Near-infrared reflectance (NIR) spectroscopy is widely used for the quantitative determination of quality attributes such as moisture, protein, fat, and kernel hardness in agriculture and food products . It is an approved method for quantitative measurement of wheat protein and moisture content. NIR instruments for grain and grain product measurement have predominantly used grating monochromators to obtain spectral information, which is measured with a single or diode array detector. NIR instruments use various hardware configurations to obtain spectral information, and Fourier-transform near-infrared reflectance (FT-NIR) instruments are only one method to do this. FT-NIR hardware is generally more complex but advances in electronics, methods, and manufacturing have significantly improved the detector sensitivities, resolution, and immunity from vibration effects .
FT-NIR records the intensity of the entire spectrum as a function of the optical path differences (OPD) between two NIR beams in an interferometer. The two beams are created by splitting the measurement beam, i.e., the beam that is transmitted through or reflected from the specimen. One split beam travels over a different optical path length, via a moving mirror, and is recombined with the second beam to create an interference signal. The total interference signal results from the mirror traveling through a range of wavelengths and is transformed to spectral components via a fast Fourier transform. Gave a detailed
explanation of FT-NIR and cited its advantages over conventional grating NIR spectroscopy as 1) higher signalto-noise ratios, 2) extremely high resolutions, and 3) fast and accurate frequency determinations. The first advantage is realized because there are fewer optical components to attenuate radiation which leads to greater power reaching the detector and a better signal-to-noise ratio. The second advantage is that resolution at discrete wavelengths is much better, and thus elements which interact within very narrow
bands can be detected. The third advantage states that spectral collection is much faster than with a dispersive instrument, since all wavelengths are measured simultaneously. While this last advantage is somewhat true, it can also be misleading since simultaneous measurement occurs only at discrete mirror positions. The total interference signal must be measured at discrete points during the mirror’s travel, which requires a finite time period. After the entire time-domain interference signal is measured, it is then
transformed to the frequency (spectral) domain via a fast Fourier transform (FFT) to obtain discrete wavelength information. When using FT-NIR at the shorter NIR wavelengths, compared to mid-infrared, the
interference pattern is more influenced by misalignment and may cancel any added advantage FT-NIR may have over grating NIR instruments. Thus small perturbations in mirror positioning or misalignment are sources of error and as a result, lasers are commonly incorporated to provide a position reference. To achieve a position reference, monochromatic laser output is directed through the interferometer onto its own detector. The precision of this alignment is not generally specified by the manufacturer. Other literature indicated that the advantages of FT-NIR/IR are in the mid-infrared region, not the NIR region, because of the higher signal/noise (S/N) ratio using detector noise- limited instrumentation. NIR instruments do not use
detector-noise limited instrumentation. Biological materials absorbs over broad regions in the NIR, not at discrete wavelengths. Thus, the advantage of FT-NIR being able to detect absorbance in very narrow bands may not be a significant benefit.
FT-NIR has been applied to study various attributes such as fat, protein, cholesterol, lactose, and internal quality in products of agriculture and food industries.