BRDF based on a microfacets theory. Only the fit of 2 parameters and one function is needed to reconstruct the entire BRDF from a limited number of measurements in the incident plane. ln a vision study, we propose a uniform visual gloss scale. We find a non-linear relation between the gloss percept and the specular gloss index. Moreover, results reveal that the judgments of observers show gloss constancy, and that binocular vision accentuates the discrimination on glossiest surfaces.PARIS-CNAM (751032301) / SudocSudocFranceF

Visual experiment on LED lighting quality with color quality scale colored samples

International audienceA psychophysical experiment developed to evaluate light quality of Light Emitting Diodes (LEDs) is described. It is based on coloured samples used in the “Color Quality Scale” (CQS) proposed by the NIST to replace the “Colour Rendering Index” (CRI). Predictions of the CQS and results from visual measurements are compared

Difference scaling of gloss: nonlinearity, binocularity, and constancy.

Gloss is an attribute of visual appearance that originates from the geometrical distribution of the light reflected by the surface. We used the maximum likelihood difference scaling (MLDS) procedure (L.T. Maloney & J. N. Yang, 2003) to estimate gloss scales over an extended range. Observers' judgments were obtained for a series of 10 black, coated samples for two directions of illumination, in binocular and monocular vision. The results showed a nonlinear relation between gloss percept and instrumental specular gloss values. Sensitivity is higher at extreme scale values than in the middle. In binocular vision, the sensitivity to gloss is higher than in monocular vision exclusively for high gloss levels. Lastly, we found that gloss difference scales, when expressed in terms of the samples rather than the photometric characteristics, vary little with the direction of illumination. Gloss scaling thus seems to be independent of the geometrical variations of the luminous flux at the surface of the sample. By analogy with the term "color constancy," we call this property "gloss constancy.

Bilateral comparison of luminous flux using lamps as transfer standards (EURAMET.PR-K4.2)

International audienceThe Laboratoire National de Métrologie et d'Essais, LNE-CNAM (France) and The National Institute of Metrology, RCM LIPI (Indonesia) agreed in December 2013 to conduct a bilateral comparison on luminous flux using a group of lamps as transfer standards. The aims of the comparison were: • To link the RCM LIPI realized lumen to the CCPR K4 reference value • To demonstrate the RCM LIPI capability in calibrating tungsten standards lamps. The comparison and its protocol were approved by the CCPR and registered with KCDB with the comparison number: EURAMET.PR-K4.2. KEY WORDS FOR SEARCH Luminous flux, PR K4, Bilateral Comparison Main text To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/ . The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA)

Metrological issues related to BRDF measurements around the specular direction in the particular case of glossy surfaces

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.status: publishe

Modeling, measuring, and using BRDFs: significant French contributions

International audienceThe 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

Investigating seasonality and season of birth in past herds: A reference set of sheep enamel stable oxygen isotope ratios

International audienceIntra-tooth sequential analysis of enamel δ18O is currently used to investigate birth seasonality in past animal populations, offering new insights into seasonal availability of animal resources, herd management and seasonality of site occupation. Reference datasets are still required to address two major difficulties: (1) inter-individual variability in the record of the seasonal cycle is affected by tooth size; (2) the season of birth cannot be directly estimated from the timing of tooth growth, because of a delay in enamel mineralization. We present a dataset acquired on the lower second molar of ten modern sheep from Rousay (Orkney) born within a few weeks in April / May and submitted to the same environmental conditions until death. All sheep have recorded a sinusoidal pattern of δ18O variation spanning approximately a year. From the difference between expected and measured time sequence, the delay of enamel mineralization is estimated to five to six months. The data set is then described using a model mainly based on a cosine function. The period, corresponding to the length of M2 crown formed over a year, averaged 35.8 mm. A very slight variation of tooth growth rate with time and no attenuation of the isotopic signal towards the cervical margin of the crown could be detected on this dataset. The lowest δ18O values, corresponding to the sheep's first winter, were tracked at a distance from the enamel-root junction varying between 23.0 and 30.3 mm (xmin mean = 27.6 mm); the highest δ18O values, corresponding to the sheep second summer, between 6.3 and 11.6 mm (xmax mean = 9.9 mm). Most of the variability can be attributed to tooth size. When normalized on the period, xmin and xmax are 0.28 (± 0.05) and 0.78 ( ± 0.05) on average, meaning that the Rousay sheep have recorded the minimum and maximum δ18O values on average at respectively 78 % and 28 % of the end of the periodic cycle recorded in the second molar

Intercomparison of visual gloss psychometric scales

The total visual appearance of a surface is of concern in a wide panel of industrial sectors. In many manufacturing processes, more than instruments, the visual system has the last word when the quality of the finished product is at stake. Psychophysical studies aim to produce scales describing our perception of stimuli. In the field of gloss appraisal, psychophysics has been of importance since Hunter’s early work, and a variety of scales describing one or more visual aspects of surface gloss have been derived. However, to our knowledge, no metrological assessment of scales constructed from one and the same set of samples through different protocols has ever been carried out. In this study, psychometric functions of a commercially available gloss scale are derived from experiments conducted under different assessment conditions, by two sets of naïve observers, and using two distinct protocols. The metrological validity of the results is discussed. The potential output of this work may lead the way towards the definition of a standard gloss observer.status: publishe

Repeatability and reproducibility of specular gloss meters in theory and practice

In order to numerically quantify and to ensure consistency of the visual appearance of the end product, many industries perform color and gloss measurements, through the results of which they define maximum tolerances during quality control. For this, they may primarily rely on precision data for repeatability and reproducibility as reported by instrument manufacturers, who develop their instruments according to standardized measurement geometries defined in international standards. This paper reports on an investigation of the inter-instrument agreement between specular gloss meters which conform to universally adopted gloss standards. Six commercially available instruments, manufactured by three different companies, and twenty-five gloss artifacts, with specular gloss values ranging between 2 and 110 gloss units (60 geometry), were selected for use in the study. The repeatability and reproducibility of the instruments were assessed according to the criteria described in ASTM D523-14 and ISO 2813:2014, and to the specifications reported by the instrument manufacturers. Ray tracing simulations were performed by use of the bidirectional reflectance distribution function measurement data of two samples, and based on the standardized optical design of the specular gloss meter, in order to get a better insight on the expected tolerances of the gloss readings from a theoretical point of view. On average, the practical results indicate that both the repeatability and reproducibility values are higher than those specified in the recommendations and by the manufacturers. Ray tracing simulations confirm this finding. While specified repeatability and reproducibility thresholds are based on ideal standards, in practice, there are parameters that can lead to significant higher deviations. Care should therefore be taken when analyzing measurement results obtained from different instruments, and when defining tolerances for evaluation of gloss measurements.status: publishe

Luminous intensity bilateral comparison using lamps as transfer standards between LNE (France) and INM-RO (Roumania)

International audienceThe EUROMET project 823, "Comparison of luminous intensity and luminous flux using lamps as transfer standards" between LNE (formerly BNM), France, and INM-RO, Romania, has linked the INM-RO real-ized candela to the CCPR key comparison reference values. The comparison was registered on BIPM key comparison data base with the numbers EUROMET.PR-K3.a.1. The comparison has been piloted by LNE. It has been carried out by successive calibrations of a group of four travelling standard lamps in the two laboratories. The lamps were first calibrated by INM-RO, then cal-ibrated by LNE and calibrated by INM-RO in order to check for any drift or instability. The measurements have been performed over the period November 2004 to June 2006. According to the results of the comparison, the Degree of Equivalence of the candela realized by INM-RO to the Key Comparison Reference Value of CCPR-K3 is - 0,72 % with an expended uncertainty (k = 2) of 2.14 %. Main text To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/ . The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA)