Interferometer transmittance

The preparation of the KBr pellet is standardized to use about 0.20 g of warm, dry, IR-spectroscopic grade KBr and about 2 mg of sample which will yield a pellet approximately 0.5 mm thick and 13 mm in diameter. The typical percent transmittance values of the 1200- to 500-cm "1 region of the spectrum range from a 70-85% base line to 5-30% for the peaks. This set of characteristics adequately allows the resolution of those peaks that can be resolved. The spectra given herein were taken on a Nicolet DX FTIR System. The resolution was 2.0 cm", the gain was set to 1, the interferometer velocity was 20, and 30 scans were taken. 

Smaller values of are obtained for interferometers operated in a double-beam mode, since the moveable mirror must be left stationary for a fraction of the cycle time to allow the detector to stabilize each time the beam is switched from the sample to the reference position. With an optical null grating spectrometer the chopper is used not only to modulate the beam but also to alternate the beam between sample and reference channels. Thus, it takes approximately the same time to measure a transmittance spectrum using a double beam optical null spectrometer as it takes to measure a single-beam spectrum with the same S/R. Hence, for this type of spectrometer may be assigned a value of 2. 

Here T v) is the power or intensity transmittance of the sample and qi(v) is the phase shift introduced by it. The wave reflected at the movable mirror can still be written in the form of Eq. (2.3b). The interferogram with the sample in one arm of the Michelson interferometer is then for monochromatic radiation... 

Figure 2.45. (a) Diffuse reflectance and (b) normalized diffuse transmittance of 1.3-mm-thick samples of Cab-O-Sil silica powders entirely covered by (4) DMB and (S) DMP.CN siloxy substituents, (a) Number of coadded scans 250, resolution 4 cm" interferometer scanning speed 0.5 cm s". (b) Photopyroelectric signals normalized to spectrum of empty sample holder/detector. Number of coadded scans 75, resolution 4 cm" interferometer scanning speed 0.03 cm s". Adapted, by permission, from F. Boroumand, J. E. Moser, and H. Vandenbergh, Appl. Spectrosc. 46, 1874 (1992), pp. 1883 (Fig. 7) and 1885 (Fig. 10). Copyright 1992 Society for Applied Spectroscopy.

Fig. 4.37. Transmittance of an absorption-free multiple-beam interferometer as a function of the phase difference 0 for different values of the finesse F ...

The wavelength position of the transmittance peak (A,) through either a Fabry-Perot interferometer or bandpass interference filter is given as... 

Figure 4.37 illustrates (4.51) for different values of the reflectivity R. The maximum transmittance is T = 1 for 0 = 2m7r. At these maxima Irj. = Iq, and the reflected intensity Ir is zero. The frequency range 8u between two maxima is the free spectral range of the interferometer. With 0 = 27tAs/A and A = c/u, we obtain from (4.47a)... 

The performance of the beamsplitter is of vital importance to the successful operation of most FT-IR spectrometers and we consider the design and performance of beamsplitters in this section. Consider a beam of monochromatic radiation at wavenumber, V, entering a two-beam interferometer. Let the intensity of the radiation be 7(v) and let the reflectance and transmittance of the beamsplitter be and TV, respectively. For a beamsplitter that absorbs no radiation, Ry- -T = 1. As mentioned in Section 2.2, an ideal beamsplitter is one for which R and TV are equal to 0.5 across the entire spectrum that is measured. Usually, however, either R or TV is slightly greater than 0.5, which has the effect of reducing the beamsplitter efficiency. 

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