Abstract
We present the results of the first systematic "round-robin" comparison of far-infrared transmittance spectra measurements, which was performed by five laboratories and piloted by Physikalisch-Technische (PTB). The transmittance spectra of four different samples were measured by the participating laboratories in the 600 cm-1 to 10 cm-1 range (16.67 µm to 1000 µm) in a blind comparison. Different types of instruments, Fourier transform infrared (FT-IR) spectrometers of Michelson type and a laser radiation-based system were used for the transmittance measurements. FT-IR spectrometers are the most popular and commonly used instruments for the spectral characterization of materials in the infrared spectral range, and are well established for quantitative measurements in the mid- and near-infrared spectral ranges. However, obtaining quantitative transmittance measurements in the far-infrared spectral range by means of these instruments is challenging, because it involves weaker radiation sources, stronger diffraction effects, significant radiation originating from the sample itself and temperature gradients inside the spectrometer that may not be given proper consideration. Therefore, this comparison was initiated to test the actual capability of and identify problems with FT-IR transmittance measurements in this spectral region. We discuss the results and the possible reasons for the observed discrepancies.
Highlights
Due to their superior properties compared to dispersive instruments, Fourier transform infrared (FT-IR) spectrometers have become the most commonly used instruments for the spectral characterization of materials in the entire infrared spectral range
Three different models of spectrometers were used to perform the transmittance measurements throughout the comparison: Commercial FT-IR spectrometers of the VERTEX80v and VERTEX70v types made by the Bruker Corporation, as well as a DA3.26 made by ABB-Bomem
The transmittance of the LP 100 filter measured by all participants in the range from 600 cm–1 to 50 cm–1 is given on the left-hand side of Fig. 2
Summary
Due to their superior properties compared to dispersive instruments (i.e., their high throughput and fast and wide spectral coverage), Fourier transform infrared (FT-IR) spectrometers have become the most commonly used instruments for the spectral characterization of materials in the entire infrared spectral range. Possible deviations in the comparison were expected from different solid angles of the radiation incident on the sample within the different applied instruments, as well as from a slightly defocused spot on the detector due to the difference in optical path length between the reference and sample measurements. In the transition region – mainly for the TE11 mode – it was expected that the spectral resolution (lowest intended spectral resolution 0.4 cm–1) would not be sufficient, possibly resulting in smoothed details of the spectral structure of the filter transmittance This filter was mounted in a tube (seen in Fig. 1(d)) and was measured under normal incidence. Difficulties with this sample may arise from back reflections in the different instruments
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