All light on the night

That said, it's a truism that most crime occurs at night – yet many system designs neglect the need for good lighting that will inevitably prove vital when (for example) the time comes to submit captured images in court. It may sound obvious, but in order for any CCTV system to achieve a workable picture there must be light. If there's no light, there'll be no picture to speak of.

The importance of achieving effective 24-hour CCTV coverage is also reflected in the Data Protection Act. The Code of Practice for CCTV users states that "when installing cameras, account must be taken of the lighting conditions in which those cameras are located". Consideration of the quality of image produced at night is no longer a desirable. It's essential.

Several options are available to the discerning end user when it comes to providing enough illumination such that their surveillance cameras will work effectively during the hours of darkness.

It's possible, for example, to provide sufficient ambient lighting, but this is all-too-often an expensive and difficult path to follow. In addition, it's worth remembering that ambient illumination on scene may appear adequate for use by pedestrians or motorists, but it will not be designed to work in harmony with the CCTV cameras. Indeed, it may actually reduce image quality (if, for example, the cameras are looking directly towards bright light sources).

A more practical approach involves the use of infrared technology, which may be located at the camera head. For CCTV purposes, infrared is the light that the human eye cannot see, but which the human eye can. Often referred to as 'black light', it lies between 700 nm and 1,000 nm (one micron).

In simple terms, an nm is a measurement of light, and visible light to the naked eye would lie between the blue (400 nm) and red (700 nm) parts of the colour spectrum. The eye cannot see efficiently beyond 730 nm, while CCD cameras are able to see up to 1,000 nm (thus enabling the use of semi-covert and covert infrared applications for CCTV designs).

Achieving 24-hour surveillance
When designing CCTV systems for effective 24-hour surveillance, there are several key elements that end users need to consider – the choice of camera(s), the use of infrared filters, lens type and the possibility of focal shift.

When monochrome CCD cameras first came onto the market, the CCD chips themselves were intended to match as closely as possible the response curve of the human eye. These remarkable chips actually did better than that, and were able to 'see' into the near infrared.

Today, the end user is faced with a large selection of monochrome cameras able to see this infrared light. However, choosing a suitable camera can be a minefield for both end user and installer. Why? Not all cameras are the same – some effectively see more of the infrared light than others.

The newer generation CCDs offer excellent low noise and high resolution operation, together with low smear characteristics and excellent infrared response. Otherwise known as Ex-View cameras, they provide the best performance in conjunction with infrared for high quality night-time surveillance.

Certain cameras offer integration as a method of improving night-time performance, multiplying the available light by several factors. However, the application of such technology may be limited to more fixed or static situations with limited movement on scene due to 'jerkiness' caused by the integration.

Several so-called 'dual mode' cameras (day-night, dual technology) have been launched over the last few years. These are intended to provide the best compromise for 24-hour surveillance – colour by day and monochrome-infrared sensitivity by night.

Again, there are different forms of dual-mode camera, some of them featuring built-in optical filters which are moved over the CCD sensor for daytime/colour operation and then removed during night-time/monochrome mode to maximise the low light sensitivity.

Other designs boast specialised filters. These do not move, but nonetheless offer good colour performance and infrared sensitivity.

Although the majority of colour cameras are claimed to offer impressive performance under low light conditions, they remain considerably less sensitive than monochrome cameras and (as a general rule) don't possess the capability to function with infrared. Additionally, many of today's surveillance schemes make use of dome cameras which may compromise the night-time performance of the system.

End users should be aware that both smoked and semi-smoked domes reduce the light available to the camera within. Indeed, the use of infrared with dome cameras will require a different approach – by providing general coverage of the scene with infrared, or specifically illuminating key targets with local infrared illumination.

At the end of the day, when choosing your cameras you should look out for: the signal-to-noise ratio (a good one will provide 'clean' images), sensitivity (with regard to low light performance) and the cameras' spectral response (in other words, their ability to see infrared wavelengths).

Determining visual appearance
The filter on each infrared lamp will determine both the visual performance of the lamp and that of the camera itself. Inevitably, there will be a trade-off between appearance and performance. Efficiencies will vary when using either a 730 nm, 830 nm or 950 nm filter.

Generally speaking, the 730 nm filter will glow in a similar way to a traffic light, while an 830 nm filter should offer a dull red glow that's only just visible to the human eye. For its part, the 950 nm will appear to be totally covert. No glow is visible at all.

The amount of so-called 'useable' light is reduced the longer the wavelength of light, which in turn will reduce achievable distance and picture quality. 730 nm filters offer the greatest levels of useable light, and thus a high level of visible deterrent. The 830 nm filters represent a good compromise for most end users, offering a discreet appearance (the trade-off being a slightly reduced level of performance). The totally covert 950 nm filters are useable over short range distances only, and must be deployed in conjunction with high performance, infrared-sensitive cameras.

Night-time performance of camera lenses is all-too-easy to overlook. Essentially, there is a compromise to be made here. At night, of course, you will want to maximise the light gathering capability of your lenses (ie have the smallest 'f' stop). This will reduce the depth of field of a picture, however, and may also cause problems with focusing. Obviously, this will be less of a problem with auto-iris lenses, whereby the lens automatically opens to its maximum aperture (ie the lowest 'f' stop) in low light operation. On a fixed lens, there may be a need for compromise between the low light operation itself and the depth of field focusing.

As you'd expect, daylight and infrared light have different focal lengths/points because the different wavelengths of light pass through the lens in dissimilar ways, and do not focus on the CCD at the same point. This may then cause what's known as a focus shift between daytime and infrared operation. The degree of focus shift may depend on a variety of factors, themselves dependent upon the quality of lenses used, the wavelength of the infrared filters – 830 and 950 nm filters offer a more exaggerated focus shift – and the infrared response of the camera.

More recently, certain manufacturers have developed a range of lenses with zero focus shift between daytime and infrared performance. This is a point of considerable interest for consultants and end users when it comes to devising CCTV solutions (in particular those designed for unmanned or non-telemetry controlled installations).

Considering the field of view
It's important to match the field of view of the camera/lens combination with the infrared light coverage provided by the infrared lamp. Mismatching in this area can lead to either poor image quality or a loss of infrared efficiency. If the infrared spread is narrower than the camera's field of view, this will result in part of the scene not being illuminated and the camera will not be able to 'see' anything in those areas not illuminated.

Conversely, if the infrared spread is much wider than the camera's/lens' field of view, this will result in a loss of efficiency and potential distance. For instance, a camera with an 8 mm lens on a half-an-inch CCD would render a field of view of around 43 degrees.

If a traditional 'spot and floodlighting' approach were to be used here, the resulting picture would more than likely have a white circle in the centre of the screen. The reason?

A spot lens will only illuminate a small selection of the field of view. The flood lens would more than likely be wasting 30% of its energy outside the field of view. Remember that you are designing the illumination for the cameras and not the human eye.

The real key to successful night-time schemes is having sufficient light, the right quality of light and the right control over that light. The best night-time solution for CCTV, then, is often infrared lighting at the camera head – lighting that's controlled by either telemetry or photocell.

What distances can you reasonably expect to achieve with infrared sources? Achievable distances do depend on camera and lens type, which can affect system performance by anything up to 50%. All cameras are less efficient at 850 nm, when achievable distances can be reduced by at least 25%. At 950 nm, the performance of many camera/lens combinations may be reduced by over 60%.

When choosing your infrared lamps, consideration should be given to their ability to generate even illumination. Additional factors such as lamp life and running costs must also be taken into account. As a general rule, end users should bear in mind that infrared lamps driven via the mains tend to have considerably less bulb life and higher running costs than low voltage infrared light sources.

Installation, costs and maintenance
The installation of infrared lighting at the camera head really does offer the most cost-effective method of providing additional lighting for night-time CCTV. Providing wide area and long range lighting over distances of up to 200 metres (long throw) and 60 metres (width) with conventional sources would mean the installation of several luminaires spread over the target area. That will entail high capital and civil works costs for the end user. The same area may be covered by just two infrared lights mounted at the camera head.

Both Government and the industry at large should focus on the running costs of infrared lamps in a CCTV system. In terms of maintenance, the key areas of concern for end users are lamp life, overall product life and actual running costs.

In recent times, Cramlington-based CCTV lighting systems manufacturer Derwent has witnessed a significant growth in retro installations – signifying that the financial case for replacing the existing infrared lights or standard lights is so great that the additional replacement cost may be quickly recovered.

Let's look at a real life example. A major industrial company was considering the running costs of around 50 separate 500 W lamps on its site. The outlay for the system's electricity would have been 500 W x 4,400 hours divided by 1,000, equating to 2,200 chargeable kW hours.

The cost of electricity for operating a suitable infrared lighting system (using only 220 W to provide a comparable level of illumination) would be 968 chargeable kW hours. A saving of over 65%. The payback period for the latter system was two years.

Bear in mind that using infrared located at the camera head is very efficient – and means that only the required viewing area is illuminated at any one time (ie illuminate where the camera is looking). This is far more cost-effective than providing wide area coverage.

Solid-state devices – including light-emitting diodes (or LEDs) – bring major advantages. These include lower maintenance levels and lower power consumption. LEDs are beginning to make a bigger impact on the CCTV scene because of the increased infrared sensitivity of emerging monochrome cameras.

Certain manufacturers combine the camera, lens and LED illumination in one package. Such products are often referred to as IDN products (ie integrated day and night performance). They are able to sense ambient light levels and switch on the infrared when insufficient light is available. Achievable lighting distances are usually in the range of 50 metres.