The reflection geometry is required for thicker samples.
Data interpretation is more tricky depending upon the nature and flatness of the analysed sample. Depending on the nature of the sample and the configuration of the optics, different types of reflectance measurements can be made, including specular reflectance (SR), diffuse reflectance (DR) - not available at MIRAS at the moment -, reflection-absorption spectroscopy or "transflectance", and attenuated total reflectance (ATR).
In FTIR microspectroscopy, one of the most common measurements is transflectance, where the infrared beam penetrates the sample, reflects off a highly reflective substrate, commonly a gold mirror, and passes back through sample to the illumination objective, see Figure. In this case the samples must be very thin, as the beam passes twice through them. There are special IR reflective and visible transmitting slides available also for this type of measurement, allowing one to view the sample in transmission, and measure in transflectance. Also highly reflective samples, or highly polished samples which cannot be cut by microtome to a thin section (eg. Bone, teeth, etc.) can be studied in the reflectance mode.
Here, flat and smooth sample surfaces are important requirements to avoid scattering, as irregular surfaces can produce artefacts in the spectral baseline.