DIFFUSE-REFLECTANCE INFRARED-SPECTROSCOPY - EXPERIMENTAL-STUDY OF NONABSORBING MATERIALS AND COMPARISON WITH THEORIES

This work presents a comprehensive study of the diffuse reflectance of nonabsorbing powders in the mid-infrared. The changes in the optical and physical parameters of the powder (particle size, granulometric di stribution, powder density, refractive index) have been used to get a better understanding of the influence of each parameter on the diffuse ly reflected intensity, as well as the mechanisms of the light diffusi on by the powder. A commercial diffuse reflectance attachment has been tested optically for the accuracy of measurements, the focalization o f the incident beam at the sample surface being taken into account. Th e experimental results have been compared to results of existing diffu sion theories. This comparison proves the pertinency of the observed v ariation of the intensity of the diffusely reflected light with the re fractive index which increases from O for n = 1 to a maximum and then decreases with n. This behavior is related to the anisotropy of the li ght diffusion both at the inside and then at the outside of the partic les and to the light trapping by total reflection inside the particles .