Essentially, without light there can be no picture. But night-time surveillance need not be a headache, says Shaun Cutler of Derwent Systems, who throws some light on the subject for us
Recent Government figures indicate that crime is on the increase, and it's a fact that large percentages of these crimes take place during the night. So, the industry needs to focus on achieving effective 24-hour CCTV, which means night-time footage must be just as effective as daytime. Infrared is the recognised choice of many leading schemes and Derwent can claim to have provided a night-time solution to at least three individuals from the world's top ten richest people – from princes to pop stars! When designing a CCTV system for 24-hour surveillance, particular attention must be paid to the night-time performance of the camera, lens and illumination.

Consider the camera

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 of zero/low lux cameras, but in essence without light there can be no picture. All CCD cameras offer some degree of IR response, 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 for CCTV were based on frame transfer chips. Now, however, some new chip sets have become available, mainly in 1/2in format. 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.

Derwent analyses and tests many monochrome cameras in actual night-time conditions, so that we can explain to installers, consultants and specifiers what kind of results they can expect to achieve when using infrared illumination. As a general rule, these cameras can see further with the equivalent amount of infrared than their predecessors, while semi-covert and covert operations become more feasible, including the use of LED illuminations.

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 past few years, which are intended to provide the best compromise for 24-hour surveillance colour by day and mono/IR sensitive by night.

There are different forms of dual modes with some incorporating small moving filters that are moved over the CCD sensor for daytime/colour operation and removed during night-time/monochrome operation to maximise low light sensitivity. Other camera designs incorporate specialised filters (non-moving) that provide both good colour performance and IR sensitivity.

The key elements to consider when choosing a camera are:

  • Sensitivity, and low light performance
  • Signal to noise ratio – a good one will provide clean pictures
  • Spectral response – the ability of the camera to see IR

    Don't overlook the lens

    The night-time performance of lenses is sometimes overlooked. You want to maximise the light-gathering capability of the lens (i.e. have the smallest 'f' stop), which will reduce the depth of field of the picture and may cause focusing problems. This 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 conditions, but with fixed lenses there may need to be a compromise between 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 at the same point. This may cause a focus shift between daytime and IR operation. The degree of focus shift may depend on a variety of factors including the quality of lenses and the wavelength of the IR filter – 830nM and 950nM may give a more exaggerated focus shift.

    However, some manufacturers have developed a range of lenses with zero focus shift between daytime and IR performance.

    Most illuminating

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

    Some of the benefits of infrared include:

  • Invisible to the human eye but monochrome cameras can see it
  • No light pollution (or unnecessary costly lighting)
  • Discreet or covert to blend in 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 cost.

    The Uniflood and Miniflood ranges are Derwent's leading established infrared illumination systems designed to give enhanced performance, bulb life and even illumination.

    Some frequently-asked questions answered

    Why do I need an infrared lamp?
    The human eye cannot see infrared light, however most mono CCTV cameras can. As such the invisible light can be used to illuminate a scene, this allows night-time surveillance without the need for additional artificial lighting. Infrared also provides many other benefits above conventional lighting; IR beam-shapes can be designed to optimise CCTV camera performance, extended bulb-life, covert surveillance, lower running costs etc. How safe is infrared?
    With over 20,000 infrared lamps installed in Europe each year, Derwent is unaware of a single safety issue raised about IR used in CCTV. However, as a basic rule of thumb, if you can feel the heat don’t stare at the lamp. Derwent produces a paper regarding safe installation and use of its illuminators. How far can a camera see at night with infrared lamps?
    The range that the camera will see in the dark will depend on sensitivity and spectral response of the camera and lens combination. Derwent produces a chart that gives an indication of the performance. Can infrared lamps work with colour cameras?
    No. Normal artificial light, e.g. sodium light, causes problems to the quality of the colour CCTV picture, not producing accurate colour quality. There is a range of dual-mode cameras now available, which will work with infrared lamps when the camera is working in monochrome mode. Can infrared be used in covert operations?
    Yes. Infrared lamps come in varying wavelengths from approximately 730nM to 950nM. The 730nM wavelengths will give the normal red glow on the lamp – like a traffic light. The 830nM lamp will greatly reduce the visible glow. The 950nM lamp is totally black and is ideal for covert use where no indication of night-time security is a requirement. However, a 950nM lamp will require a highly sensitive night-time camera. What is the average bulb life of an infrared bulb?
    Bulb life is dependent on filament ruggedness and design. Derwent bulb life is considerably greater than the industry average due to the rugged halogen quartz bulb. Uniflood 500: average 3,000 hours; Uniflood 300: one year normal use; Miniflood: average 8,000 hours. Can infrared lamps face the camera head?
    No. Although the light emitted from the lamp is not visible to the human eye, the camera sees the light in much the same way as we see white light. As a result, the camera will ‘white out’ if the lamp is situated within the camera’s direct field of view. Can the lamps be operated remotely?
    Yes – most Derwent power supplies have a built-in remote switch that allows on/off facilities via telemetry or PIR.

    • Source

    Security Installer

    Postscript

    Derwent Systems Ltd, Derwent House, Colborne Crescent, Nelson Park, Cramlington, Northumberland, NE23 1WB. Tel 01670 730187; fax 01670 730188.