Basic Principles of Night Vision

To understand the principles of night vision, it is necessary to know something about the principles of light. The amount of energy in a light wave is related to its wavelength: the shorter the wavelength, the higher the energy. In visible light, violet light has the highest energy, while red light has the lowest. Adjacent to the visible light spectrum is the infrared spectrum. Infrared light is divided into three categories:

Near infrared (near IR) - 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) - Mid-infrared has a wavelength range of 1.3-3 microns. Near infrared and mid infrared are used in a variety of electronic devices, such as remote controls.

Thermal Infrared (Thermal IR) - Thermal infrared occupies the largest portion of the infrared spectrum and has 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 an object rather than reflected off of it. An object is able to emit infrared light because its atoms have changed in some way.