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Guide to Closed Circuit TV (CCTV)

So would you like to play "Spot the CCTV Camera?". Eagle eyed visitors may recognise the profusion of street furniture, surrounding a single heritage dome camera keeping watch over part  of Londons' prestigious Oxford Street shopping area.

Making light of a dark place ...
the inside track on Infra Red.

Using lenses with Infra Red

It’s important to remember that although humans, and indeed most other animals eyes are completely insensitive to Infra Red light, most Black and White capable CCTV cameras are varyingly efficient at producing images at covert wavelengths.

Although we cannot see this type of light, in practice it behaves in exactly the same way as visible frequencies.

That’s not to say that there aren’t a few interesting characteristics which need to be considered. In practice, many everyday objects appear very differently when illuminated purely with IR.

If you point a monochrome camera at a dense green tree in daylight, the foliage will appear extremely dark on screen; try the same exercise at night using only covert Infra Red illumination, and the tree may well look almost white.

If a fox happens to walk past, you may well think it’s been to the hairdressers to have blonde highlights put in! The fact is that apparently dark objects (such as vegetation) may well be highly Infra Red reflective, and so appear significantly brighter than under daylight conditions.

This in itself has benefits and disadvantages.

On the one hand, if an object is highly reflective to Infra Red, there is a strong possibility that the night image will be grossly overexposed in parts, and the camera / lens ‘over compensation’, may make it increasingly difficult to produce a clearly recognisable image of the target.

Though careful positioning of the Infra Red illuminator(s) and indeed some considered selection of the most appropriate type and power of lamp, may well restore some of the overall quality which is present under normal daylight conditions.

One obvious trick which Doktor Jon has been known to use on the odd occasion is perhaps also worth considering.

Continued >>

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