Solutions

Considerations to be made

 

 
Do you need it?
 

When thinking about buying or selling a CCTV system there are some considerations which should be made first of all:

 

  • What is the value of the asset to protect?

  • What is the risk to be the target of wrongful removal?

  • How will the video surveillance system be used?

    • For Observation?

    • Forensic Review?

    • Identification?

 

Based upon the answers to these basic questions the work deciding the camera quality and the kind of system to choose can commence.

 

 

Camera resolution
 

Having done the above the next step will be to consider which picture quality is needed. At start it can be a good idea to have a look at the UK Home Office Guidelines for CCTV systems:

 

  • Detection of Intruders Into an Area

    • Persons Should be at Least 10% of Screen Height

  • Recognition of a Known Individual

    • Persons Should be 50% of Screen Height

  • Identify an Unknown Person

    • Present an Image at 120% (knees to head)

 

The standard height for a person that these standards are based on is 1.6m (5’ 4”). Furthermore, these guidelines are based upon VGA format (640x480 pixels) and do not take into consideration the resolution of the camera. Hence, one can interpolate the UK guidelines when using cameras of higher resolution

 

Example:

For recognition of a known person the person should cover 50% of screen height for a 640 x 480 camera. This imply that the target occupies 240 pixels height of the screen height. Assuming a person is 6 foot high there will be a need for 240/6=40 pixels per foot.

Now, comparing the VGA camera with a 1920 x 1080 (HD) camera the person only need to cover 23% of the picture in order to get the same degree of recognition.

 

However, these guidelines were made while the CCTV systems had a high cost and the recorders normally could handle up to 16 channels only. Today one can get NVRs handling more than 100 cameras at VGA resolution, or systems handling 32 channles with 5Mpxl resolution at 30 fps each.

 

Enough light?

 

Infrared illumination is used to provide light over scenes that would otherwise be too dark for a camera to create an image. It is a compromise because the best results can only be obtained by providing sufficient white light but of course this is not always possible. In many cases using powerful floodlights would cause a considerable nuisance and could be dangerous where there is road traffic moving towards the lights. It is also difficult to cover a large area when a pan tilt camera is being used, in this case the illumination is only required where the camera is directed and infrared lights provide the answer.

 

Focusing with infrared light. The distance to an object in focus will be different under infrared light compared to natural light. This is due to the different angle of refraction and the fact that a lens compensates for refraction mainly in the visible part of the spectrum. Therefore focusing of cameras illuminated by infrared light requires that they must be set up and back focused at night under the infrared light. The focusing is also more critical under infrared light because the aperture will be at its maximum with resulting decrease in depth of field. This will generally be compensated for in daylight with smaller apertures. Another factor is that the angle of the lamp must be accurately adjusted to the angle of the camera, just a couple of degrees difference can lose a large amount of the available light energy.

There are lenses available that will focus visible light and infrared light at the same focal point, however the price penalty (or benefit!) is a factor of about four times the cost.

 

System integration

 

Is there a need integrating the video system with other systems, i.e. access control, point of sales (POS), license plate recognition etc?