The electric field standing wave effect in infrared transmission spectroscopy

When in IR absorbance spectra of films, which were converted from transmission, bands ratios are compared, then it can be noted that even after background correction and removal of interference fringes these band ratios change with the thickness of the films. The main goal of this work is to show that this effect is a consequence of an electric field standing wave based on the coherent superposition of light waves in the film. We further investigate how transmittance and reflectance, as well as absorbance and finally the from absorbance regained index of absorption depend on the thickness of the film and the influences on the positions of bands. We compare the results with those for the incoherent case and the case where a single pass of light through the film without reflection loss is assumed.