We have made quantitative infrared intensity measurements by FT-IR transmission and attenuated total reflection spectroscopy of liquids and by specular reflection spectroscopy of single crystals. Knowledge of the accuracy of the measurements is necessary, and this paper emphasizes how this was determined for liquid samples. It is concluded that a value of an absorption coefficient that is determined from 4 to 6 transmission spectra of a pure liquid is likely to vary by ±0.6% to 1.5%, depending on the cell used, the beamsplitter used and the optical alignment of the interferometer and detector, provided that the cell is mounted firmly. The average value is likely to agree with the available calibrated value to within 3%, which is within the approximately 5% accuracy of the calibrated value. We believe the main source of error to be the effect of the infrared cell on the light path through the instrument. An attenuated total reflection method has been developed for the easy evaluation of the absorption and dispersion parameters of liquids to an accuracy of about 6% for bands of medium intensity. This accuracy is compatible with typical literature data and is useful to current theory. Specular reflection measurements were calibrated against a silicon standard. They are reproducible to about 3% of the reflectance value, largely due to the surface and position of the sample.
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