Strategic Planning Key for CCTV
By Hadi Rezvani, P.E., RCDD, Glumac International, Sacramento, Calif. -- Consulting-Specifying Engineer, 5/1/2004 1:00:00 AM
Have you ever watched fuzzy pictures or videos of unidentified criminals, recorded by surveillance cameras and shown on TV programs such as "America's Most Wanted"? Some of these pictures, even after extensive image enhancements, are extremely indistinctive.
One of the keys to an effective CCTV system is proper video collection, which depends on two factors: location and camera settings.
Location, location, location
There is no general rule for the ideal location and mounting height of a security camera; it all depends on the application. For example, in a convenience store with a 12-ft. ceiling, a camera mounted at the ceiling near the entrance may not capture the face of a robber wearing a baseball cap. A camera mounted on the wall behind the cashier at seven to eight ft. would do a much better job.
The most important portion of the camera's field of view is the target area, and this should be taken into account during design. For example, when a camera is viewing an entrance door to a building, the people walking through the door are the target of the monitored scene. It is important to work with the end user to understand what specific function is expected: a detection, assessment or positive identification. In most applications, a positive identification of a person or an object such as a car is required.
Normally, the following rules of thumb can be used: For positive identification viewing, the target area should be about 25% of the monitored scene. For other viewing purposes, such as detection or assessment, this number should be around 10%.
Camera components
When it comes to the camera itself, the two most important components are the charged couple device and the lens. The CCD is an electronic memory made of a special transistor. It holds a variable charge proportional to the variable shades of light. The output of the CCD is an analog signal that can be converted to digital for transmission or processing purposes.
Selecting the correct lens for each application is one of the most important steps of CCTV design. The lens format should be equal or greater than the imager size. For example a 1/2-in., 2/3-in., or 1-in. lens size can be used for a 1/2-in. camera.
To calculate the required focal length for the lens, factors such as scene area, target area and distance should be taken into account.
Besides the CCD and lens, light level is another camera consideration. The level of required light to capture a good picture is referred to as a camera's lux. The lower the lux, the better the camera can see in the dark.
A final decision is whether to go color or black and white. For obvious reasons, color cameras have an advantage over monochrome cameras in applications requiring positive identification. However, black and white cameras are more suitable for extremely low light levels.
