Department of Electrical Engineering and Automation

BxDiff (2019-2022)

European Metrology Programme for Innovation and Research (EMPIR) project BxDiff – “New quantities for the measurement of appearance”.

Product appearance and visual branding are important drivers for consumer purchase decisions, as they underpin perceptions of ‘quality’ and ‘desirability’. Therefore, the project aimed to advance primary metrology in spectrophotometry to meet industrial needs for the quantitative measurement of appearance. The main objectives of the BxDiff project included advancing spectrophotometric practices by putting effort in development of metrological quantities. These quantities benefit the industry by providing reference materials and traceable standard artefacts of metrological quantities of reflectance, transmittance, and surface scattering events.

The Metrology Research Institute (MRI) at Aalto university put efforts into the BxDiff work package 2 (WP2) by focusing on measurements of the Bidirectional Transmittance Distribution Function (BTDF). The MRI developed a facility for measuring BTDF values and provided traceable measurements of reference materials requiring vertical and horizontal planes of incidence.

A fused silica sample, HOD-500
Figure 1 A fused silica sample, HOD-500

The BxDiff WP2 included an intercomparison piloted by the Danish Institute for Fundamental Metrology (DFM) and a bi-lateral comparison with the Physikalisch-Technische Bundesanstalt (PTB). In the intercomparison [1] Aalto measured five samples for their BTDF in the visible wavelength range using the reference gonioreflectometer and a transfer standard instrument, Cary 7000. Furthermore, Aalto’s facility was validated for its performance of BTDF measurements by comparison of measurements with the former two instruments [2]. Figure 1 shows a HOD-500 sample that was included in the intercomparison. The sample has strong Lambertian properties and a flat spectral response in BTDF.

picture of round sample in a holder
Figure 2 Roughened glass consisting of N-BK7, which has a Gaussian-like transmittance distribution.

The bi-lateral comparison with PTB [3] included out-of-plane measurements of a roughened glass sample in the visible wavelength range. The comparison required the use of a transfer standard 3D gonioreflectometer, which has the ability to move the detector out of the illumination plane. Figure 2 shows the roughened glass sample consisting of surface treated N-BK7. Figure 3 shows the 3D gonioreflectometer in its configuration for BTDF measurements

a scema of a gonio setup
Figure 3 Aalto's 3D gonioreflectometer in its configuration for BTDF measurements. (1) Illumination optics, (2) incident zenith angle arm, (3) viewing azimuth angle arm, (4) viewing zenith angle arm, (5) collection optics, and (6) horizontal sample holder


[1]     J. Fu, A. Ferrero, M. Esslinger, T. Quast, A. Schirmacher, P. Santafé, N. Tejedor, J. Campos, L. Gevaux, G. Obein, R. Aschan, F. Manoocheri, E. Ikonen, G. Porrovecchio, M. Šmid, E. Molloy, A. Koo, S. A. Jensen, R. Oser and J. Audenaer, "Survey of BTDF measurement facilities by multilateral scale comparisons," Submitted for publication in 2024. 
[2]     R. Aschan, D. Lanevski, F. Manoocheri and E. Ikonen,” Measurement of bidirectional transmittance distribution function in the visible and near-infrared spectral range”. Submitted for publication in 2024.
[3]     J. Fu, T. Quast, E. Velke, M. Esslinger, M. Pastuschek, A. Schirmacher, R. Aschan, F. Manoocheri and E. Ikonen, "Development of primary reference facilities and measurement comparison of standard artifacts for the bidirectional transmittance distribution function," Review of Scientific Instruments, vol. 94, no. 8, 083102, 2023.


More information can be found on the project website:

Contact persons: Farshid Manoocheri and Robin Aschan

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