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System Design Considerations:
Basic Design Criteria:
To minimise response times, cameras, monitors, video recorders, etc. should be continuously on, and the system should not generate more information than the operators can effectively manage.
Surveillance Zone Determination Criteria:
A CCTV surveillance installation is designed to monitor events of fundamental importance. These events might be hold up theft, sabotage/vandalism, hazard, evacuation, etc. Typical examples of monitoring applications are:
- a) perimeter surveillance
- b) access control
- c) safety
- d) property protection
Although one should consider all the possibilities in respect to the current installation further consideration should be given to the possible need for future system expansion.
Criteria for Determining the Number of Cameras & their Location:
Once zones and objects requiring surveillance have been identified the number of cameras required can be determined by the fields of view of the cameras equipment and system resolution and the nature of the zones to be surveyed. Picture detail should be consistent and adequate for the required level of service. The operational requirement of the system and its use, as well as site constraints and service access, may restrict the camera location and increase the possibility of further cameras being required.
The selection criteria should take into account the following:
the camera equipment should satisfy the operational requirement under all specified environmental conditions
(b) safety regulations relating to the site
Points to take into consideration:
white balance of colour cameras
electronic iris in relation to picture degrading for highlights
long exposure times in relation to movement blur
spectral sensitivity in relation to the type of illumination
back-up power supply
Selection of the correct lens type is as important as the camera selection. A poor lens performance can significantly detract from the overall performance of the system. When selecting the lens the following should be taken into account:
the lens field of view may be reduced by any overscan in the display device
the illumination of the sensor in the camera is defined by the aperture number of the lens and its transmission number,
which is dependent on the design of the lens
internal lens reflections and ghosting can significantly impair the image
some zoom lenses are subject to ramping whereby the effective aperture number of the lens increases as the focal
length is increased
accuracy of the preset shots if fitted
NOTE: Having selected the camera-lens combination, it is recommended that, for difficult scenes, a camera of the type selected should be evaluated in conditions similar to those to be encountered at the installation.
Recommended Object Sizes:
The size of an object (target) on the monitor screen should have a relation to the operator task, e.g. identification, recognition, detection or monitoring. If the target is a person and the CCTV system has an installed limiting resolution of better than 400 tv lines, the recommended minimum sizes of this target are:
for identification the target should represent not less than 120% of screen height
for recognition the target should represent not less than 50% of picture height
for detection of an intruder the target should represent not less than 10% of picture height
for crowd control (monitoring) the target should represent no less than 5% of picture height
Evaluation of Scene & Illumination:
The existing lighting should be evaluated for the level, direction and spectral content. Optimal light sources are those which have a spectrum that best matches the camera imaging device response. If additional lighting is required, the number, type, siting and power of the light sources should be determined.
Selection of the Video Transmission System:
There are many methods of video transmission and the suitability of a particular method for a given application should be carefully evaluated. Video transmission may be as simple as a single coaxial cable or as complex as a microwave link or a mix of different transmission means. Consideration should be given to prevailing and potential environmental conditions in those areas where the equipment and/or cables are to be installed.
Selection Criteria for Video Transmission:
Considerations for the selection of a transmission means or a combination of different means are:
bandwidth of the transmission path
signal to noise ratio
distance to be covered
security of communication
constraints in physical installation
NOTE: Signal to noise ratio and signal distortion are cumulative and can cause picture degradation.
Main Video Transmission Systems are:
Coaxial cable, twisted pair transmission, micro wave and radio frequency transmission, infra red and laser transmission and fibre optic transmission.
Control Centre Configuration:
The operational requirement and personnel available are the factors determining the number of control locations and equipment configuration of the control centre.
Number of Monitors & their Screen Size:
The number of monitors should be sufficient to display the maximum number of simultaneous alarms as stated in the operational requirement.
The monitor screen size should be selected in relation to the viewing distance. The recommended viewing distance is approximately 5 times the displayed picture diagonal.