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 N_D = 2.46×10¹⁶ cm^-3 （InAs） and N_D = 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.