The bench test 2: Seeing in the dark

What our experts say …
Steady improvements in the low light performance of video surveillance cameras have tended to overshadow an older and simpler method of seeing in the dark, namely infrared illumination or floodlighting. Virtually all video cameras are sensitive to IR light, some more so than others, but that coupled with the human eye's relative insensitivity to IR makes it the ideal choice for illuminating areas of interest, without necessarily drawing attention to the fact.

The principal drawback of IR lighting is the maintenance and running costs due to the relatively short life of the bulbs (typically just a few thousand hours or around one to two years) and power consumption, which can be significant in a large installation.

There are other ways to produce infrared light though, and IR light emitting diodes (LEDs), originally developed for use in remote control systems, have been around for at least 25 years. They consume comparatively little power and as solid-state semiconductor devices, they have long working lives (at least five years in continuous operation) and, unlike incandescent bulbs, waste less energy in the form of heat.

Unfortunately it has not been possible to scale up LED technology to the same sort of light output as conventional light bulbs and so their use in surveillance has been mostly confined to short range/small area applications, such as door viewer cameras, and then only by using several LEDs together to create a useable field of illumination.

Nevertheless, the light output levels of single LEDs continues to increase and it has now reached the stage where the combined effect of several hundred of the latest high-power LEDs compares quite favourably with conventional IR illuminators for lighting larger areas. That's the cue for the Derwent SuperLED, a high performance LED illuminator capable of providing useful illumination over distances of around 100m when using conventional mid-range surveillance cameras or up to 175m on models with extended IR sensitivity. It is an elegantly simple design comprising the light head, mounted on a sturdy bracket, and a matching power supply unit with built-in light sensor and remote switching option.

The light head, which measures 24 x 18 x 9cm is basically made up of just three components: an extruded alloy heat sink, the LED board and a protective cover/filter. The black coloured heat sink is also the main body of the unit, with a flat font surface and a row of heat-dissipating fins on the back. Onto that is mounted a single PCB containing the array of LEDs, arranged in a 20 x 28 matrix, which comes to a grand total of 560.

Two types of LED are available: semi-covert 850 nanometre (nm) types, which emit a dim red light, and covert 940nm LEDs that are invisible to the naked eye. There is some trade-off with the covert type and this includes a slightly narrower beam spread (20 – 30 degrees, compared with the 30 – 45 deg beam of the semi-covert light), and it works best with cameras with a wider range of IR sensitivity. A transparent front cover fitted with an IR filter completes the whole ensemble. The cover surround bolts directly to the heat sink (separated by a gasket) to create a solid and weatherproof structure.

The power supply cable (4m supplied as standard) emerges from a grommet on the rear. It is fairly hefty and all-up weight is around 5.8kg, which is worth bearing in mind when it comes to mounting the unit.

The PSU is a dedicated design and built into a tough, grey-coloured weatherproof alloy box measuring 28 x 18 x 10cm. Inside there are two circuit boards. The larger one is a multi-mode mains power supply that operates over a range 85 - 264 volts AC or 110 – 370v DC.

The second smaller board handles the day/night switching, photocell sensitivity, light output and remote switching. Two cables emerge from the case, one for the mains connection, the other carrying the low voltage DC supply for the light head. The standard of construction is very high and appears to be well able to withstand the worst the British climate can throw at it.

  Mounting the PSU and light head should pose no particular problems for installers. The only point to consider regarding the power supply is that the photocell needs to be positioned horizontally, or pointing downwards, and away from any sources of artificial light. The light output can be varied over the range of eight to 140 watts using a small potentiometer on the control board inside the PSU. The manufacturer recommends using the lower to mid-range power level setting for interior use and ambient temperatures of up to 45 deg C. The high-power setting should only be used on outside installations in temperatures of up to 40 degrees C.

During our tests the lamp never got uncom-fortably warm. Nevertheless several references in the instructions to operating temperature and environmental conditions suggests that cooling is important and the light head should be in the path of freely circulating air to prevent the build-up of heat, which can damage the LEDs. Remote control is disabled by default but it's a simple matter to remove the shorting link on the telemetry terminal on the control board inside the PSU and install a control cable (0 volt latched contact).

The only other adjustment is photocell sensi-tivity. It is factory-set to come on at around 20 lux and switch off at 70 lux. A fair amount of lag or 'hysteresis' is built into the switching to prevent spurious operation in marginal lighting conditions or areas subject to brief fluctuations in lighting level.

Positioning the light head shouldn't take long. The mounting bracket allows the lamp to pivot through a full 360 deg. Bear in mind it will probably be a two-person job, and of necessity carried out in darkness as the illumination pattern can only be viewed on a monitor connected to a camera looking at the scene.

  SuperLED produces a noticeably more diffuse and 'softer' field of illumination than most comparable single-bulb illuminators. We tried our sample with a couple of ordinary cameras plus one supplied to us by Derwent (Extreme EX27 All Weather camera). It works reasonably well over distances of 50 to 60m, providing enough light for most sub 0.5-lux mono models to produce a useable image (depending on lens and exposure settings of course). We suspect that Derwent's claim of 100m is a tad optimistic with the majority of general-purpose cameras. Obviously a lot depends on the individual camera's IR response and highly sensitive models may well achieve the kind of results in ideal conditions. But we suggest taking the conservative view over what these lamps can do when planning an installation.

What the manufacturer says ...
SuperLED is a long life, high reliability infrared illuminator, which uses solid state high efficiency LEDs. Designed for night-time CCTV schemes which demand a high level of reliability and minimum maintenance costs, SuperLED achieves distances unrivalled in the LED market and has been developed for external applications with ranges up to 175m.

   The SuperLED utilises infrared technology providing high quality, even illumination to ensure that today's CCD cameras can provide the effective night-time images required.

  Previously LED technology was limited, chiefly in distance. Derwent engineers developed the technology to overcome this limitation.

  Solid state devices such as Light Emitting Diodes bring major advantages, including lower maintenance and lower power consumption.

  Furthermore, SuperLED can reliably provide up to ten years of life. The method by which the solid state LEDs are powered allows for greater performance per LED.

   In addition, the design utilises several unique features, which ensures maximum heat dissipation, and channels the heat directly from the source to a specially designed heat sink.

  While LED technology is not new to the surveillance industry, application of the technology has historically been limited. The market for the SuperLED can be defined as any long range (including wide area) CCTV schemes using monochrome or dual mode cameras, which are otherwise known as day/night cameras or colour/mono cameras.

  Typical external applications include: city centres; long range, wide area perimeters; car parks; barriers and gates; toll booths; airports and military applications. Internal applications include: warehouses; factories; large office complexes; sports halls/arenas; and leisure centres.