Characterization of the structure of low-e substrates and consequences for IR transflection measurements

In mid-infrared (IR) spectroscopy, the choice of substrate and corresponding illumination method are critical considerations. Transflectance measurements with reflective metallic substrates are commonly used as a cost effective approach. Reflective substrates, however, are widely known to lead to spectral distortions and variability making it difficult to interpret complex spectra. Understanding these effects is a topic of much recent interest. Previous studies have focused only on spectroscopy using incoherent light, which is likely to present distinct distortions from measurements that might use coherent light sources such as lasers. Hence, there is a need to specifically understand spectral data recorded with these emerging sources. Here, we first experimentally determine the structure of an inexpensive and commercially available substrate referred to as low-emissivity glass and then we compare it with a gold-coated glass, using analytical calculations to help understand spectral differences. Transflectance calculations reveal significant differences between the two substrates for both incoherent and coherent light propagation through the multilayer structure. These results highlight the importance of substrate choice for mid-infrared spectroscopy and illustrate that reflective substrates are not universally equivalent. Using a model based on the experimental data, we study the consequences of the low-e structure on the acquired spectral signal and validate the differences between gold and low-e substrates experimentally using polymer films.