Spectrum distortion in far-infrared Fourier spectroscopy by multiple reflections between sample and Michelson interferometer

Testing the radiometric accuracy of Fourier transform infrared transmittance measurements

“…There is a minimum near the center of the measured spectral range, and prominent structure near 8 m and 3 m. The sample-interferometer interreflection should produce contributions of alternating positive and negative signs, respectively, to the apparent transmittance for odd and even multiples of a given frequency component of the spectrum. 3 This behavior is qualitatively borne out in the data, which shows considerable structure of both signs, especially for Fig. 1.…”

“…Interreflections involving multiple passes through the interferometer result in false signals of alternating sign at harmonics of the actual ͑physical͒ wave number of a particular spectral component. 3,4 Other interreflections, if they involve the sample, merely add to the apparent transmittance of the sample. Errors as great as 10% were noted for reflective neutral-density filter samples without the beam blocks, but with the beam blocks the change in apparent transmittance of a NiCr͞Lexan sample was less than 0.2%.…”

“…However, some of the same optical design elements that are responsible for the superior performance also lead to more complicated and, in some cases, more severe sources of radiometric error in FT-IR spectra compared with their dispersive counterparts. Extensive theoretical and experimental literature exists on the subjects of detector nonlinearity, 2 interreflection effects, 3,4 ambient thermal emission, 5 and many other error sources. 6 Unfortunately, it is still not un-common for the high signal-to-noise ratio and good measurement repeatability afforded by FT-IR spectrophotometers to be mistaken for low absolute radiometric uncertainty.…”

Source of a problem with Fourier transform spectroscopy

“…Third, the cure suggested by Fig. 8 also will correct the error described by Jongbloets et al, 31 because light reflected from the sample is not allowed to return to the detector.…”