QUANTITATIVE DIFFUSE REFLECTANCE AND TRANSMITTANCE SPECTROSCOPY OF LOOSELY PACKED POWDERS

A self-consistent discontinuum theory describing the dependence of the diffuse reflectance and diffuse transmittance of powders on their optical absorption coefficient is presented. The theory is valid for particle sizes large compared to the exciting wavelength. It constitutes generalization and evolution of the statistical theory of absolute diffuse reflectance of powders put forth by N. T. MELAMED [J. Appl. Phys. 34, 560 (1963)]. Experimental evidence of the spectroscopic utility of the theory in determining absolute optical absorption coefficients self-consistently from both the reflectance and transmittance measurement channels sequentially is provided for two powders, and the new photothermal technique of Diffuse Transmittance Infrared Fourier Transform Spectroscopy (DTIFTS) is introduced. It is shown that the transmittance channel can be used advantageously to measure small changes in optical absorption spectra, as it is more sensitive to such changes than the diffuse reflectance. Practical experimental limits of the validity of the discontinuum theory are also established.