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Covert operations often take place during the night or in environments where the illumination is not designed to work in conjunction with the CCTV camera. Infrared can be a key component to many covert schemes, providing illumination that monochrome cameras can see but the human eye cannot. It is sometimes called black light and lies approximately between 700 and 1,000nm (1 micron). The diagram below shows what the human eye can see and what a normal mono camera can see.

Graph B demonstrates the increased sensitivity of more specialised IR sensitive mono cameras. These cameras are required to achieve effective covert operations in total darkness.

It is important to notice that the camera sensitivity drops as the wavelength increases. This means that the camera sees less of the light distri-buted on the scene. Installers should be careful when specifying cameras – size isn't everything.

There is often a requirement for semi-covert or covert installations. Graphs C, D and E demonstrate a 730nm, 830nm and 950nm filter. Generally speaking, the 730nm filter will glow similarly to a traffic light, an 830nm will give off a dull red glow only just visible to the human eye, and a 950nm will appear totally covert – no glow is visible. The key factors in choosing the type of IR lamp often relates to the required viewing distance and scene. As seen in the diagrams, the sensitivity of the camera is an important element. Additionally the lens sensitivity and type play an important role. The traditional approach often involves cleverly hidden cameras in everyday objects such as wall clocks, pictures and office box files. However there are some key technical barriers to achieving high quality pictures.

   All cameras are not the same, and some are better suited to providing effective coverage at night. It can be a minefield for installers with impressive claims or zero or low lux cameras, but in essence, without light there can be no picture. All CCD cameras offer some degree of IR response, though some have enhanced IR performance which makes them more suitable for longer range applications or for use with low power IR sources such as LEDs.

Until recently, the most IR sensitive cameras were based on frame transfer chips. Now some new chip sets have become available. These offer excellent low noise, high resolution, low smear characteristics together with excellent IR response. They also overcome some of the drawbacks of the frame transfer cameras.

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

Several 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/IR sensitive by night.

There are different forms of dual-mode cameras with some incorporating optical filters which are moved over the CCD sensor for daytime/colour operation – and removed during night-time/monochrome to maximise the low light sensitivity. Other camera designs incorporate specialised filters which do not move but which have both good colour performance and IR sensitivity.

The key elements to consider when choosing your camera are:

• Sensitivity: low light performance
• Signal to noise ratio: a good s/n will provide clean pictures
• Spectral response: The ability of the camera to see IR wavelengths.

Lens: The night-time or low light performance of lenses is sometimes overlooked. There is a compromise to be made: at night you want to maximise the light-gathering capability of your lens (i.e. have the smallest f stop) but this will reduce the depth of field of the picture, which may cause focusing problems.

This obviously is less of a problem with auto iris lenses where the lens will naturally open to its maximum aperture (lowest f stop) in low light operation but on fixed lens there may need to be a compromise between the low light operation and its depth of field focusing.

Daylight and IR light have different focal lengths/points because the different wavelengths of light pass through the lens differently and do not focus on the CCD at the same point. This may cause a focus shift between the day-time and IR operation.

The degree of focus shift may depend on a variety of factors depending on the quality of the lenses, the wavelength of the IR filter (830nM & 950nM will give a more exaggerated focus shift) and the IR response of the camera.

However, more recently some manufacturers have developed a range of lenses with zero focus shift between day-time and IR performance. This is of considerable interest to CCTV designers – especially for unmanned or non-telemetry controlled installations.

Derwent overcomes the effect of focus shift in its combined lighting products with the use of high quality components and optical filters.

Illumination: The key for a successful night-time or covert scheme is having sufficient light, the right quality of light and the right control over the light. The best night-time solution for CCTV is infrared lighting at the camera head, controlled by either telemetry or photocell.

Advantages of infrared

• Covert opportunities: invisible to the human eye, but monochrome cameras can see it
• No light pollution or unnecessary costly lighting
• Discreet or covert to blend with surrounding environment
• Intruders are unaware of surveillance (semi- discreet and covert)
• No glare for pedestrians or drivers
• Long range beyond normal lighting systems
• Low installation and capital costs
• Low running and maintenance costs
• Increased return from investment in system by giving 24 hour coverage instead of just daylight coverage
• Designed to work within a CCTV system.

In conclusion

• 730nm filters are brighter in appearance than 830nm or 930nm but provide more useable infrared radiation for cameras. In certain applications the red appearance of 730nm filters may provide an additional deterrent to 830nm and 950nm applications
• When using 830nm or 950nm filters, ensure IR sensitive cameras are used for maximum performance
• Match the field of view of camera/lens with lens on infrared lamp
• Designers need to provide even illumination to allow a CCTV camera to work within its dynamic range
• Low cost of ownership is important to the end users – consider the power consumption, bulb life and bulb costs when specifying your infrared products
• Solid state LED devices offer ultra long life and low power consumption – but are significantly less powerful than standard quartz halogen products. They should therefore be used with very sensitive IR cameras for anything but the shortest of ranges.

* This article is based on a presentation to the IFSEC 2001 Conference, organised by United Business Media International. The conference session also heard that the Data Protection Act has implications for the legal use of covert CCTV. "It has teeth and it will bite – but it is not installers who will be liable, but the owner of the system," said Andrew Holmes of Data Compliance.