The heart of the modern CCTV camera is the Charge Coupled Device (CCD) sensor. A CCD consists of a flat array of tiny, light-sensitive photodiodes that convert light into an electrical signal. Each diode produces a voltage that's directly proportional to the amount of light falling on it. No light would produce zero voltage, and therefore, a black level. Maximum light would produce a maximum voltage (a white level). In between these extremes are shades of gray. In the case of a color camera, a chrominance signal is superimposed onto the luminance signal to carry the color information.
The camera processes this electrical signal and converts it to a video signal output, which is then recorded or displayed on a monitor. The range of light levels that a CCD can handle is somewhat limited, so the light range tht the CCD receives must be restricted within certain limits.