Infrared or thermal imaging is a heat-sensing technology that can see in complete darkness without the need for illumination. All objects emit infrared energy, but it cannot be seen by the human eye without the use of technology. Infrared is a part of the electromagnetic spectrum (see diagram below) with wavelengths that are longer than visible light. Infrared is typically divided into near, mid, long and extreme and is measured in units known as microns or nanometers. Thermal technology detects infrared energy in the mid-infrared spectrum at 3-5 microns or long-infrared at 8-12 microns.

The warmer an object, the more energy it emits. Infrared energy emitted by a viewed scene is focused through the specialized objective lens assembly of an infrared camera on to the camera's focal plane array (FPA). The FPA uses materials that respond by generating electrical impulses when infrared energy strikes it. These electrical impulses are then sent in the form of temperature values to an image signal processor that turns them into video data for presentation on a display.