GaSb and InAs demonstrate significant potential photoelectric applications in mid-wave infrared (3-5 μm) and long-wave infrared (8-12 μm) spectral regions. However, their weak optical absorption properties hinder accurate determination of absorption coefficients via conventional transmission spectroscopy due to multi-pass transmission effects. This study introduces a combined reflection-transmission analysis method based on Fourier-transform infrared spectroscopy (FTIR), achieving enhancement in measurement accuracy within the low-absorption regime (α<10 cm?1). For samples with doping concentrations of ND=2.46×1016 ^cm-3 ^{(InAs) and} ND=8.76×1016 ^cm-3 (GaSb), the analysis reveals that free carrier absorption in the 8-18 μm range is predominantly governed by acoustic phonon scattering and ionized impurity scattering. Notably, GaSb exhibits significantly enhanced scattering intensity compared to InAs, with a 10-fold increase in ionized impurity scattering coefficient and a 450-fold amplification in acoustic phonon scattering coefficient. The developed methodology provides a technical framework for determining absorption coefficients in low-absorption materials.