The phenomenon in which light of a single frequency changes its frequency and is inelastically scattered when it interacts with the vibration, rotational motion, or other excited states of a material system is called Raman scattering. In general, the Raman spectrum represents the vibration of molecules or the vibrational energy of a crystal lattice. Most of the light scattered after interacting with a material undergoes Rayleigh scattering, which is elastic scattering, where the frequency does not change, but a very small number of light undergoes inelastic scattering, where it is scattered at a lower or higher frequency than the initial frequency. Light that raises the vibrational energy state of a material and is scattered at a lower frequency is called Stokes scattering, and light that obtains energy from a material that was originally in a high vibrational energy state and lowers the vibrational energy level of the material and is scattered at a higher frequency is called anti-Stokes scattering. If the light scattered with a changed frequency is spectroscopically analyzed, the Raman spectrum unique to that material can be measured. At room temperature in a state of thermal equilibrium, the probability of Stokes scattering is much higher according to the Boltzmann distribution theory, so the Raman spectrum is mainly obtained by spectroscopically analyzing the Stokes scattered light.

The unit of the x-axis of the Raman spectrum is cm-1 and is expressed as Raman shift or Wavenumber, which means the difference between the initial frequency of the light source and the frequency of the scattered light that interacts with the substance. Therefore, the larger the value, the higher the vibrational energy of the vibrational mode of the molecule represented by the corresponding band (peak). The vibrational energy of a molecule varies depending on how it vibrates. If it is the same type of vibrational mode, the value of the Raman band is higher if it is a covalent bond or a bond between lighter atoms. The y-axis represents the intensity of Raman scattering and is not expressed as a specific physical quantity, but rather as Intensity, Count, etc.

Since the Raman spectrum is unique to each substance, it is used to identify unknown substances. In addition, the physical and chemical properties of a substance can be observed by analyzing the Raman spectrum that changes when an external stimulus is applied to the sample.

A laser is used as a single-frequency light source, and a spectrometer, a photodetector (CCD or PMT), a microscope, and various optical devices connecting them are combined to form a micro-Raman spectroscopy system that can measure even extremely small samples.