Near infrared has a wavelength between 750 and 3000 nm, The radiation of which is a result of molecular vibrations, transitions or rotation in solids, gases and liquids.
When the molecules absorb radiation energy, they are elevated into an excited state where the molecules vibrate and create an absorption spectrum in the infrared region, the intensity of this absorption energy increase or decrease depending on the overtone of the absorption bands in NIR region.
The frequency or wavelength of the molecular vibration has a primary peak and a series of harmonics or overtones based on the molecular structure.
The spectrum shape of any material depends on these vibration characteristics. Electromagnetic response in the NIR region is primarily from the vibration response of O–H, C–H, C–O and N–H molecular bonds.
Hydrogen bond has more spectral stability in NIR region, which helps greatly in food quality analysis and information of the chemical composition present in the food.