To understand the principle of night vision, we must know something about the principle of light. The energy of light wave is related to its wavelength: the shorter the wavelength, the higher the energy. In visible light, purple light has the highest energy and red light has the lowest energy. The infrared spectrum is adjacent to the visible light spectrum. The infrared is divided into three categories:
Near infrared (near IR) - near infrared and visible light, its wavelength range is 0.7-1.3 micron (1 microns equals to one millionth of a meter).
Mid infrared (middle IR) - the wavelength range of mid infrared is 1.3-3 micron. Near infrared and mid infrared applications are applied to various electronic devices, such as remote controls.
Thermal infrared (thermal IR) - Thermal Infrared occupies the largest part of the infrared spectrum, and its wavelength range is 3-30 microns.
The main difference between thermal infrared and two other kinds of infrared rays is that thermal infrared rays are emitted from objects rather than from objects. The reason why an object can emit infrared light is that its atom has undergone some change.