In this study properties of a contaminant material on the surface of a probe window (prism) were investigated. The probe window, i.e., the prism, is part of the reflectometer apparatus exploited in this study. The contaminant layer was considered to originate from a pulping solution containing lignin contacting the surface of the prism. In these experiments the contact time was up to 5.0 hours. To find the values of the contaminant material properties, i.e., refractive index (<i>n</i>), extinction coefficient (<i>k</i>), and effective thickness (<i>d</i>), an optical model was proposed assuming that the contaminant layer is continuous and homogenous. In addition, the model includes an inter-diffusion layer existing between the contaminant layer and the lignin solution. From the fitting procedure using the optical model of depth profiling, it was found that the growth rate of the contaminant layer was approximately 60 nm per hour. The results obtained by theory and experiments were consistent for both <i>s</i>- and <i>p</i>-polarizations and for the various wavelengths of the light source (300, 400, and 500 nm).
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