Get a more detailed look into the world of CCTV.

Where to use CCTV systems
CCTV (Closed Circuit TeleVision) is often used in connection with an existing security system or in process technical systems for fast verification of alarms / events. Such a fast verification may save expensive capital goods such as buildings, machinery and development projects and may also improve the product quality. Equally CCTV can be used as documentation when it comes to company quality politics.

CCTV systems are an important part of documentation of incidents such as rubbery, sabotage etc.

Today you will find CCTV systems within for instance

• Industry and industrial processor
• Power plants
• Prisons
• Harbours
• Airports
• Hospitals
• Parking lots
• Banks
• Stores and shopping centres
• Military areas
• Traffic surveillance

The use of CCTV constantly develops due to new technologies which for instance make the cameras smaller, less expensive and more suitable for integration with other systems.

Choice of camera
In recent years the variation of camera has grown considerably and it may be difficult to find out which type of camera to use in a specific application

To achieve optimum result there are three things to consider. These are: how does the camera reproduce details, how are the light conditions where the camera is mounted and where is the camera placed

Resolution
The camera’s resolution is stated in TV-lines and informs of the camera’s ability to reproduce details, in general you could say that there are two type of cameras:

Cameras with standard resolution

• Typically appr. 380 TVL in black/white quality and
• Typically appr. 330 TVL in colour quality

Cameras with high resolution

• Typically appr. 600 TVL in black/white quality and
• Typically appr. 540 TVL in colour quality

A camera with standard resolution would typically be used with systems for general surveillance where identification or recognition is not a must.

A camera with high resolution and lens with a focal length adapted to the actual situation would typically be used where recognition of a person, reading of a number plate etc. is essential..

Light conditions
When choosing a camera you have to consider the light conditions in the area where the camera is to be mounted.

Below list may be useful when you wish to find out how the light conditions are:

• Summer day appr. 50.000 lux
• Cloudy day appr. 5.000 lux
• Well lighted office appr. 500 lux
• Min. light to be able to read appr. 300 lux
• Parking building appr. 100 lux
• Street light appr. 15 lux
• Dusk appr. 1 lux
• Full moon appr. 0,3 lux
• Starry night appr. 0,001 lux

If light conditions are fine, meaning much light (min. 150 lux scene light) it is possible to use colour camera, if not, you should choose black/white cameras.

If light conditions are not sufficient it is necessary to add mere light. When using colour cameras you need to add white light. It is not possible to add infra red light in connection with colour cameras because a colour camera isn’t sensitive to infra red light.

Black/white cameras may be used both with white and infra red light. It should be noticed though that you have to use a lens which is colour corrected towards IR light if you use infra red light. Otherwise you may risk that the camera cannot focus correctly when the IR-light is lit.

If you mount a camera where the light conditions are poor, the camera will start to intensify the video signal to achieve the necessary level. Unfortunately both video information as well as electrical noise will then be intensified and it may be impossible to identify a person or read a number plate once the light is gone simply because there is too much noise in the picture

Where to place the camera
To achieve optimum result it is necessary to investigate where you can place the camera.

Avoid to place the camera as described below:

• The camera points towards a window with daylight
• The sun shines directly into the camera (for instance especially when the sun rises)
• Close to street light or any other kind of light (attracts insects)

Lenses
An area which may cause a great deal of concern is which lens to choose for the CCTV system.

If you choose a wrong lens the system may not live up to the customer’s expectations.

The following should be considered:

Lens mounting:
The lens you choose has to be compatible with the chosen camera. As a standard two mounting forms are used today:

• C-Mount
• CS-Mount

Most new cameras are now supplied with the possibility to choose either C or CS mounting. For an older camera the lens mount would probably be C-Mount.

For a C-Mount camera you can use C-Mount lenses and CS-Mount lenses together with a C/CS Mount adapter.

For a CS-Mount camera it is only possible to use lenses with CS-mount.

Manual, auto-iris or direct drive?
In general it can only be recommended to use lenses with manual iris in systems with constant light conditions

If the camera is placed where the light conditions varies you should use a lens with auto-iris. Today there are two types of lenses with auto-iris:

• Auto-iris
• Direct Drive

The difference of these two types is that an auto-iris lens measures the video signal’s size and adjusts the iris until the pre-adjusted video level has been reached. Due to internal automatic adjustments in the camera you may risk that it takes some time for the lens to find the correct iris position. In worst case the lens may not find a correct iris position and the iris continues to open and shut (hunting phenomena). The Direct Drive lens is controlled directly from the camera and this kind of control offers optimum iris control.

Since most cameras today offers the opportunity of Direct Drive lens control it is recommended to use this type of lenses. Further you have the bonus advantage that Direct Drive lenses are supplied mounted with a standard connector.

Calculation of focal length
In order to calculate the focal length of a lens you must know the following data:

• Size of the CCD sensor used by the camera, in millimetres.
• Distance to the object, in metres
• Height or length of the object, in metres

Today the following CCD sensors are available formats:

With this information it is possible to calculate the focal length using the following formula:

Where Sw is the sensor width, Od the distance to the object and Ow the width of the object.

Where Sh is the sensor heigth, Od the distance to the object and Oh the height of the object.

Example
Using a camera with a 1/3 sensor with a sensor width of 4,8 mm (se above) we should see an object with a width of 4 m, the distance to the object is 12 m. The result will be the following focal length (fl):

A lens with fixed focal length of 14,4 mm is not available.The closest standard lens is with focal length 12,0 mm.
There are lenses with variable focal lengths of 5 – 40 or 5 – 50 mm which could cover the object in question.

If you don’t want to use the more expensive solution (variable focal length lens) you will have to move the camera closer to the object. In this case the camera should be mounted appr. 9 m from the object.