Correlation between surface roughness and spectral features in IR-reflection spectroscopy

Modifications of IR spectral features were reported in the literature when surfaces with diverse morphologies were analysed. In this study, five surfaces at different average roughness (Sa) were created on a massive slice of malachite with the final aim to investigate possible variations of spectral features collected through non-contact infrared reflection spectroscopy. Modifications were investigated as a function of Sa, with five frequency intervals being considered. The spectral acquisition was carried out by two portable spectrometers equipped with specific non-invasive optical heads, operating in External Reflection (ER) and Diffuse Reflection (DR) modes, respectively. A comparison study of the collected spectral profiles is proposed in this work. The analysis of spectra produced by surfaces at different roughness allowed for evaluating the effectiveness of each device in the different spectral ranges. The ER spectra were generally more resolved and cleaner compared to the DR ones. The ER module was able to return spectra whose features - both specular and diffuse - consistently follow the variation of Sa in the investigated spectral ranges. A lack of sensitivity was instead registered when working in DR mode, especially in the frequency intervals where the contribution of the diffuse component dominates the spectra.