Near-infrared (NIR) diffuse reflectance has become an accepted method for quantitative analysis of a variety of organic solids. The technique, associated instrumentation, and applications have been comprehensively reviewed by Wetzel. NIR reflectance analysis usually utilizes mathematical correlation techniques to extract compositional and property data from poorly understood spectra, and has been regarded as suspect by spectroscopists and chemists.
Stuart A. Fysh, Dom A. J. Swinkels, and Peter M. Fredericks, "Near-Infrared Diffuse Reflectance Spectroscopy of Coal," Appl. Spectrosc. 39, 354-357 (1985)