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the dark greys in the picture to distinguish them from black level. It is possible to increase the depth of field by increasing the level of illumination, thereby allowing the aperture to be reduced. Security cameras are often mounted in a dome, with remote control of pan, tilt and zoom. The camera can then pan and tilt over a wide angle without anyone in the scene being able to see it inside the dome. Outdoor CCTV cameras are often mounted in a sophisticated weatherproof housing with a sunshield, heater, windscreen wiper, as well as pan, tilt and zoom options. Try to aim CCTV cameras below the horizon. They should not be located where they look directly into the sun. Several video pictures can be displayed concurrently, either on a single monitor as shown on the “quad” display alongside, or using separate adjacent flat panel displays mounted on a suitable frame. Digital video CCTV signals can be recorded using a Digital Video Recorder (DVR) and discs with a capacity of 1 terrabyte are now commonplace. The signals are compressed using MPEG techniques. The recordings can be initiated using movement detection where the scene is divided into cells which trigger the recording – it is sometimes possible to mask some areas of the picture where movement is allowed and others where movement initiates the recording. These techniques relieve an operator from the stress of watching CCTV pictures over a long period of time. “Time lapse” recording is a technique whereby the frequency at which pictures are captured is much lower than that used to view the sequence. When played at normal speed, time appears to be moving faster and thus lapsing. 45 CAI Feedback Spring 2018