When a sample is illuminated with broad-band white light some of this energy may absorbed at discrete wavelengths by constituent molecules. The measured deficiencies in exiting energies can provide the identifying 'finger print' of the constituent molecule. In the NIR region the observed absorption bands are either overtones or combinations of overtones of fundamental bands originating in the mid infrared region (2500 to 25000 nm).
These overtones and combination bands are much weaker (typically by a factor 1000) than their counterparts in the IR region and characteristically have broad bandwidths ">(typically 30-40 nm FWHM). Low absorptivity in the NIR allows the use of long path lengths and making for minimal or no sample preparation; a significant advantage for in-line analyses.
Some typical spectra show the need to use chemometrics.
Figure 2: Typical IR Spectrum
Figure 3: Typical NIR Fruit Spectrum
