We investigate the optical phenomenon responsible for the colored shine that sometimes appears at the surface of ink layers in the specular direction, often called bronzing or gloss differential. It seems to come from the wavelength-dependent refractive index of the ink, which induces a wavelength-dependent reflectance of the ink-air interface. Our experiments on cyan and magenta inkjet inks confirm this theory. Complex refractive indices can be obtained from measurements of the spectral reflectance and transmittance of a transparency film coated with the ink. We propose a correction of the classical Clapper-Yule model in order to include the colored gloss in the prediction of the spectral reflectance of an inked paper. We also explored effects of scattering by the micrometric or nanometric roughness of the ink surface. The micrometric roughness, easy to model with a geometrical optics model, can predict the spreading of the colored gloss over a large cone. Electromagnetic models accounting for the effect of the nanometric roughness of the surface also predict the attenuation of short wavelengths observed under collimated illumination.

Date Published: 13 March 2015
PDF: 15 pages
Proc. SPIE 9398, Measuring, Modeling, and Reproducing Material Appearance 2015, 93980U (13 March 2015); doi: 10.1117/12.2076446

Published in SPIE Proceedings Vol. 9398:
Measuring, Modeling, and Reproducing Material Appearance 2015
Maria V. Ortiz Segovia; Philipp Urban; Francisco H. Imai, Editor(s)