If you want to understand the principle of night vision, you must understand the principle of light. The energy of a light wave is related to its wavelength: the shorter the wavelength, the higher the energy. Among visible light, violet light has the highest energy, while red light has the lowest energy. Adjacent to the visible light spectrum is the infrared spectrum. Infrared is divided into three categories:

Near Infrared (NIR) - Near infrared is adjacent to visible light and has a wavelength range of 0.7-1.3 microns (1 micron equals one millionth of a meter).

Mid-Infrared (Mid IR) - The wavelength range of the mid-infrared is 1.3-3 microns. Near-infrared and mid-infrared are used in various electronic devices, such as remote controls.

Thermal Infrared (Thermal IR) - Thermal infrared occupies the largest part of the infrared spectrum, with a wavelength range of 3-30 microns.

The main difference between thermal infrared and the other two types of infrared is that thermal infrared is emitted by objects rather than reflected from them. Objects emit infrared light because of a change in their atoms.