The ability to film in low light conditions or even complete darkness is essential for many CCTV applications. Part of the that ability is down to using the right electronics which are sensitive enough to work in low light, the other tool used is infra red or IR lighting. We have a guide to infra red in our camera buying guide.
Infra red light isn't visible to the human eye but CCTV cameras switch to black and white as light levels fall and can film the IR light spectrum. This means that to the camera its like shining a torch. IR has to bounce off something but providing there is a hard surface to reflect the infra red your camera is able to see in the dark.
Previously cameras had separate infra red light lamps bolted onto the side of their housings. It made the whole unit very large and industrial looking. In more recent times cameras have been designed with IR lighting built into them. This greatly reduces their overall size and visual impact. It also reduces cost when compared to modular cameras using separate lighting units.
Many people selling CCTV cameras describe them in terms of infra red range. Do not confuse this with the optical range of the camera or the distance at which a camera can record detail. That is determined by the lens. Wider angle lenses will have a shorter optical range, more telephoto lenses a longer range. The IR range is a largely immaterial figure, often made up and is supposed to represent the distance at which the camera's infra red can be detected. In reality the ability of a camera is a combination of the lens, how powerful the IR is and how sensitive the electronics are.
Something you might see mentioned is an IR cut filter. This is a mecanical filter which moves in front of the camera lens during the daytime. The filter is designed to improve colour rendition during daylight filming. Electronic adjustments can also me made to the camera to improve colour representation.