Radiative transfer analysis on the internal optical-path of FTIR spectrometer for transmission measurements of absorbing media

Fourier transform infrared (FTIR) spectrometer has been widely used in spectral measurements, especially for transmission measurements of absorbing media, but most researchers assume that the internal light source is collimated, while it is convergent in fact. Therefore, it is necessary to explain the applicability of FTIR spectrometer in transmission measurements of absorbing materials. Based on this, influence of the deviation distance caused by the dispersion effect on the final detection is analyzed. Then expression of the deviation distance for single-layer absorbing media is first deduced, which is related to the convergent angle of incident beam, the refractive index and thickness of sample. The maximum measurable thicknesses of typical optical windows are calculated. Besides, influence of the random error in experiments on transmission measurements and radiation property identification of absorbing media are analyzed.