UV-Vis absorption spectra of floating layers were measured using an AvaSpec-2048 optic fiber spectrophotometer equipped with deuterium-halogen light source AvaLight-DHC (Avantes) by placing a reflectometry probe (400µm fiber, six irradiating and one measuring fiber combined) orthogonally to a water subphrase over a deep well in the center of a Langmuir trough (to quench mirror reflections from white surface of a trough) at a distance of 2-3 mm from the monolayer.