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The new definition supports traceable measurements needed to quantify the optical properties of different materials

The project

The visual appearance of materials is directly linked to their perceived quality and, in some applications, can impact the effectiveness or efficiency of the products they are used in.

An object’s ‘total appearance’ is defined by the International Commission on Illumination (CIE) as the collective contribution of colour, gloss, texture and translucency. The appearance of an object depends on its shape and size but also on factors such as viewing distance and lighting environment. New materials designed to produce complex visual effects such as iridescence, sparkle or retroreflection are more difficult to characterise and require the development of novel methods. For a number of key measurands, standard definitions and traceable references have previously been unavailable.

EMPIR project ‘New quantities for the measurement of appearance’ (18SIB03, BxDiff) has defined and developed traceable references and reference facilities for two spectrophotometric quantities required for the characterisation of transparency and translucency, Bidirectional Transmittance Distribution Function (BTDF) and Bidirectional Scattering Surface Reflectance Distribution Function (BSSRDF). It has also improved the measurement uncertainty and traceability across different scales on the Bidirectional Reflectance Distribution Function (BRDF), the quantity requested for the acquiring iridescence, sparkle and gloss.

CIE definition

Following a review by the project of theoretical and experimental work on BSSRDF, the CIE assigned a Division Reporter on “Definitions for bidirectional scattering surface reflectance distribution function (BSSRDF)”. This reporter proposed a standard definition for BSSRDF, including name, symbol, quantity and SI unit, to be incorporated into the International Lighting Vocabulary (ILV), which exists to promote international standardisation related to light and lighting. Based on this work, a paper (“Fundamental scattering quantities for the determination of reflectance and transmittance”) describing names, symbols and measurement equations related to both BSSRDF and BTDF has also been published by the project in Optics Express.

Measurement campaign

In addition, project partners have completed a campaign measuring 12 translucent reference samples fabricated by project stakeholder Covestro. The samples cover a wide range of translucencies and have been measured for BSSRDF, BTDF, refractive index, roughness and microtopography. The results obtained have shed light on the extent to which BSSRDF, BRDF and BTDF models can be used to represent translucent samples, which will be important to the development of improved appearance rendering models and simplified techniques for estimating these models’ parameters.

Project coordinator Gael Obein (LNE - CNAM) has said of the project’s work:

“BxDiff has made it possible to define and implement spectrophotometric quantities traceable to the SI, which are needed to pave the way for measuring the transparency and translucency of materials. Thanks to this work, we now have a solid basis for taking a further step towards SI-traceable measurement of total appearance of surfaces.”

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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