"It is known that the amount of incident light absorbed provides the composition information. If no light is absorbed there is no information available about the sample. On the other hand, if all the light is absorbed there is no information available about the sample. Hence there must be an optimum level of absorbance for acquiring the best information. At high absorbance, the detector noise and path length errors will dominate. Due to the very low band intensities, successful NIR measurements require extremely high signal-to-noise ratios, typically on the order of 50,000 to 1. Typically modern NIR instruments have noise around 10 to 100 micro absorbance units. The theoretical optimum absorbance for detector-noise-limited systems is log (1/R) = 0.434 (T = 37%).
Modern NIR instruments are also capable of adjustment to control the amount of absorbance. Instruments are capable of adjusting the path length, changing the sensing mode, changing the wavelength region, or samples can be diluted with a non-absorber or the particle size can be adjusted. The reason NIR has an advantage over IR is that NIR has lower absorbance coefficients. Due to this, the optimum path length without dilution in the IR is in micrometers, where as long NIR is in millimeters and short NIR is in centimeters. This means that with NIR it is possible to measure representative sample portions with simple or no sample preparation. It is this lack of sample preparation makes this method preferred for analysis of agricultural and food materials. Fig. 5. 90° - 45° Reflectance Configuration"
(Near-Infrared Spectroscopy for Analysis of Agricultural Material
Eric C. Newgard)
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