Cameras respond to light, however adequate light is not always possible. In those cases another source is needed to allow the camera to "see" in the dark. Infrared Illuminators are designed to light an area for the cameras without the need for artificial lights.
This also helps in areas where covert surveillance is required, because the light waves are not in the visible range and cannot be seen.
Note: The closer the nanometer rating to the 700nm barrier, the more visible the light may be. The lowest nanometer rating, 850nm, is just visible at the light source itself. The 940nm models are invisible to the naked eye.
IR illuminators are a little more expensive than regular lights, but require less maintenance.
