1851 SITO/City & Guilds masterclass [part 25] CCTV [paper 1] - Focusing on Cameras

Welcome to the first paper on CCTV. This series of articles, twelve in all, will provide an insight into an area of security often misunderstood. There are many different elements in today's closed circuit television systems. The camera and monitor, however, are probably the most significant. The camera views the scene and converts that scene to an electronic signal that can be transmitted down a cable. The monitor then converts that signal back into a coherent picture.

Modern CCTV technology is complex and can involve many different types of equipment. But the heart of every system is always the camera and monitor.

In these first articles we shall concentrate on the camera, the general principles of how it works and the different types. We shall look at the lens later, but it is, in many respects, part of the camera.

CAMERAS IN USE

CCTV cameras can be used in a variety of applications. Traditionally they have been used in security surveillance but the introduction of microchips into cameras has widened their scope.

They can also be found in industry, used in applications from monitoring processes and quality control to harsh and unsafe environments such as furnaces and nuclear reactors.

The introduction of microchip viewing devices has meant that cameras can be made extremely small and, therefore, they can be used in areas where space is limited or in covert applications where they can be hidden.

There are two main types of cameras in general use. These are:

• The tube camera
• The solid state camera

While microchip cameras have replaced tube cameras to a very great extent, tube cameras are still in general use.

Both types of cameras work on the same principle in that they convert light into an electronic signal. This is done by focusing the light from the scene through a lens on to an imaging device which converts the light to a signal. The camera can then process this signal (See Fig 1).

The imaging device will either be an electronic vacuum tube, "the tube camera", or a sophisticated integrated circuit "the microchip camera".

THE TUBE CAMERA

The tube used is a glass vacuum sealed tube (see Fig 2). Within it there is an electronic "gun" assembly which emits a beam of electrons. This beam is focused on the tube front plate, known as the target plate. The inside of the target plates is coated with a special material which will generate a small voltage, known as the target voltage, when light falls on it.

The voltage is in proportion to the amount of light falling on that part of the plate. This voltage forms a pattern of charge which builds up on the faceplate. This is converted to a signal when struck by the beam from the camera gun. The signal from the tube imaging plate is then fed into the camera video amplifier circuit via the target ring.The signal voltage increases with light striking the target plate. Therefore the voltage will be higher when the camera views a bright scene.

Because of the way in which the tube operates it is possible that the target plate can be damaged by either too much concentrated light for a period of time or by the target voltage being set too high. For these reasons it is important to ensure that the scene being viewed has, if possible, overall equal lighting without any high spots, that is, small concentrations of bright lights.

The sensitivity of the target face material to light will affect the camera's ability to convert the image into an electronic signal. There are two basic levels of sensitivity used in general CCTV applications.

These are referred to as standard and low light.

The material used for the standard camera tube or videcon, as it is known, enables the camera to work at normal light levels, that is, bright daylight and well-lit areas.

If the camera is to operate in lower light levels then a more sensitive low light camera will be required.

Fig 3 shows the operating ranges of these two cameras.

Scanning (see Fig 4)

As mentioned earlier, light is converted to a signal by the action of a beam fired from the tube's electron gun. This beam does not remain static but is pulled across the face plate of the tube in a process called scanning. This is how a picture is developed. The video signal corresponding to any point of the face plate is determined by the light falling on that part of the face plate.

The scanning process is achieved using a set of electromagnets which are placed around the tube neck. These are known as the deflection or scan coils. the deflection coils are synchronised so that the scanning process is precise.

The Video Signal

The signal output from the target plate is too small to be usable so it is amplified and processed through video circuits.

Towards the end of the processing, the video signal is merged with a synchronised pulse that controls the camera scanning process, the pulse tells the monitor where each line of scan starts and ends.

Synchronisation of the camera and monitor is vital if the picture is to be reproduced correctly.

The signal which is then sent to the monitor is called the composite video signal.

The composite signal is not just the signal from the camera. It also contains the synchronisation information and a ground reference for the video signal called the black level (see Fig 5).

This reference is slightly higher than true earth and is designed to hide the effect of unwanted signals appearing on the picture.

The composite video signal is a standard used throughout the industry and the total value of the signal is one volt peak to peak. This means the overall value of the system from the top to the bottom is one volt.

Each of the signal elements within the composite video signal has a specific value.

Camera Set-Up

While the tube camera is a complex unit, most of the general setting up is limited to four main controls. These are: Mechanical Focus, Electrical Focus, Beam and Target.

Mechanical Focus (see Fig 6)

The mechanical focus alters the focal length between the lens and the imaging device face plate. This is done by moving the lens mount assembly or moving the camera tube assembly. The latter method is used commonly on tube cameras.

Focal length will be discussed in more detail later on in the series.

Racking extends the lens focusing range by altering the distance between the lens and the imager face.

Electrical focus

There are two types of electrical focus, often referred to as beam focus. These are electrostatic focus and electromagnetic focus.

More expensive tube cameras will use both, while cheaper cameras may only use electrostatic.

The electrostatic focus sets the voltage level to the electrostatic electrodes which are inside the camera tube. This is a high voltage which controls the width of the electron beam.

If the beam was to spread out, unwanted areas of the face plate would be hit causing a blurring of the image. The electrostatic focus keeps the beam as narrow as possible.

The electromagnetic focus achieves the same effect of controlling the width of the beam but uses a magnetic coil. This is fitted inside the deflection coil assembly.

This type of focus could be likened to invisible rings keeping an arrow on a true course to ensure it hits the target, dead centre.

Beam

The beam adjustment sets the intensity of the electron beam. The adjustment should only be carried out by experienced personnel as adjusting the beam incorrectly may damage the tube.

Target

The target adjustment sets the charge voltage on the target plate of the tube. Incorrect setting of the target will result in tube damage and no attempt should be made to set the target voltage without the correct instruction.