Abstract

The Kubelka-Munk relationship is the equation most commonly used to describe the reflectance from a scattering medium. This equation breaks down, however, in several important situations in the mid-infrared. Most notably, the relationship does not account for specular reflectance, which is simply the energy reflected from the front surface of the sample. When a sample reflects diffusely and specularly, as do all real-world samples, there is a specular reflectance spectrum superimposed on the diffuse reflectance spectrum, and deviations from this equation occur. In the case of organic samples, this causes a curved line (concave downward) for the plot of <i>f</i>(R_∞) or Kubelka-Munk units vs. sample weight percent in a nonabsorbing matrix. For inorganic samples, the effect can be more severe. Since the specular reflectance of inorganic bands can be much greater than organics, often complete band inversions (<i>reststrahlen</i> bands) or derivative shaped peaks result. In either case, quantitative analysis is not straightforward, and in the latter case, even qualitative interpretation is difficult.

Diffuse reflectance infrared spectrometry: characteristics of the diffuse and specular components

Paul W. Yang and Henry H. Mantsch
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Reflectance of Perfect Diffuse and Specular Samples in the Integrating Sphere

Bjarne J. Hisdal
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Diffuse and Specular Reflectance from Rough Surfaces

Bram van Ginneken, Marigo Stavridi, and Jan J. Koenderink
Appl. Opt. 37(1) 130-139 (1998)

Infrared specular reflectance of pressed crystal powders and mixtures

Frederic E. Volz
Appl. Opt. 22(12) 1842-1855 (1983)

Model to subtract contributions of scattered radiation from measured direct transmittance and specular reflectance by light diffusing materials

William E. Vargas, Diego M. Jiménez, and Mavis L. Montero
Appl. Opt. 61(34) 10197-10206 (2022)