A BASIC GUIDE TO CLOSED CIRCUIT TELEVISION
Camera / Lens
Monitor
Video Recording
Position
Summary
Closed Circuit Television (CCTV) has proved to be a very effective weapon against crime. There is however a great deal of confusion amongst users, and differences between installation companies as to the most effective CCTV solution to address a specification problem.
A governing factor is almost always the budgetary consideration. This is obviously a very important concern however if this is the primary deciding factor, it is probable that a system tailored exclusively to a budget will not adequately do the job required of it.
When considering CCTV it is important to decide on the objectives of the system. If you wish to record visually an incident such as a possible armed robbery, this would seem to be relatively simple, however if you watch the TV program Crimeline you will see what can go wrong. In many incidences the camera position is too high so that rather than getting a full facial view of the thieves we get the tops of heads and a bit of a nose or often there is a lot of bright light behind the perpetrator sometimes the subject may be in view for too short a time, or worse, the camera is positioned correctly, the picture on the monitor is excellent but the recording i9s a very poor because the least expensive part of the system, the video recording tape has been used too many times.
So what so we need to achieve a good recording of the incident. Colour is important in aiding identification; therefore we should go for a colour system.
A good quality camera is required. Cameras are rated by resolution which is indicated in terms of pixels (active picture elements) and line resolution. The higher the number of pixels and the higher the line resolution the greater the camera’s ability to resolve detail and the sharper the picture. In many incidences a relatively low-resolution camera (say 330tvl) will be sufficient to identify a person if we have a good head and shoulder shot (fills 1 / 2 to 3 / 4 of screen). To read number plates when using zoom lenses a higher resolution camera (say 450tvl plus) will be required.
A good quality lens is also needed. In an area where the light is fairly constant throughout the day or night (i.e. office with good artificial light and no windows) a standard lens may be used. A vari-focal lens, adjustable between 3.5 and 8mm is ideal, with the iris adjusted to the light the camera is focused on the target area and that’s it. If however the camera is being used in an area which is subject to changes of light it will be necessary to use an auto-iris lens. This type of lens will automatically adjust the iris in the lens so that when it becomes bright the iris will close and when it becomes darker the iris will open so that a more or less constant amount of light is allowed through the lens.
If the camera is covering a door through which sunlight shines ( avoid if at all possible) a camera with backlight compensation is necessary. The function of this feature id to suppress the high lights behind a subject coming through the door which will give a reasonable detail of the subject.
To view the picture captured by the camera a monitor is required. The monitor should have at least as high a resolution as the camera.
To store the picture for review a video recorder id needed. Standard VHS video recorders will have resolution of approximately 240 tvl in colour and S-VHS, 400 tvl in colour. Obviously it will be better to use S-VHS recording if possible whatever camera is being used, if however high resolution cameras are being used it is a waste to use anything but a S-VHS recorder. When using a time lapse recorder in say, 24 hour mode it is capturing one frame every 0.32 seconds while a standard recorder will record 50 fields or 25 frames per second.
Position the camera to get the best possible view of your target, if the target is a face ensure that it fills at least 1 / 3 of the screen.
Ensure the correct lens is used and that the monitor is at least the same resolution as the camera. Use the best recorder affordable (S-VHS if possible). It is important not to use videotapes more than 12 times. Keep 32 tapes and label them for each day of the month. The recorder should be kept in a secure area.
All the major manufacturers of CCTV products such as Panasonic, Sony, JVC, and Burle will normally be accurate in their specifications, and these will have been verified by independent bodies and can be relied upon.
There are many other products coming into the market from Taiwan and Korea and to be fair some of them are quite acceptable, however they may not be subject to the same type of quality control as the Majors and their quality is not always consistent. If in doubt stick to a name you know with guarantees you can rely on.
