Colour cameras are preferable from an evidential point of view, if the lighting is good then colour cameras should be your No.1 choice. However mono cameras work down to much lower light levels than colour cameras and are normally slightly less expensive. to obtain the best of both worlds mono/colour auto switching cameras are an alternative although colour correctness is sometimes sacrificed by certain makes of this type of camera.
Camera Location
External cameras should be 1/2 inch format as the larger CCD chip diameter is preferable when dealing with low light conditions, and auto iris lens should always be fitted to external cameras.
Internal cameras can be 1/3 inch format and may use fixed iris lens unless facing a variable light source such as an external glass door or window then an auto iris lens should be used.
Camera Mounting Type
Fixed or static cameras as they are known are less expensive than fully functional pan tilt and zoom cameras by a ratio of approx. 5 to 1 i.e. 5 static cameras equal the cost of 1 pan tilt zoom camera PTZ camera. There is little point in specifying PTZ cameras unless an operator is to be employed or unless sensors wired to the PTZ's presets are installed to automatically pan the camera to a detection point. Sometimes to obtain the best view it is often better to erect a tower on which to mount the camera and view a building from a distance as opposed to installing the camera on the building itself.
Objective of Camera
This is without doubt the most important part of any CCTV system design. You should always establish each Cameras objective prior to writing a specification. Do not fall into the common trap of assuming one camera can achieve all four objectives at once! Although pan tilt and zoom cameras (PTZ's) can meet all four objectives they can only do so under operator control and even then only one at a time. For simplicity a cameras field of view can be said to be determined by :
1. The focal length of its lens
2. The distance from the camera to the object being viewed
3. The format size of the camera
Each of the above factors require careful calculation for selection and installation of the correct equipment to achieve the desired objective. However you can place the onus of compliance on an installing Company by simply stating the objective of a camera at a given distance and make them do the calculations. These have been broken down into four categories by the UK Police Scientific Development Branch publication CCTV System Operational Requirements Manual 17/94 as being:
Monitor
Definition
An observer should be able to determine the number, direction and speed of movement of people whose presence is known to them; i.e. they do not have to be searched for.
What does this mean in practice?
When viewed on a CCTV monitor screen (of any size) the height of a person 1.6 metres tall should fill not less than 5% of the monitors screen vertically.
Detection
Definition
Following an alert an observer can after a search ascertain with a high degree of certainty whether or not a person is visible in the picture displayed.
What does this mean in practice?
When viewed on a CCTV monitor screen (of any size) the height of a person 1.6 metres tall should fill not less than 10% of the monitors screen vertically.
Recognition
Definition
An observer can say with a high degree of certainty whether or not an individual shown is the same as someone they have seen before.
What does this mean in practice?
When viewed on a CCTV monitor screen (of any size) the height of a person 1.6 metres tall should fill not less than 50% of the monitors screen vertically.
Identification
Definition
Picture quality and detail should be sufficient to enable the identity of a subject to be established beyond reasonable doubt.
What does this mean in practice?
When viewed on a CCTV monitor screen (of any size) the height of a person 1.6 metres tall should fill not less than 120% of the monitors screen vertically. In other words 1 metre from a subjects head to their lower body of a 1.6 metre tall person should fill at least 75% of the monitors screen vertically.
What is a Multiplexer?
A Multiplexer permits one time lapse VCR to record multiple camera images all at once. It achieves this by digitising each frame from each camera and recording them to tape in turn. i.e. Imagine 8 Cameras numbered one to eight If you could hold the VCR tape up to the light and view it as you could a piece of movie film you would see the pictures from each camera recorded as 1-2-3-4-5-6-7-8-1-2-3-4-5-6-7-8-1-2-3-4-5-6-7-8- and so on.
In order to replay the images stored from just one camera you have to play back the VCR tape via the multiplexer so it will display just the camera you wish to view.
Simplex Multiplexer
A Simplex Multiplexer records full screen size images from multiple cameras at once using the above method, but can only display one full screen live image whilst recording. It can however sequence through the cameras if required and can display multiscreen images on playback only.
Duplex Multiplexer
A Duplex Multiplexer is in effect two simplex multiplexers in one case. Hence it is more exspensive than a simplex unit. This is because it has the unique ability to record full image screen pictures from multiple cameras at once. Whilst at the same time displaying multiple live camera images on a split screen while still recording all cameras at thier full screen size and resolution.
