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CCTV
Cameras and Infra-red
This
is always a difficult area as the camera manufacturers use different
types of specifications to make claims for their equipment to gain
a sales advantage. There is nothing like testing a camera and lens
combination on the actual job they are to be used on - unfortunately
in the real world this is rarely possible.
Things to be aware of:
Resolution of the picture - usually quoted in "lines per mm".
Generally seek out a high enough spec for the task in hand but beware
this specification can be fudged and often is! Remember that resolution
is sometimes at the expense of sensitivity as the pixel size has
to be reduced to allow more to be fitted into the same area.
Lowest operational light level - usually quoted in
LUX. Some cameras that appear very sensitive may suffer from large
amounts of "noise" or "shash" where the AGC
(automatic gain control) is amplifying the video signal from the
CCD so much that the pictures become severely degraded and very
"grainy". Some manufacturers quote the lux level at the
CCD sensor or faceplate and not the light on scene . It is not always
made clear! If lux on scene is quoted then you need to know what
lens and f. stop was also used. This may well be a totally impractical
value if you are using a zoom lens. Remember that for every full
f. stop you need to double the amount of light.
IR sensitivity - this is rarely quoted as a figure.
The best guide to this is to check if the CCD sensor type - a Sony
X-wave or X-view type has an extended IR response. Other manufacturers
are now producing sensitive CCD sensors which perform well in IR.
CMOS sensors at the moment are generally not very sensitive to IR
but developments in the future may well see cheap IR CMOS sensors.
Detail, Aperture correction or sharpness - This is
a circuit within the camera that make the pictures appear sharp
and watchable! It works by enhancing the contrast of the edges within
the image, making objects appear sharper. Some cameras overdo the
edges and this creates the effect of a black/white line drawn around
the objects. VCRs and monitors can also add to this effect! Any
noise in the picture is also regarded as an "object" and
is enhanced, so at night, with a lot of AGC in operation,you can
end up with accentuated "noise" on the pictures .
Camera output level and termination - most cameras
that give a composite video signal are designed to produce a signal
of 1 volt peak to peak into a load of 75 ohms. This may seem academic
but has a real impact on signal/picture quality. Remove the termination
and look at the picture difference, it may well appear brighter
and more contrasted. Double terminate and the picture will be darker
and "muddy". Correct signal level is of major importance
when recording pictures via a digital multiplexer as these do not
tolerate high level signals. Colour signals suffer too, when levels
are not correct.
More....
Infra-red light, Lenses and Infra-red
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