CCTV: Back to basics: Part 1

The progression of CCTV in monitoring and control applications appears to be non-stop. In fact, it looks like we have reached a stage at which the role and growth of closed circuit television technology using specialist cameras, custom-designed equipment and communications links seems only to be limited by the imagination of the end user.

As a huge range of products has become available to the installer, with diverse specifications and capabilities, the time has surely come for us to step back and assess the fundamentals. With this in mind, the ideal starting point is for us to detail the foundations of camera and lens technologies before looking at the main selection criteria.

Cameras and lenses

Modern day cameras, in the main, use a solid state charge coupled device (CCD) which stores an electrical charge and operates in the same way as a photodiode. Many thousands of these are contained in the camera imaging chip which is used to create the picture elements, or pixels. The greater the number of pixels, the higher the picture resolution or definition.

The lens is an optical device attached to the camera and used to gather light from the scene being viewed. This collected light is focused onto the camera CCD pick-up device and then converted into proportional electrical charges.

The voltages that are generated as a result of this are in proportion to the brightness of the image at a given point on a horizontal line. The 'white level' or brightest part produces a level of one volt, whilst the 'black level' corresponds to a voltage of approximately 0.3 volts, with all the shades of grey in between these values.

The colour video signal components are also captured as voltage levels and converted to a luminance signal. Every colour used in CCTV lies between black and white.

The video signals that contain all of this information are processed and transmitted from the camera to the monitoring equipment for conversion back into light energy. This is then formed into pictures of the scene which is being viewed. It should be noted that black and white (monochrome) cameras use the full bandwidth of the video signal and outperform equivalent colour cameras in terms of resolution and low light ability.

The selection of camera and lens combinations is based on a series of time honoured considerations.

In the first instance we will refer to the more general aspects of the camera and lens specifications.

We can then extend our observations to other essential criteria that relates to camera and lens parameters, before giving a few practical examples.

Camera specifications

Cameras are specified and available for internal or external duty, or they may be board cameras intended for mounting on the printed circuit board of another system, such as video entry. See Table 2 (overpage) for advice on camera choice.

Internal cameras do not need environmental protection, but may need some protection against vandalism. If the light levels are stable and the scene of view can be easily established, then a manual iris is satisfactory.

External cameras need a level of protection from a weatherproof housing, plus electronic sensitivity control to cater for changes in the light level.

It is difficult to compare the performance of competing cameras because of the wide specifications available, and quoting 'high resolution' may have little relevance.

In general, monochrome cameras are 380 lines for low end and 570 lines for high end. Colour cameras are 330 lines and 470 lines respectively.

At the basic level it is necessary to confirm that the video output is 1 volt peak to peak, and if the lighting and spectral response is matched to establish how the camera will perform under the particular site's light levels.

A signal to ratio should be quoted, as for video signals a value less than 40dB will form 'grain' in the pictures. The signal to noise ratio also has an effect on the camera sensitivity, as the better the ratio is at a given light level, then the lower the minimum light level at which the picture is usable.

Lens specifications

The lens has an enormous influence on the performance of any CCTV system. The installer must have an understanding of formats and F-numbers, together with focal length, angle and field of view, as well as depth of field. See Table 3 (overpage) for advice on choosing a lens.

The format describes the size of the active area of the pick-up device in relation to the lens and camera, typically as ½, 1/3, 2/3 or ¼ inch. The lens and camera formats must be correctly matched. If the lens format is smaller than the camera format, the image will not fill the display. Therefore, a lens designed for a larger format camera may be used on a smaller camera, but a lens designed for a smaller format camera may not be used on a larger camera.

F-numbers denote the lens aperture size - for example, F1.2. This not only indicates the size of lens aperture, but also the brightness of the image formed by the lens. A smaller F-number means a brighter image and a larger aperture. Therefore, F-numbers increase as the aperture size decreases. If choosing a lens for an application where low light is an issue, clearly an F1.2 lens would be a better choice than an F1.4 device.

The focal length value determines the field of view at a given distance and needs careful selection to ensure that the correct area is in view and the detail of the scene is acceptable.

The angle of view varies according to the sensor size, and is the angle that the two lines from the second principal point make with the image sensor edges. The field of view is the area viewed through a lens.

Depth of field is the distance in sharp focus immediately in front of and behind the target.

From this, we can provide general examples and illustrate typical site coverage. See Table 1.

It is clear that for the larger and more complex CCTV system that the competing camera/lens combinations from different manufacturers may need to be type tested in actual site conditions over varying light levels to gauge their true performance.

Hopefully this will give installers wishing to enter this sector a basic understanding of cameras and lenses and provide a starting point.