Design of an instrument for measuring the spectral bidirectional scatter distribution function
ABSTRACT The spectral bidirectional scatter distribution function (BSDF) offers a complete description of the spectral and spatial optical characteristics of a material. Any gloss and color measurement can be related to a particular value of the BSDF, while accurate luminaire design with ray tracing software requires the BSDF of reflectors and filters. Many measuring instruments, each having particular advantages and limitations, have been reported in the literature, and an overview of these instruments is included. A measuring instrument that allows for an absolute determination of the spectral BSDF with a full three dimensional spatial coverage in both reflectance and transmittance mode, a broadband spectral coverage, a large dynamic range, a reasonable acquisition time, and a large sample illumination area is presented. The main instrument characteristics are discussed, and the measurement capabilities are illustrated.
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ABSTRACT: Multispectral and polarized light reflectance measurements are very useful to characterize materials such as paint coatings. This article presents an overview of an automated high-angular resolved, in-plane multispectral polarized reflectometer and its calibration process. A compre-hensive study based on multispectral BRDF and DOLP measurements is conducted on different colour and glossy aspects of paint coatings. An original inverse method from in-plane measurements is used to model the out-of-plane BRDF and to investigate the role of the surface and subsurface scattering phenomena in its components.Optica Applicata 01/2012; 42(1). DOI:10.5277/oa120101 · 0.64 Impact Factor
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ABSTRACT: This study reports on a psychophysical experiment with real stimuli that differ in multiple visual gloss criteria. Four samples were presented to 15 observers under different conditions of illumination, resulting in a series of 16 stimuli. Through pairwise comparisons, a gloss scale was derived and the observers' strategy to evaluate gloss was investigated. The preference probability matrix P indicated a dichotomy among observers. A first group of observers used the distinctness-of-image as a principal cue to glossiness, while a second group evaluated gloss primarily from differences in brightness. It could therefore be questioned if surface gloss can be characterized by one single quantity, or that a set of quantities is necessary to describe differences in gloss.
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ABSTRACT: The reliability of ray tracing simulations is strongly dependent on the accuracy of the input data such as the bidirectional reflectance distribution function (BRDF). Software developers offer the possibility to implement BRDF data in different ways, ranging from simple predefined functions to detailed tabulated data. The impact of the accuracy of the implemented reflectance model on ray tracing simulations has been investigated. A light-emitting diode device including a frequently employed diffuse reflector [microcellular polyethylene terephthalate (MCPET)] was constructed. The luminous intensity distribution (LID) and luminance distribution from a specific viewpoint were measured with a near-field goniophotometer. Both distributions were also simulated by use of ray tracing software. Three different reflection models of MCPET were introduced, varying in complexity: a diffuse model, a diffuse/specular model, and a model containing tabulated BRDF data. A good agreement between the measured and simulated LID was found irrespective of the applied model. However, the luminance distributions only corresponded when the most accurate BRDF model was applied. This proves that even for diffuse reflective materials, a simple BRDF model may only be employed for simulations of the LID; for evaluation of luminance distributions, more complex models are needed.Optical Engineering 09/2013; 52(9):art.nr. 095101. DOI:10.1117/1.OE.52.9.095101 · 0.96 Impact Factor