Publishable summary – January 2021

Presentation – overview

Project structure

The project consists of five technical work packages and two work packages for creating impact and management. The technical work packages are described briefly below.

WP1: Advanced metrological issues related to BRDF

A first large comparison of BRDF scales has been performed between 8 NMIs during EMRP project xD-Reflect. Results have shown that the uncertainty was not well controlled for all cases, particularly at the extremity of the visible wavelength range. Further investigations have been carried out in EMPIR project BiRD, which have allowed better understanding of the role of polarisation in BRDF measurements. This workpackage will aim to implement a full control of polarisation on primary goniospectrophotometers at NMIs and study speckle effects in BRDF measurements. A new comparison will be carried out. The optical properties of materials will be measured at different scales: from the macroscopic to the microscopic.

WP2: Traceable BTDF Measurements

Workpackage two aims to provide a European measurement landscape capable of offering traceable BTDF (Bidirectional Transmittance Distribution Function) calibrations and measurements for versatile standards and at different measurement configurations. Two primary reference facilities will be developed for measurement of the BTDF at Aalto and PTB. Intercomparison measurements will be carried out. The results of the intercomparisons will be used to evaluate if modifications on the agreed definitions must be performed.

WP3: Development of the first BSSRDF references

The aim of this work package is to provide traceability to the subsurface scattering of materials. In order to accomplish this, the relevant quantity BSSRDF (Bidirectional Surface Scattering Reflectance Distribution Function) and its corresponding unit need to be thoroughly defined, the practical realisation of the unit developed and validated, and transfer artefacts and procedures supplied. Two primary facilities for the BSSRDF measurements will be developed, aiming for an expanded uncertainty of 5 %, and their performances will be compared.

WP4: Modelling and interpolation tools

The aim of this work package is to provide theoretical support to the experimental work. The measured data from WP2 and WP3 will be analysed using models for light propagation and light-matter interaction in order to interpret the data and improve comparison between the various measurement setups employed. Additionally, model(s) developed in this WP will be used to ensure scalability in WP1, where traceable BRDF measurements ranging from the µm to the mm scale will be performed. The model(s) developed will link the appearance of a sample, as measured through the BRDF, BTDF, and BSSRDF, to the material properties and small scale topographic structure. This will aid in achieving quantitative control of the appearance of surfaces.

WP5: Reference samples (standard artefacts)

The aim of this work package is to gather a suitable selection of reference samples (standard artefacts) to be used in the project. The work will be carried out in close collaboration between Nation Metrology Institutes and industrial partners in order to obtain samples covering a large range of appearance with structure parameters compatible with the type and geometry of measurement. Additional characterisation of the samples will include evaluation of surface topography and roughness, and different dimensional measurements including flatness as well as determination of the refractive index.