The laboratory of Photometry, Radiometry and Fiber Optics of the Instituto de Óptica has conducted a study to improve the accuracy of the measurement system of bidirectional reflectance (how light is reflected on a surface)

In order to improve the measurement of material reflectance, national metrology institutes and other research centers have developed in recent years complex robot-based goniospectrophotometers to measure the bi-directional reflectance of surfaces with the least possible geometrical restrictions.

When measuring the distribution function of the bi-directional reflectance (BRDF) of characteristic materials, the effect of the polarization of the light source used in the measurement or the detector's dependence on polarization is usually not taken into account, although many studies have shown its impact on the uncertainty of the BRDF of most materials, including the highly diffusive patterns used for the measurement of diffuse reflectance.

The main objective of this work was to evaluate the effects related to polarization in the measurement of BRDF and to provide a new approach to take into account the systematic errors it generates. This approach was experimentally tested on four typical materials for the realization of diffuse reflectance patterns (ceramic tile, barium sulfate, Spectralon and Russian opal glass).

In these experiments, the relative systematic error due to the polarization conditions has been calculated, as a combination of the coefficients that quantify the effect of the instrument and the coefficients that quantify the effect of the surface itself to be characterized, and the impact of the new proposed methodology on the uncertainty of the measurement obtained has been tested.

With these data, a methodology has been proposed for this type of measurement that allows to estimate the relative systematic error due to the polarization conditions from the coefficients related to the surface to be characterized and to the instrument.
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