Closed-circuit Television (CCTV)

Part of a forensic investigation can be to record the events that take place at a scene. If for example, a suspicious fire takes place at a factory, forensic investigators could examine tapes from surveillance cameras to see if anyone was on the property near the time of the fire. Thus, closed-circuit television (CCTV) can play an important role in forensic science .

CCTV involves the use of video cameras to produce images for display on a limited number of screens connected directly to a non-broadcast transmission system. Commercial cable TV is, technically, an example of CCTV, but the term "closed-circuit TV" is generally reserved for systems serving a small number of screens that are monitored for security purposes.

CCTV is a ubiquitous feature of institutional security systems. It is employed by prisons, banks, urban police forces, airports, military organizations, utilities, large corporations, various other organizations, and wealthy individuals. Examples include:

• X ray baggage-inspection devices at airports.
• Remote viewing of dangerous industrial processes, rocket liftoffs, and other operations.
• Perimeter security around power plants, military installations, warehouses, police stations, and other defended facilities.
• Intrusion or theft monitoring of secure spaces, whether indoors (halls, lobbies, specific doors and rooms, etc.) or outdoors (parking lots, automatic teller machines, loading docks, etc.).
• Monitoring of vehicular traffic for traffic-control purposes or detection of illegal activity.
• Identity-checking of persons desiring entry into a building.
• Computerized recognition of individual faces, with possible identification of "wanted" persons.

Prior to CCTV, in order to secure the perimeter of an area, it was necessary to post guards in such a way that their collective line of sight covered the entire circumference of the area. With CCTV, it is possible to reduce the number of personnel needed to secure a perimeter by placing TV cameras at strategic points and transmitting the resulting images to a control room where a few guards can monitor many screens. Ideally, these observers will note any suspicious event on their screens and alert a response team. CCTV has, thus, for decades been a component of the typical perimeter intrusion detection system (PIDS), which combines CCTV with devices designed to detect intrusion by other means, including ultrasonic motion detectors and window alarm-contacts.

CCTV technology, however, has not proved as effective in PIDS applications as was once hoped. As vigilance studies by psychologists confirm, guards who spend hours "screen gazing" at static scenes (>20 minutes, in tests) tend to become bored and less efficient, and are then likely to miss low-frequency events, such as a figure running up to and climbing over a fence.

Starting in the 1980s, designers sought to combat the bored-guard effect by using automatic video motion detectors (VMDs). These devices are designed to automatically detect scene action by comparing successive image-frames for changes. When change is detected that exceeds a predetermined threshold, an alarm is sounded. A guard then judges whether the alarm is false or valid.

VMDs, however, have not turned out to be a security panacea. There are too many sources of image change, especially in outdoor scenes, for a simple circuit to distinguish meaningful intrusions from nuisance alarms. VMD use is therefore restricted to artificially-lighted indoor spaces or to expensive systems that employ computer processing to reduce the false-alarm rate.

see also Aviation security screeners, United States; Cameras; Fire investigation.