Theory of NIR Spectroscopy
The word “spectroscopy” is derived from the Latin root spectrum (appearance, image) and the Greek word skopia (to view). This definition is rather descriptive of the spectroscopic measurement itself i.e. to view a light image coming from a specimen (Miller, 2001).
In essence, NIR technology involves light interacting with matter where electromagnetic radiation occurs in the form of waves. The wavelength of a wave is the distance between the two peaks or high points and is indicated by the symbol λ (Shadow, 2000; Figure 1). A wavelength in the NIR spectrum is normally measured in nanometer (nm) where
1 nm = 10-9 m or 1000 nm = .001 mm.
That part of the spectrum visible to the human eye extends from about 400 nm to 800 nm, while the infrared spectrum extends from about 2 500 nm to 25 000 nm. Near infrared is considered as that part of the spectrum lying between the visible region and the infrared region. The range of wavelengths NIR covers are from 750 nm to 2 600 nm (Figure 2).