1851 SITO/City & Guilds masterclass [part 35] CCTV [paper 11] - Location, lighting and lenses

When choosing which camera is best suited to which application, there are a number of factors to consider. They are: size (covert/overt); monochrome/colour; sensitivity/lighting/imager size; voltage supply; tube or CCD; and environmental operating conditions.

Operational conditions

If size is a consideration, in covert applications for example, it may be necessary to use a small CCD camera. Life expectancy, reliability and environmental operating conditions will determine the type of camera required. Supply voltages will also need to be considered.

Lens requirements

Any camera is only as good as the lens fitted to it. There are three main types: fixed, auto-iris and motorised. If a telephoto (zoom) facility is required, then a motorised lens will be necessary. The size of zoom lens will be determined by the viewing requirements.

Having established the type of lens required, the viewing angle and focal length must be determined. There are three basic methods: manual calculation, lens viewfinders and lens calculator.

Deciding the specification for a zoom lens is a little more complex as there are two calculations to consider: the focal length and the magnification factor. The focal length is calculated in the same way as for standard lenses, which will reflect the angle of view when the lens is zoomed out. The magnification factor is obtained by calculating the focal length for the required scene when 'zoomed in' and dividing the first figure by the second.

For example, a camera is required to view a scene that is 10m wide at a distance of 25m. The camera has a 1/2in imaging device, resulting in a 12mm focal length. When zoomed in, the camera is expected to view a doorway and identify anyone passing through it. This would require a scene window 2m high at the same distance, a focal length of 50mm.

If we divide 12 by 50 the answer is 4.16, so we require a zoom lens with a magnification of 4 to 1 (expressed as 4:1). The nearest readily available standard lens size is 5:1 and this would normally be used (fig 1, previous page).

Lighting

The lighting conditions that the camera will be operating under will influence the type used. Take into account the lighting during the whole operating time, not just at the time of the survey. Select a camera with suitable sensitivity. If unsure, use a standard light meter to measure the site illumination. If the camera is operating under artificial lighting, this must also be considered.

While all lighting provides illumination, it does not always do so at the same frequency – different types of lighting have differing frequency ranges and this is an important consideration with some types of camera.

Colour cameras are designed to work best across the full white light range. Lighting for these can be provided by fluorescent lamps, standard white light bulbs and daylight – all provide acceptable results. If a sodium vapour lamp was introduced it would cause an imbalance in the illumination because it does not cover the blue end of the spectrum. This would affect the colour reproduction.

If the existing lighting is not sufficient you may need to consider additional lighting – the type is important and will depend upon each situation.

Tungsten halogen floodlights provide a good quality, high-profile illumination across the spectrum, but may cause annoyance to the neighbourhood. Infrared is more discreet, covers a wide range of frequencies, and allows a subject to be illuminated without their knowledge.

However, IR light is highly reflective and details can be lost on light surfaces. It also has range limitations, so the correct wattage of lamp should be used for the distance involved.

As IR lamps are normally available with a range of lenses, it may be necessary to balance distance and illumination by mixing lenses. For example, two IR lamps could be used, one fitted with a wide angle flood lens to provide overall scene illumination, the other fitted with a narrow spot lens to give a longer, more accurate beam to penetrate dark areas. IR lights are available with a range of specifications, so it is important to match the camera to the light.

Ancillary equipment

If the camera is to be located externally or in an exposed environment it will need a housing. The important considerations are:

For particularly harsh or dangerous environments a specialist housing may be required. High temperatures may require a cooled housing, while explosive and hazardous environments may require an explosion-proof or flameproof housing.

Camera mobility

The next consideration is whether the camera is to be static or movable. While a moving camera offers greater flexibility, it is better to use static cameras where possible as they are cheaper and more reliable.

Moving cameras require controlling and if the system is not manned there may be little benefit from the motorised unit. Where a situation does call for a motorised unit, its selection must be carefully considered and how it will be controlled, either by telemetry or hard wire.

Fixings

If the camera is to be secured to a wall, a bracket capable of supporting the camera unit's total weight will be required. The total weight includes the camera, lens, and any housing, lights and pan units that may be fitted. If the camera is external and mobile, allowances should also be made for wind, snow, ice and cable drag. The fixing must be able to carry the same weight plus that of the bracket without risk or damage to the host wall.

Clearance between the wall and camera plus clearance between the camera assembly and bracket must also be taken into consideration.

In some situations, special brackets will have to be built, or it may be necessary to use an extension post.

If there is no suitable wall, a pole or tower may need to be erected, capable of supporting the unit weight safely. Safe access to these for maintenance is important and should be catered for when specifying the post or tower. Speed of service response may also be affected, as there may be scaffolding or hydraulic equipment to be hired, at extra cost.