Near-infrared interferograms

The photograph presented in Figure 13.10 shows a typical interface used to collect these noninvasive spectra. Light is incident on one side of the skinfold and a fraction of the transmitted light is collected directly from across the input fiber. Bundles of low-hydroxy silica fibers are used to deliver and collect the near-infrared radiation for the measurement. For the experiments described here, the noninvasive spectra were collected with a Fourier transform spectrometer set for a resolution of 16 cm-1 and 128 coadded interferograms. Each recorded spectrum required approximately 60 s to acquire and save. A total of 370 spectra were collected over a period of nearly 7h while in vivo glucose concentrations varied from 6 to 33 mM (108-594 mg/dL). 

There is a real chance of a breakthrough of Raman spectroscopy in routine analytics. Excitation of Raman spectra by near-infrared radiation and recording with interferometers, followed by the Fourier transformation of the interferogram into a spectrum -the so-called NIR-FT-Raman technique - has made it possible to obtain Raman spectra of most samples uninhibited by fluorescence. In addition, the introduction of dispersive spectrometers with multi-channel detectors and the development of several varieties of Raman spectroscopy has made it possible to combine infrared and Raman spectroscopy whenever this appears to be advantageous. 

Fourier transform instrument Instruments based on interferometers for wavelength separation. Near infrared light passes through a scanning interferometer and Fourier transformation gives intensity as a function of frequency. When samples are placed in the beam (before or after the interferometer), the sample absorbs at some frequencies, and the intensities are reduced into an interferogram. The mathematical FT function is then used to convert the interferogram to an absorption spectrum of the sample. 

Fourier Transform-Near infrared (FT-NiR). Only within the last 20 years has FT-NIR instrumentation (Fig. 4.1.14) become available. Even then, the first commercial instmments had a distinct disadvantage compared to grating-based scanning instruments. FT-NIR spectrometers employ an entirely different method for producing spectra. There is no dispersion involved. Energy patterns set up by an interaction with a sample and a reference and moving mirrors (or other optical components) produce sample and reference interferograms that are used to calculate the absorbance spectrum of the sample. 

Most near-infrared (NIR) spectra are measured from 2500 to llOOnm (4000 to 9100 cm ). To record a spectrum over this range, the interferogram must be sampled twice per wavelength of the HeNe laser interferogram to give a bandpass... 

The response time of the detector should also be borne in mind when the time resolution is to be reduced well below 1 s. If the firequency of the HeNe interferogram is raised much above 10 kHz, the response of the DTGS detector is too slow and a faster detector must be used. In the mid-infrared, this is not a major problem, since MCT detectors operate optimally for modulation frequencies above 1 kHz. For near-infrared measurements, however, while InSb has a very fast response time, other quantum detectors, such as InGaAs, cannot be used at data acquisition speeds much above 5 kHz (see Section 18.2.5). 

The most accurate measurements of the CMB spectrum to date have come from the Far InfraRed Absolute Spectrophotometer (FIRAS) on the COsmic Background Explorer (COBE) (Boggess et al., 1992). In contradiction to its name, FIRAS was a fully differential spectrograph that only measured the difference between the sky and an internal reference source that was very nearly a blackbody. Figure 9.2 shows the interferograms observed by FIRAS for the sky and for the external calibrator (XC) at three different temperatures, all taken with the internal calibrator (IC) at 2.759 K. Data from the entire FIRAS dataset show that the rms deviation from a blackbody is only 50 parts per million of the peak Iv of the blackbody (Fixsen et al., 1996) and a recalibration of the thermometers on the external calibrator yield a blackbody temperature of... 

The Michelson two-beam interferometer records the autocovariance function of the observed radiation, the interferogram, as a function of optical path difference (delay) between both beams. The spectmm is obtained by Fourier analysis of the interferogram. The operation of a Michelson interferometer as an infrared spectrometer is discussed with the help of Fig. 5.8.1. The essential part of the instrument is the beamsplitter, which divides the incoming radiation into two beams of nearly equal intensity. After reflection from the stationary and the movable mirrors, the beams recombine at the beamsplitter. The phase difference between the beams is proportional to their optical path difference, including a phase shift due to the... 

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