Welcome to the second paper in the CCTV series. Having considered tube cameras, let's now look at CCD (charged coupled device) cameras. The tube camera has, in many applications, been superseded by the microchip camera. The major difference between them lies in the imaging device. This is not a vacuum tube but a silicon microchip device known as a charged coupled device or CCD (Fig 1). There are two types of CCD in use: Frame Transfer and Interline Transfer. These terms refer to the way in which the devices process the image, not affect the overall construction of the camera.
THE CCD CHIP
Both types are similar in construction, with the imaging area made up of a large number of individual photosensors, often referred to as pixels (short for Picture Elements). Each pixel will charge when light is focused upon it.
The amount of charge built up in each cell is in direct proportion to the light landing on it. Therefore, the brighter the scene at that point, the greater the charge will be. The number of pixels and their size will affect the overall resolution or detail of the picture.
Earlier CCD chips contain an average of 250,000 pixels but this has now increased with new generation devices.
In solid state devices the formats used are usually 1/2in, 2/3in and 1/4in. Unlike the tube camera, the size of the imaging area is not important. In CCD, the number of pixels, their structure, size and sensitivity are the important factors.
The CCD chip operation is similar to that described for the tube camera in that each cell is discharged in a set sequence electronically producing a type of scanning.
The discharge produces the signal which is then processed in the same way as the tube camera signal.
CAMERA ADJUSTMENTS
Unlike tube cameras, solid state cameras do not have beam, target and electronic focus controls. In fact, there is no external adjustment required other than mechanical focus (Fig 2). While some CCD cameras use a racking system, most use a focus ring which is located on the lens holder. As this is adjusted it moves the back of the lens towards and away from the imager plate.
Some manufacturers build a focal ring adjustment on to the lens itself.
COLOUR CCD
Colour cameras have now become popular due to improvements in their price, size and stability of operation, due to the development of the imaging chip.
The colour CCD is the same as the black-and-white CCD except that it has a special filter built into the front of the imaging face. This allows the colour from the scene to be broken down into three primary elements and converted into an electronic signal.
WHY USE CCD?
The major advantages of using CCD are:
Operational life The average life of a camera tube is two to three years depending on the operating conditions. The expected life of solid state imaging devices is significantly longer than this.
Reduced size and stronger construction The CCD device is much smaller than the bulky camera tube which means that camera sizes can be much smaller and neater. They also take up much less space.
As CCD cameras are electronically scanned there is no need for deflection coils, reducing size and weight. While camera tubes are reasonably strong in construction, they are susceptible to damage through vibration and knocks. CCD cameras are virtually impervious to these, making them suitable for use in harsh environments.
No image burn in Tube cameras can suffer from target damage known as image burn. This is where the image being viewed becomes permanently burned on to the target material. This is common in situations where the camera is constantly viewing one scene or if there is a severe contrast in lighting within the scene. Solid state devices do not suffer these problems.
Better light sensitivity The CCD camera has greater light sensitivity than the standard vidicon tube camera, giving it greater operational range (Fig 3). It is also sensitive to infra-red light. For normal use, a special filter, called a cut filter, is sometimes fitted to prevent the infra-red affecting the picture. While CCD has a greater sensitivity to light, it is unlike some tube cameras in that it is not possible to adjust the sensitivity. Because of this, CCD cameras should always be fitted with auto iris lenses which automatically adjust the light input level.
Reduced power consumption The CCD camera requires less power to operate than the tube camera. The camera tube generates voltages in excess of 400v while the CCD device operates on a range of 5 to 12v.
Accurate geometric image The CCD camera produces a much more accurate geometric image than the tube camera. While this may not be important for standard surveillance, it can affect specialist applications such as detailed measurement.
High stability colour The introduction of the CCD device has made the use of colour more practical as tube cameras can suffer from colour drift caused by age and conditions. In CCD, colour reproduction is kept stable by advanced circuitry providing high stability and minimal changes in colour hues.
POWER SUPPLIES
The power to both tube and CCD cameras may be supplied in one of three ways: Direct connection to the mains supply (Fig 4): A mains transformer and regulating circuit is fitted inside the camera to obtain the required voltages.
Low voltage supply (Fig 5): This method normally employs a separate power supply unit which converts the mains supply to a low voltage, regulated supply of either 12 or 24 volts DC or AC.
Integrated supply (Fig 6): These cameras receive a low voltage supply from a separate unit which transmits the supply through the coaxial signal cable.
Because CCD cameras draw much less current than tube cameras the power supplies can be much smaller and neater.
INTENSIFIED CAMERAS
In some situations the sensitivity range of the low light camera may not be sufficient to cope with the available light (Fig 7). For example, it may not be possible to illuminate an area. In this situation a camera fitted with an image intensifier may be required.
This may be either a tube or CCD camera fitted with a sensitive imaging device which magnifies the available light input by up to 85 times. The high sensitivity makes it susceptible to damage and the cost can be as much as ten times the price of a standard camera, limiting its use to specialist applications.
In our next session we will look at lenses.
Source
Security Installer
