By definition, optical quantities transmittance and reflectance can basically be determined as the ratio of two flux measurements. One measurement is performed with, and the other without, the sample under test in the optical path. However, at longer wavelengths the temperature radiation of the sample itself as well as of the applied spectrometer and detector increasingly contribute to the detected radiation budget. This leads to growing systematic errors in the determination of the transmittance and reflectance of samples with Fourier transform infrared spectrometers at longer wavelengths. We present an effective method to overcome this problem by measuring a sequence of four measurements at two different flux levels. Results obtained with this method are compared to the basic ratio method over a spectral range from 200 cm-1 to 30 cm-1 (0.9 THz to 6 THz).