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Sensitivity
Colour cameras generally perform less well in low light than monochrome cameras,
all other things being equal. Sensitivity is usually measured by reference to the lux
level at which a camera can produce an image (see page 9).
The lux valuation, although usually referred to as a metric unit of measurement
of light, is very often applied subjectively to the ability of a camera to produce
images.
Comparisons based on the lux levels given by different manufacturers do not
usually lead to a valid determination. A more valid comparison can be made where
the F-stop of the lens is also quoted together with the video level, although the best
solution is to make a live visual evaluation of the cameras under consideration.
i) Lux level at the face plate
This measures how the camera performs when all light falls directly on the chip
faceplate. Whilst technically valid, this never actually happens. You cannot get a
picture unless you use a lens and the lens cuts down the available light.
ii) At the lens
This is closer to the specification we are looking for because it goes some way to
describing the specification in real life.
Which lens?
The lower the ‘F’ number of the lens used the better the result. For example:
■ Camera one quotes 0.8 lux full video with an F1.0 lens
■ Camera two quotes 0.8 lux full video with an F1.2 lens
Camera two is the more sensitive camera as it quotes 1 lux with a slower F1.2
lens. With the faster F1.0 lens, it would probably give a full video signal at only
0.6lux.
Colour Rendition
Any assessment of a camera’s operating capabilities should include how accurately
it can reproduce colour. If, for example, a target subject is dressed in a brown
jacket and blue trousers it will not be helpful for the police if they are looking at a
CCTV image of a person apparently wearing an orange jacket and turquoise
trousers. This will also have severe limitations on the evidential value of such an
image. Cameras offering better colour rendition will have both auto and selectable
white balance to handle varying lighting conditions.
12
The Sony Guide to CCTV
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Issue 3
Choosing a Camera