In organic molecules, atoms that make up chemical bonds or functional groups are in a state of constant vibration, and their vibration frequency is equivalent to that of infrared light. Therefore, when organic molecules are irradiated with infrared light, chemical bonds or functional groups in the molecules can be absorbed by vibration. Different chemical bonds or functional groups have different absorption frequencies and will be in different positions in the infrared spectrum, so that the information of which chemical bonds or functional groups are contained in the molecule can be obtained.
In 1960s, with a lot of work by Norris and others, the theory that the content of substances has a linear relationship with the absorption peaks of several different wavelength points in near infrared region was put forward, and the moisture, protein, fat and other components in agricultural products were determined by near infrared diffuse reflection technology, which made the near infrared spectroscopy technology widely used in the analysis of agricultural and sideline products. In the middle and late 1960s, with the emergence of various new analytical techniques, coupled with the weakness of low sensitivity and poor anti-interference exposed by the classical near infrared spectroscopy, people became indifferent to the application of this technology in analysis and testing. Since then, near infrared spectroscopy technology has once again entered a silent period.
Multivariate calibration technology is an important part of chemometrics. The successful application of multivariate calibration technology in spectral analysis in 1970s promoted the popularization of near infrared spectroscopy technology. In the late 1980s, with the rapid development of computer technology, the digitalization of analytical instruments and the development of chemometrics were promoted. The good effect of chemometrics in solving spectral information extraction and background interference, coupled with the unique characteristics of near infrared spectroscopy in sample measurement technology, make people familiar with the value of near infrared spectroscopy again, and the application research of near infrared spectroscopy in various fields has also begun. In the 1990s, the application of near-infrared spectroscopy in industrial fields has been fully developed, and the research and application literature on near-infrared spectroscopy has grown almost exponentially, becoming the fastest developing and most striking independent analysis technology. Because of the good transmission characteristics of near-infrared light in conventional optical fiber, near-infrared spectroscopy technology has also been well applied in the field of online analysis, and achieved good social and economic benefits. Since then, near infrared spectroscopy technology has entered a new period of rapid development.