In the infrared absorbance spectra of carbon impurities in silicon, baseline variations are found in the range 630-610 cm(-1) near the carbon vibration band at 605 cm(-1). The variations appear when the resistivities of test and reference samples are: different, it has been shown that if there is a difference in resistivity between test and reference samples, the variations are, caused by the contribution difference of multiple reflections of the incident light to absorbance between within and outside the range, reflecting the strong two-phonon absorption in this spectral range.