INFRARED-OPTICAL TRANSMISSION AND REFLECTION MEASUREMENTS ON LOOSE POWDERS

A method is described to determine quantitatively the infrared-optical properties of loose powder beds via directional-hemispherical transmi ssion and reflection measurements. Instead of the integration of the p owders into a potassium bromide (KBr) or a paraffin oil matrix, which would drastically alter the scattering behavior, the powders are place d onto supporting layers of polyethylene (PE) and KBr. A commercial sp ectrometer is supplemented by an external optics, which enables measur ements on horizontally arranged samples. For data evaluation we use a solution of the equation of radiative transfer in the 3-flux approxima tion under boundary conditions adapted to the PE or KBr/powder system. A comparison with Kubelka-Munk's theory and Schuster's 2-flux approxi mation is performed, which shows that 3-flux approximation yields resu lts closest to the exact solution. Equations are developed, which corr ect transmission and reflection of the samples for the influence of th e supporting layer and calculate the specific extinction and the albed o of the powder and thus enables us to separate scattering and absorpt ion part of the extinction spectrum. Measurements on TiO2 powder are p resented, which show the influence of preparation techniques and data evaluation with different methods to obtain the albedo. The specific e xtinction of various TiO2 powders is presented.