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Guide to Closed Circuit TV (CCTV)

CCTV Camera Operation - in a little more detail

CCTV Camera Performance - Sensitivity

Camera sensitivity is normally indicated on a specification sheet as a measurement of 'Lux'.
Of all the various bits of information shown on the manufacturers data documentation, sensitivity is the one thing which for a number of reasons, must be treated with a strong dose of scepticism.

Firstly, in purely practical terms, most manufacturers will carry out their tests under varying conditions, so whilst one set of measurements may be absolutely spot on, another could be miles off the mark.

The quoted 'lux' sensitivity figure must first be interpreted - does it refer to a measurement of the target or 'scene' illumination, which may for arguments sake be quoted as perhaps 1 lux, or does it instead refer to the illumination level measured at the imager (CCD) which would normally be 1/10th of scene value, or in this example 0.1 lux. Two different figures, exactly the same result on screen!

Then we have the IRE mire to consider.! IRE is actually a standardised method of measuring ... how can Doktor Jon put this simply? .... well, let's consider an example instead. If you look at a monitor reproducing a near perfect picture under ideal conditions, the overall quality when measured on test equipment may come out at something in excess of 90% IRE. If the same view is then reproduced under poor lighting conditions, so that you can still see all the relevant components of the picture, but the overall effect is rather snowy ('noisy' being the technical term), then the greatly reduced IRE rating may be around 40%.

Under still worse conditions, the noise may have become so intrusive that you can barely see anything other than a moving blob, the resultant IRE rating may have dropped still further to perhaps 20%. So when a manufacturer quotes a sensitivity figure, how do you know whether its appropriate unless they also indicate the IRE figure alongside ( as an example, 0.25 lux at 90 IRE would be preferable to 0.1 lux at 25 IRE).

Just to complicate matters even further, there is of course the important consideration of precisely what type of lens was used when taking the laboratory measurements. One manufacturer in particular, almost always uses an f 0.8 maximum aperture lens (extremely sensitive [and expensive] and hardly ever used for general CCTV work), whilst others use f1.2 or f1.4 optics which are more readily used in the real world.

So, simply quoting a small 'lux' figure may look impressive, but without all the other information, it really doesn't tell you anything at all.

Information on camera exposure control>

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