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The Building Blocks of CCTV
From veri-focal to megapixel, lens selection is essential to a properly functioning surveillance system
There is a great deal of excitement in the video sector of the security market. Cameras are becoming more sophisticated. IP cameras are playing a key role in many security installations — they provide megapixel-quality pictures and digital processing capability. Intelligent video (IV) systems are enabling security system operators to provide better response without having to watch a wall of monitors.
With all the advances in technology and capability in today’s video systems we can, at times, overlook the more elementary components that enable today’s systems to provide the results that we have come to expect. The basic components of video systems, such as lenses, mounts, environmental enclosures and pan-tilt-zoom (PTZ) hardware, affect the overall quality, performance and integrity of the system. For this article, lenses and their selection is the focus. A future article will address lens adjustment, back focus, depth-of-field, different types and operations of auto-iris lenses, lens mounts and automatic light control.
This article should be used as a guide to selecting the best lens for a given application. I will differentiate how lenses affect the camera’s quality of video based on the lighting, field-of-view and video detail.
The Right Lens for the Job
Without the proper camera lens, the video used by an IV system, for example, will not provide the proper field-of-view and detail to enable the software to properly process video information and notify the control center operator when a potential problem exists. It is important to make a very basic decision early on in the lens-selection process — whether or not the CCTV system will be used for either identification or detection. This seems at first to be an unimportant distinction, but in fact, it will impact the selection of many of the components that make up the CCTV video function, including the lenses.
If the desire is to identify someone, the field-of-view and image quality must be robust enough to actually provide video that can be used to pick out a specific person. On the other hand, if the video is used to detect that a person is present, the resolution and associated field-of-view should be such that movement is detected, but the individual that is moving through the scene does not have to be identifiable. To identify a person or vehicle requires the object to have enough detail to allow identification. This means that the object must be detailed, large enough on the monitor and viewable long enough within the field-of-view to identify them.
Lighting Issues
Lighting is very important in selecting a camera lens. There are three critical issues with lighting: the variations of light; the amount of light; and the color characteristics of light.
The problem of light variation is exacerbated when viewing exterior scenes. Exterior applications will often range from bright sunlight to starlight. The ambient light level will be about 10,000 foot candles (FC) during the day and about 1FC during a moonless/starless night. To properly handle such extremes, a lens with a variable iris is often used. The variable iris is normally automatic and adjusts to the changes in light level by sensing the electronic circuitry in the camera. A small circular area in the center of the lens is often covered with a filter that limits the amount of light that can pass through the lens at the maximum FC level during the day. This spot filter is called a neutral density filter.
