An improved double-thickness method combined with genetic algorithm was developed to determine the optical constants of liquid hydrocarbon fuels. Different from traditional transmission measurement, it obtained the total transmittance of a window–liquid–window three-layer structure, which is the ratio of the transmission intensity of the filled cell and that of the empty sample compartment. Also, the change of the surface reflectance at the interface of the liquid sample and the optical window and the difference between forward reflection and back reflection are considered. Experiments were operated to measure spectra in the wavelength range of 0.83–2.2 µm using a Fourier transform infrared (FT-IR) spectrometer. To verify the reliability of this method, optical constants of the distilled water were determined from its experimental transmittance spectra and the results agreed well with published data. On the basis of verification, the transmittance spectra of Chinese No. -35 diesel fuel were measured at 300 K, 350 K, and 400 K with liquid sample thicknesses of 0.5 mm and 1.0 mm. Then the optical constants of the diesel sample were obtained and the temperature dependence was analyzed. Analytical results indicate that the biggest change of the extinction coefficient between 300 K and 400 K can reach 30%, while that of the refractive index is 4.7%.
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