The attenuated total reflectance spectroscopy method of determining the complex refractive indices of materials which occur only as small particles was applied at a 10.6-µm wavelength to numerous pressed powder samples. Fresnel relations were used to obtain best fit values for the complex refractive indices of the samples. Good fits were obtained only when the particles were small compared to the wavelength. For such samples, several effective medium theories were used to predict values of bulk material refractive indices from those of samples with different volume packing fractions. Only the Bruggeman theory produced consistent results.
James B. Gillespie and George H. Goedecke, "Refractive indices of powdered materials using attenuated total reflectance spectroscopy," Appl. Opt. 28, 3985-3992 (1989)