Light scattering and absorption in pigmented coatings : Theory and experiments

Abstract: Radiative transfer models are used to describe optical properties of polymericthick films containing spherical inhomogeneities with sizes comparable to thewavelength of the incident radiation. From Lorenz-Mie theory various parametersrelated to elastic light scattering and absorption by single particles are calculated, i.e.the scattering and absorption cross sections, the forward scattering ratio undercollimated illumination, the asymmetry factor, and the single-particle phase function.Multiple scattering effects are taken into account by means of a novel approachwhich solves the radiative transfer equation for semi-infinite media perpendicularlyilluminated with unpolarized radiation. The angular dependence of the orders ofscattering of the diffuse radiation is described in terms of generalized phasefunctions, and the optical depth dependence is specified by weighting factorscorresponding to the successive scattering orders. From this multiple scatteringapproach, the explicit evaluation of average pathlength parameters and forwardscattering ratios characterizing anisotropic diffuse radiation is carried out.Optical properties of polymeric thick films containing SiO2, TiO2 (rutile), or TiO2(anatase) coated SiO2 pigments are measured and compared to theoreticalpredictions. A four-flux model is used to calculate the collimated and diffusecomponents of reflectance and transmittance from these coatings. Good agreementbetween measurements and computed reflectances and transmittances is displayedwhen pigment porosity is taken into account.