CCTV:back to basics (part two) | Archive Titles

Table 1
General Overt Lighting	Considerations
Incandescent Rely on the passage of an electrical current through a metallic filament. Do not have starting circuits.Can be used with automatic detection.	General filament lighting Acceptable for internal applications. Uneconomic when run over long periods of time. Restricted output without the use of halogen – the gas used to protect and prolong the life of tungsten filament bulbs. Tungsten halogen A harsh instantaneous white light with good colour rendering. Uneconomic when run over long periods of time. Becoming less popular due to needs for energy management and reduction in light pollution.
Fluorescent Generated as light emissions as a result of gas discharges.Have starting circuits.	Good colour definition as a white light. Equivalent illumination uses up to 85 per cent less power than incandescent lighting.
High intensity discharge (HID) The light output is produced as a result of an electrical discharge. Such lamps consist of discharge tubes containing a gas or vapour. Intended for extended period lighting only, due to starting circuits. Not to be used for short term demand illumination. Restrike times following switch off periods must be taken into account as they require several minutes to allow cooling.	Low pressure sodium (SOX) A high efficiency monochromatic light with a yellow tinge used predominantly in urban street lighting schemes. Not capable of colour discrimination with CCTV cameras as most objects in its presence appear as shades of grey. High pressure sodium (SON) The dominant light source for lighting over long periods. Has an efficient golden white light source. Capable of respectable colour rendering. Produces acceptable images with CCTV cameras but inferior to white light. Metal halide (HPI) A cool clear white light with excellent colour discrimination. A good range of options available so can be used in those applications needing efficiency and good colour rendering. High pressure mercury vapour An alternative to metal halide. A cool white light with adequate colour rendering.

Table 2
Infra-red (IR)	Considerations
Match the camera field of view Design the illumination for the camera and not for the human eye.	Reference the camera lens to the beam pattern of the IR light. The lens and field of view of the camera are matched to the lens and area of illumination of the lamp by reference to the manufacturers 'Lens Selector Charts'. The manufacturer will produce a range of beam patterns within the product range. Combination systems use twin infra- red illuminators and a single camera with a zoom facility with the illuminators matched to the two different optimum illumination distances, i.e. the overview and the zoom facility.
Determine the range. Governed by sensitivity and spectral response.	The sensitivity – low light performance and spectral response – ability of camera to 'see' IR wavelengths govern the range. Guidance is given in the manufacturers' data.
Select the filter Important for semi-covert or covert applications.	730 nm – slight red glow. 830 nm – dull red glow. Use IR enhanced camera. 950 nm – no visible glow. Use IR enhanced camera.
Specification/maintenance Accessibility of illuminator for maintenance and bulb changes.	Compare life expectancy of bulb life for mains and low voltage illuminators. Life expectancy figures are included in the manufacturers' data. Solid state LED array illuminators have low maintenance costs.
Control	Telemetry or photocell. User selectable power levels on power supplies enable selection of an efficient IR output on scene.

Table 3
Cabling and free space transmission systems	Considerations
Cables Coaxial An unbalanced signal cable where the signal level is a voltage reference to ground (earth). The video signal from a camera is between 0.3 V (black) and 1V (white) above zero (ground). The shield is the ground level. The centre copper conductor must always be well screened by the outer copper braiding.	The principal cable for carrying the video signal. Attenuation is a problem over long runs. RG59 (solid core) and URM70 (braided core) are of 75 Ohm impedance and predominant in UK. Over a cable run of 200 metres a loss of 50 per cent of signal strength may be experienced so standard coaxial cable cannot be used beyond a theoretical 267 metres. Special amplifiers required over long runs. Earth loops may be formed in coaxial cables and need to be removed using an isolation transformer.
Twisted pair A balanced signal cable comprising of a pair of copper wires twisted along their length and made in several impedances. The signal voltage is the difference between the voltage in each conductor. With twisted pair the video signal is travelling in opposite directions in the two conductors. Interference may be balanced by means of a differential amplifier.	Used extensively in large cable networks where there would be an unacceptable loss of signal strength using conventional coaxial cable. Long cable runs are a key advantage typically 1.5 - 2 km. At the camera end a launch amplifier should be fitted and, at the monitor end, an equaliser unit installed. The equaliser balances out the signals.
Optical fibre Relies on the passage of a light signal through a fibre optic medium.	Immune to outside interference. Signals can travel long distances without a need for amplification. Used in transmission systems up to distances of 30 km. Unaffected by electrical interference.
Telephone: PSTN & ISDN Public switched telephone network (PSTN): Analogue signals. Integrated services digital network (ISDN): Digital transmission. Used for transmitting data and speech within the telephone system network. Tend to be used with Alarm Receiving Centre monitoring.	Use transmitters that convert the video signals to RS232 serial data at high speed. PSTN: A technique now referred to as a video transmission system. It uses transmitters linked via modems to a receiver. ISDN: A technique using the digital network with a speed three times that of the PSTN. We also see rapid moves in networking with links to internet protocol (IP) working with other transmission methods.
Free space transmission Use a transmitter and receiver. Infrared beams An infra-red light source is modulated by a video signal and the light focused to a receiver unit. Infra-red links are often used to bridge gaps in hard wired CCTV systems.	Used for distances up to 1km. A direct alignment path is needed between the transmitter and receiver. Affected by the weather.
Microwave Need to be licensed as it can cause interference with other transmission systems and radar.	Unaffected by the weather to the same extent as infra-red. Capable of distances up to 50 km.
UHF RF Use radio frequency as the transmission medium.	Wireless systems that may be used for local signals.