3. CCTV Camera Information

What is a CCD?

CCD stands for Charge-Coupled Devices. These are light-sensitive chips. The image from a scene is focused through the lens onto the chip surface and a charge is built up in each pixel symmetrical to the depth of the light falling on it. An electrical depiction of the visual image is formed. Typical active pixel counts for CCD sensors is between 250,000 and 500,000 pixels. Both monochrome and colour variants of cameras are available.

Camera performance
A camera’s performance are dependent on its sensitivity and resolution. Sensitivity is a measure of the minimum amount of light required by the camera to give a ‘useable' image. Resolution defines the amount of picture detail in the image produced by the camera.

Sensitivity

A camera’s sensitivity performance is measured by various ways. Most important is the minimum scene illumination required to give a proper picture at a particular lens aperture e.g. 11.4. Another, more objective definition is the minimum scene illumination required by the camera to give full video output i.e. 1v peak to peak. Few manufacturers use this definition! Most manufactures provide a figure in LUX which is actually a light level (see chart below) the figures quoted are often like this 0.1 LUX.

Typical sensitivities (defined as minimum scene illumination required for useable picture with lens aperture at fI .4) for current CCD cameras are as follows:

• Entry Level (General purpose) Monochrome 0.5 - 0.25 lux
• Performance Level (Low-Light/Outdoor) Monochrome < 0.20 lux
• Entry Level (General purpose) Colour 3.0 lux
• Performance Level(Low-Light) Colour < 2.5 lux

Note that monochrome CCD cameras are still approx. 5 times more sensitive than equivalent colour cameras. Reference light levels are as follows:

Resolution
Resolution is measured in lines. Of more interest in measuring camera performance is horizontal resolution. This is the maximum number of black or white areas that can be identified across the picture. This is always specified per monitor height. A typical general purpose monochrome camera has a horizontal resolution of 380 lines. This means that it should be possible to resolve 380 x 1.33 i.e.. 505 individual actual lines of black, white or grey along any one line of a CCTV picture image. (The factor of 1.33 comes from the fact that a TV picture has an aspect ratio of 4 : 3)

Typical resolution performance for CCD cameras are as follows:

• Entry Level (General purpose) Monochrome 380 lines
• Performance Level 600 lines
• Entry Level (General purpose) Colour 380 lines
• Performance Colour 480 lines
• High Definition Colour 1080 lines.
  

Camera formats
Cameras will be quoted as 1/2", 1/3" or occasionally as 2/3". Newer cameras will be referred to as 1/4". This refers to the format of the camera and defines the size light-sensitive area of the imaging device used in the camera. It is provided for information to enable the installer to match the camera to the correct lens. Camera formats are getting smaller in order to reduce the costs of the imaging devices and hence the cameras themselves as well as the lenses put onto them.

Camera Lens Mounts
A CCTV lens will be specified as either C or CS mount. Both types of lens look very similar and there is nothing that can be physically measured on a lens. The difference between the two types is the position of the focused image behind the lens. CS lenses focus 12.5mm behind the lens whereas C lenses focus 17.5mm behind.

CCTV Lens Types & Uses
There are two main types of lens used in CCTV, these are fixed focal length types and zoom lenses i.e. with variable focal length (vari-focal).The focal length of a lens defines its effective viewing angles both horizontally and vertically. Hence the focal length of a lens determines the size of a particular image on the monitor screen or the area of the scene being covered by the camera.

CCTV Cable Information
Video signals used in CCTV systems must be transmitted over 75R ‘low-loss’ coaxial cable in order to give acceptable picture quality at the receiving end. The use of co-axial cable prevents interference to the quite small (1v) peak to peak) video signal from external sources and because the cable is low in capacitance then the high frequency elements of the signal are not attenuated to the same degree as would be the case with say a standard twisted-pair type transmission cable. Distance limitations do have to be set for different types of coaxial cable but these are subjective since different end-users will accept different levels of picture quality and hence levels of signal attenuation. ‘Acceptable’ pictures should be obtainable over the distances as shown for the following common types of cable used in the CCTV industry:

Coaxial cable is generally supplied either on 100, 200 or 500m drums. Always remember that a video signal is only 1volt in amplitude and is thus very vulnerable to external interference and losses in poor quality cables. Never use aerial down lead type coaxial cable for CCTV signals ! Always keep video signal cables as far away as possible from mains cables and wiring that may carry high-frequency signals.

CCTV Monitor Information
Whilst the camera + lens combination is pivotal to providing a decent quality image, it’s down to the humble Video Monitor to actually display a high quality picture.

In terms of monochrome monitors (Black and White), the technology which is based on the good old fashioned Cathode Ray Tube (CRT), has barely changed over the last twenty years. The components may have improved, and production methods become more streamlined, but the resulting picture quality is on average, pretty much the same. It’s only when we look at Colour Monitors, that there's now a noticeable shift starting to take place, towards the latest generation LCD flat panel displays. For much larger displays, particularly in custom built control rooms, Plasma Screens are now creating a bit of a stir.