In low-light conditions, some security cameras use Automatic Gain Control (AGC) to artificially improve their “dynamic range” and produce usable images. AGC is basically a from of amplification where the camera will automatically boost the image received so that objects can be seen more clearly.
In normal light conditions the camera will display a normal picture. However, when the light quality drops below a certain level the camera will begin to boost the signal to compensate for the lack of light.
The advantage of this technique is that your camera will produce images in much lower light conditions than standard.