Diffuse Reflectance Infrared Spectroscopy: Experimental Study of Nonabsorbing Materials and Comparison with Theories

Abstract

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 distribution, powder density, refractive index) have been used to get a better understanding of the influence of each parameter on the diffusely reflected intensity, as well as the mechanisms of the light diffusion by the powder. A commercial diffuse reflectance attachment has been tested optically for the accuracy of measurements, the focalization of the incident beam at the sample surface being taken into account. The experimental results have been compared to results of existing diffusion theories. This comparison proves the pertinency of the observed variation of the intensity of the diffusely reflected light with the refractive index which increases from 0 for n = 1 to a maximum and then decreases with n. This behavior is related to the anisotropy of the light diffusion both at the inside and then at the outside of the particles and to the light trapping by total reflection inside the particles.