CCTV Technical guideWhat is a CCTV?
CCTV is a closed circuit television in which images are captured and are converted into electrical signals with the help of a sensor. It is much like a television system, but instead of broadcasting its signals to public, it transmits them to a number of specific monitors for surveillance. A cctv system may consist of a simple network of multiple cameras linked together by various switches or a complex network connected together by Internet or wide area computer technology.
A cctv system mainly uses CCD video cameras for capturing videos. CCTV is now considered the industry standard in terms of surveillance security cameras. It works best in small enclosed spaces like offices, school laboratories, shops and production lines in factories. A cctv camera can be both monochrome or color type.Role of sensors in cctv camera:
Sensor has a very important role to play in a cctv camera. In a CCTV, images captured are converted into electrical signals via image sensors called CCD or CMOS. CCD is a changed coupled device and is ideally used in CCTV cameras. These have the ability to produce high resolution images compared to CMOS.
However, CMOS are more economical than CCD. These sensors are actually light sensitive chips that split into individual pixels. These are light-sensitive chips, the sensitive area being split into a large number of individual pixels. An image focused through a lens gets into this chip area and as a result an electrical representation of an image gets formed. A typical CCD sensor has 250,000 to 500,000 pixels.Know about a CCD chip:
A CCD is made up of many pixels on a tiny chip of 1/2″, 1/3″, or ¼”. These pixels are arranged a precise grid, with vertical and horizontal transfer registers carrying the signals to the camera’s video processing circuitry. This transfer of signals occurs sixty times per second.
Among the three chip sizes, 1/3 inch CCD chip is the most widely used one. It is usually 5.5mm (diagonal), 4.4mm (horizontal) and 3.3mm (vertical) in its dimension. The 1/4″ sensor format is more used in color CCTV cameras. It is easy to calculate the focal length of a lens required in a CCD chip camera. For example, if you want to watch a subject of 5.5ft width at 10ft distance with a 1/3″ CCD camera, you will need an 8.0mm FL lens.Understand Optical speed and concept of illumination
Optical speed is all about how fast a lens collect light. The speed is determined by factors like focal length and diameter of the leans used. Optical speed is denoted by f-number. F-number is demoted by FL/D. Larger focal length means less collection of light whereas a lower f- number means more lights to a camera sensor. Illumination is an important aspect of security system. Light levels or illumination is measured in LUX i.e. Lumen's per square meter.Typical illumination is the following in a single day:
- Full daylight- 10,000 LUX
- Very dark day- 100 LUX
- Twilight-10 LUX
- Deep twilight- 1 LUX
- Full moon-0.1 LUX