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Guide to Closed Circuit TV (CCTV)

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CCTV Camera Operation - in a little more detail

CCTV CameraPerformance - Exposure Control


The interesting part of Electronic Iris (EI) exposure control, is that in practice, the amount of light actually landing on the imager is not varied.

So the options for controlling exposure in a modern CCTV camera are:-

a) to manually set a lens iris for the ambient light level, provided of course it is going to remain constant,
b) to still use a manual iris lens but switch on the 'Electronic Iris' function, in order to cope with quite extreme changes in lighting level (with the iris set to f2.8, most cameras can cope from low light to full summer sunshine,
c) use a more expensive, and technically lower (optical) performance auto iris lens, which will correctly vary the amount of light reaching the imager (in other words, provide the widest dynamic rang, but not necessarily the best quality pictures under all conditions.

At this point, a little needs to be explained about the hidden benefit of using Electronic Iris, to produce really stunning quality images. If the camera is located in an area where the light level is likely to vary throughout the day, then EI can provide a key improvement over the conventional use of 'auto iris' (AI) lenses. With a standard AI lens, the camera will always be sampling images at the normal scanning system rate (i.e. 1/50th or 1/60th of a second), depending on the country where the equipment is in use.

Now, if the images contain a reasonable degree of movement, either people walking or running swiftly past the camera, or vehicles travelling at speed, the resultant video playback can show varying degrees of blurring in the image, which can frequently obscure vital information .

With a camera using the Electronic Iris function to control exposure, images will be captured at varying speeds dependant on the ambient light level, so under the darkest of conditions, the sampling rate will almost certainly be 1/50th (1/60th) of a second, with no major technical advantage, save for possibly better lens performance, and lower unit cost. However, when the light level improves and the camera starts to sample images in 1/1000ths of a second, there is a very marked improvement in the appearance of moving targets.

The images are no longer blurred, but instead appear crisp and very, very sharp.
At the opposite end of the scale, there is a facility on some 'high end" cameras, known as "Frame Integration". With this technique, in very low light conditions the camera can sample an image over a longer period, perhaps 0.5, 1 or 2 seconds, to give a perfectly exposed full colour image (which is seriously not good for moving objects!).

More information on Electronic Iris exposure control>

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