Substrate effects in standoff detection of chemical layers

The standoff reflectance spectrum for methyl salicylate (MS) layers was shown to vary systematically with substrate type. For high-reflectance substrates, the reflectance decreased with increasingly thick MS layers, but for low-R substrates, the reflectance increased non-linearly with increasing MS layer thickness. To understand these effects, the MS refractive index was evaluated at each wavenumber value by minimizing the sum of the squared variances for low-R substrate reflectance and known-thickness transmittance, by varying the complex refractive index. The dielectric constant was expanded as a sum over oscillator modes in the harmonic approximation in terms of the oscillator frequencies, line widths, and mode strengths. For each mode, these parameters can be obtained by fitting a calculated dielectric constant to the one obtained with the present refractive index analysis.