The principle of an online infrared Sensor.
Near-infrared is electromagnetic waves ranging from 1.0 to 4.0 micrometers (µ).
Short-wavelength infrared (1-2.5µ) can be measured using ordinary optical glass and lead sulfide detectors. Longer wavelengths of infrared above 2.5μ can be measured using different optical materials and lead selenide detectors.
Molecules containing hydrogen atoms, such as: -OH, -CH, -NH. They have multiple external absorption wavelengths in the red spectrum. Generally, absorption is stronger at longer wavelengths, but considering factors such as background substrate interference, sometimes shorter wavelengths are more suitable for measurement. The infrared absorption wavelengths of different heteronuclear molecules are attributed not only to their molecular bonds but also to associated molecules.
According to quantum theory, molecular bonds only absorb infrared energy at specific wavelengths. When infrared light of a "specific wavelength" irradiates a molecule, the energy absorption is a linear function of the molecular concentration, and the reflected (non-absorbed) energy is detected, thereby indirectly testing the concentration of that molecule.
Figure: Water absorption spectrum
C α Io/It C = concentration
Io = initial incident radiation
It = transmitted or backscattered radiation
Since measurements are taken at wavelengths where the initial incident radiation cannot actually be absorbed, one or more wavelengths close to the absorption region are selected. Other wavelengths with minimal or no infrared absorption are used as reference wavelengths to eliminate or reduce the impact of background variations.
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