The absorbance studies of the optical radiation reflection from the boundary of two soft-matter media with a thin monolayer between are performed for a number of angles of incidence. The reflectance and absorbance spectra are described in terms of a unique spectrum invariant with respect to the incidence angles. The angular dependence of the absorbance for spolarized radiation is shown to not provide any extra information as compared with a single-angle study in line with the previously developed theoretical considerations. We verify it experimentally performing the multi-angle infrared reflection-absorption spectroscopy measurements at the air-water interface with a thin lipid film.