To ensure 24-hour surveillance of objects in places where the conventional (visible) lighting is insufficient, infra-red illuminators are being used. Infra-red illuminators emit light within the range of wavelength of 700-1000 nm. The bigger wavelength, the light is less visible to the human eye. The use of this type of lighting in video surveillance ensures a discreet surveillance of objects.
Light is the electromagnetic radiation. Human eye receives radiation (commonly known as visible light) within the range of 380-780 nm. As opposed to human eye, industrial camcorders can observe the wavelengths to approx. 970 nm. In order to see the image, the light must be reflected by the observed object. After the reflectance, light returns with information about the shape of the object observed.
Industrial camcorders that can see in infra-red switch to black and white mode, because infra-red light does not provide information about the colour.
Fig. 1 An example of camcorder with built-in infra-red illuminator
Fig. 2 External infra-red illuminator
The range of used reflector is affected by the types of illuminated surfaces and objects, as well as ambient environmental conditions, e.g. air clarity. These factors can be described as scene reflectance coefficient.. Each material has its own light reflectance coefficient, which defines the ratio of reflected wave intensity to the incident wave intensity. It allows determining the intensity with which the material reflects the light.
Lighting of object is also affected by the distance from the light source. Light intensity is inversely proportional to the square of distance between the light source and illuminated surface.
Fig. 3 Example of an image from camcorder with enabled IR illuminator.