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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.

Cameras Suitable For Infra Red

To start off with ..... this is actually nice and easy.

All dedicated colour CCTV cameras are deliberately filtered (remove the lens, and you’ll see a blue coloured blocking filter over the imager) to prevent IR light from reaching the CCD; the reason being that without this filter, the colours would all appear false, so in effect, it would be useless for recognising a blue car, or a red headed suspect!

There are a limited number of switchable colour cameras that can automatically (or manually) change over to Black and White (Monochrome) operation; as well as making the camera sensitive to Infra Red, it also significantly increases the cameras general sensitivity to light, so it will see better under visible illumination as well.

Almost without exception, Black and White cameras are mostly sensitive to Infra Red, although some more so than others. In general terms, most cameras will produce very clear images under IR illumination at the 715nM (nano meter) range, although the illuminator will also be giving out a very bright red glow, which some may argue visibly increases the deterrent effect.

The next filter setting available is 815nM, which apart from providing only a very dull red glow, does actually effectively reduce the amount of (camera visible) IR being produced.

So for example, whilst most cameras may be about 60% sensitive at 715nM, they are only 30% receptive at 815nM; in other words, you'll need a lot more light energy output at the higher frequency to achieve the same result.

850nM (although very difficult to get hold of) is a very useful compromise lamp, as its light is effectively invisible to us mere humans, but most B/W cameras can still see it.

Continued >>

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