Just like your "dusk to dawn" porch light, night vision capable cameras use a photo cell to detect low light conditions.
Without getting into too much detail we'll cover the basic concepts of what happens
next. First, the camera is switched to black and white mode because transmitting color requires a stronger signal.
Besides, at night there is little color information available anyway. Also, the iris is opened up to let in the maximum amount
of light onto the CCD image pickup sensor. Normally, this sensor transmits 30 complete frames of video each second
at a set voltage (signal) range. With low levels of light, the sensor can not produce an acceptably strong signal and
still maintain a 30 frames per second output rate. Therefore, special circuitry tells it to spend more time charging up each frame
in order to achieve a suitably strong
signal. This circuitry is called automatic gain control (AGC) and results in a stronger night time signal at the
cost of a slower frame rate. This is why live video will get choppy when light in the scene is reduced. Finally,
infrared light is added to the scene.
Many cameras used for low light viewing have small LED lights placed around the
lens. Although infrared light is invisible to the human eye, it can play havoc with the camera's
imaging system under full light conditions, requiring use of a what is called called an IR cut filter. But for night viewing to take place, the camera must be able to remove this filter in order to see the IR light that is produced by its LEDs emitters.
Once light levels return to normal, color mode returns and the cut filter is placed back into position.
Also note that sometimes dedicated illuminators (also photo cell activated) are installed to add infrared light.
One last important
thing to know is that even with AGC, camera video signal strength will be at its
lowest during the night. This is when the most demand is placed on the CCTV system. Cameras will reqiure more power,
cable and connector integrity will be tested along with the DVR's ability to construct and display video from a weaker signal.
Often an installation will work great during the day when there is lots of light, but have problems after the sun
goes down. This means that an installation is not complete until it has been tested at night.