Exploring the bronzing effect at the surface of ink layers (Orale)

Overview & Synopsis

We i nvestigate t he optical phenomenon responsible f or t he c olored shine that s ometimes a ppears at the surface of ink layers i n t he s pecular direction, o ften ca lled bronzing or gloss di fferential. I t seems to co me from the w avelength-dependent refractive i ndex of t he i nk, which i nduces a wavelength-dependent reflectance of t he i nk-air interface. Our experiments o n c yan and magenta i nkjet i nks confirm this t heory. C omplex refractive i ndices ca n b e ob tained from measurements of t he s pectral reflectance a nd transmittance of a t ransparency film coated with t he i nk. We pr opose a correction o f t he cl assical C lapper-Yule model in order to in clude th e c olored gloss in t he pr ediction of th e s pectral 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 ov er a l arge cone. E lectromagnetic models a ccounting for t he e ffect of t he na nometric roughness of t he surface also predict the attenuation of short wavelengths observed under collimated illumination.