We have presented a practical method to collect vibrational absorption spectra of hydrogenated amorphous thin films free of interference fringes and free of absorption deviation induced by interfacial reflection. The experimental setup description and theoretical explanation given in this paper have provided adequate information for spectrum collection. Based on the same setup, we have also demonstrated that a vibrational absorption spectrum free of shape distortion can be obtained by using a regular light source. Through the comparison between proposed and conventional methods, we have shown that a pure vibrational absorption spectrum is the most reliable spectrum for absolute thin film microstructure assessment, while a shape-undistorted spectrum is reliable for relative thin film microstructure assessment.