CCTV:back to basics (part two)

Take care at installation stage

Lighting conditions can be a major issue for CCTV, says Allan Collinge, Director of RemGuard's Remote Video Response Centre.

Writing on detector-activated CCTV in a recent issue of Security Installer, he said "The BS 8418:2003 Code of Practice recommends that 'there should be sufficient lights on site to illuminate the camera's field of view.' The reality is that on a typical site at night there is going to be a considerable variation in light levels, with the frontage as the public face, tending to be illuminated most whilst in comparison areas at the back are prone to neglect, in the worst examples being pitch black ... Too much light in the wrong place can also be a bad thing, particularly if that light is shining directly at a camera blinding it, so care should be taken at the installation stage to ensure this does not happen."

With detector activated CCTV, "ideally the on-site transmission equipment should be able to drive a relay which can turn lights on for five minutes on command or when there is an alarm.

"The best transmission systems should also offer the potential for a manager to turn lights on remotely …

“The cost of illuminating an extensive site overnight can be prohibitive so it definitely makes sense in terms of the customer's bottom line that the ability to turn lights on and off is built-in from the very start."

In last month's edition of Security Installer we looked at CCTV cameras and lenses at foundation level. In this article we will stay at the same level and turn our attention to the basics of illumination used in CCTV systems together with the cabling and free space transmission techniques employed.

Illumination

Without sufficient light at the scene being viewed by the camera there can be no picture at the monitor. As the camera can only gather light energy that is reflected from the scene, the quality and quantity of light at the scene is of vital importance.

In broad terms, we provide illumination for CCTV purposes, for the periods when natural light is unavailable, by installing general overt lighting (visible radiation) or infra-red (IR) covert illumination. IR is of a wavelength that is not visible to the human eye but which the monochrome camera can 'see'.

First we will overview general lighting (See Table 1). The most popular light forms are listed along with the principal considerations. These are all available at different ampere ratings at 230 Vac and in a variety of fittings.

If high values of colour discrimination must be provided by the CCTV system this can only be achieved by using true white light, but the need for this must be balanced against the control of light pollution within the environment.

There is a need to provide even illumination over the scene – not to let the camera view the light source direct as this will cause bright patches or light spots – and not to create over exposure by using excess lighting levels. Remember also that the lighting used with CCTV cameras forms an essential part of the system budget.

Mono cameras

Infra-red (IR) lighting is designed specifically for use with black and white CCTV cameras (See Table 2 overpage). Traditionally it has used a tungsten halogen bulb mounted in a special housing and fitted with a filter to cut off the visible light. These may be mains powered or low voltage with a dedicated power supply. However new generation solid state high efficiency LED infra-red array illuminators are beginning to make inroads.

With all IR lighting the angle of illumination is matched to the camera lens combination. As an example, an 8 mm lens on a 1/3 inch CCD provides a horizontal field of view of 33 degrees, so to fully illuminate the scene a 30 degree lamp would be employed.

If a typical narrow spot or flood lamp device was used, the illumination would be too intense.

There are many aspects that must be considered in respect to using either traditional general overt lighting schemes or employing infra-red illuminators which are specifically matched to the scene.

In order to make a value judgement, each installation must be viewed as a separate entity with consideration given to the level of security required, the needs for covert illumination, budgetary constraints, ongoing costs and maintenance, energy management and the control of light pollution in specified areas.

Signal transmission.

The cables and protective devices for use with lighting systems are very much governed by the IEE Wiring Regulations used within the electrical contracting industry. The cables used as the connecting medium for the equipment and components carrying the video signals can be rather more difficult to define and specify. Table 3 (overpage) looks at cabling systems for the video signal and at free space transmission for those areas in which it can be difficult to physically install hardwired links. In practical terms coaxial cable is good for short distances whereas twisted pair is superior for relatively long distance transmission.

In every installation the joints in a cable produce a change in impedance and a potential loss in signal. If the joints are poorly made, standing waves are produced causing a "ghosting" effect. Free space transmission systems are more involved and confined to those areas where cabling is difficult.

• In the next issue we will stay at foundation level and cover recording equipment and ancillary devices.
• Gerard Honey is the leading writer of electronic security industry textbooks including the best selling Intruder Alarms, published by Newnes, an imprint of Elsevier Science