Colorimetric and spectral evaluation of the optical anisotropy of metallic and pearlescent samples

Abstract

The visual and optical anisotropies of metallic and pearlescent samples have been evaluated using a multi-gonio-spectrophotometer and by adapting a gonio-template over the instrument aperture for measuring the relative spectral reflectance factor when the sample is rotated around an axis normal to its surface. The results show that spectral data greatly vary as the azimuth angle changes, especially in pearlescent samples. Spectral and colorimetric data of the same sample for the interference and aspecular lines seen in complementary angle pairs have been compared, resulting in hue angle being kept constant, but lightness L* and chroma variations are high. In particular, the colour differences ΔE_ab calculated for pearlescent and metallic samples show that these visual differences can be clearly appreciable, even in metallic samples, but especially in pearlescent ones. Therefore, these types of optical materials do not verify the Helmholtz reciprocity, which is usually applied for digital modelling of material appearance.

Keywords:

• optical materials
• gonio-colour appearance
• spectrophotometry
• bi-directional spectral reflectance function (BRDF)
• object colour measurement

Acknowledgements

This research was supported by the Spanish Ministry for Education and Science by means of the grant number DPI2005-08999-C02-02, and the Spanish Ministry of Science and Innovation by the Grant number DPI2008-06455-C02-02.