When designing CCTV systems, one of the most
important decisions is choosing the right camera.
If the camera or cameras are to be used outdoors
then black and white cameras are more suitable, as they need less
light to form a useable picture.
Colour cameras are more suitable for indoor
applications where light levels are higher.
Most CCTV cameras use a CCD (Charge-Coupled
Device) light-sensitive chip to capture the image The sensitive
area is split into a large number of picture-elements or ‘pixels’.
The size of the CCD chip is known as the ‘format’
which may be ½”, 1/3”, ¼” or 1/6”.
In general, the larger the format the better the low-light pictures
will be. With the latest generation of CCDs, 1/3” cameras
will give good results in most situations.
An image from a scene is focused through a lens
onto a chip surface, charge is built up in each pixel proportional
to the intensity of the light falling on to it. Hence an electrical
representation of the optical image is formed. Typical pixel counts
for CCD sensors are between 250,000 and 500,000.
Note that the majority of manufacturers supply
their cameras without lenses fitted – REMEMBER TO QUOTE
FOR THE LENS!
The main performance criteria of a camera
are its sensitivity and resolution.
Sensitivity is a measure of the minimum
amount of light required by the camera to generate an acceptable
image. Resolution defines the amount of picture detail in the
image produced by the camera.
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When selecting cameras for surveillance applications
there is a third choice available. In addition to standard monochrome
and colour there are ‘Day-Night’ cameras.
Day-Night cameras provide a colour picture
when there is sufficient light to do so, normally during daylight
hours. When light levels fall, the picture reverts to black
and white, to take advantage of the inherently better sensitivity
performance of monochrome cameras.
Monochrome cameras typically require approximately
1/10 of the amount of light to make a useable picture, compared
with a good quality colour camera.
The switching from colour to monochrome at
dusk, or from mono to colour at dawn can either be automatic,
using the camera itself as a light level meter or it can be
controlled via an external photocell or a contact from suitable
telemetry equipment.
In order for colour cameras to reproduce colours
accurately they are normally equipped with an infrared cut filter.
This prevents infrared light getting onto the CCD imaging device.
Natural daylight is a mixture of both
visible and infrared light. If a colour camera is exposed to
infrared light, the image will be lighter than if it was producing
it from the available visible light. Hence colours would appear
whitened; red objects would appear pink. Grass, being reflective
to infrared light, would appear a pale shade of green.
Not all Day/Night cameras are the same; there
are three distinct types.
One does not have an infrared cut filter fitted.
Digital or other signal processing is used to compensate and
make colours look correct when in day mode. This type of camera
has good sensitivity at night and can be used with infrared
lighting.
A second choice of Day/Night camera comes
fitted with an infrared filter. This gives good colour rendition
in daylight, however the camera is not as sensitive as a monochrome
camera. The camera is not as sensitive to infrared and cannot
be used with infrared illumination. If lighting is required,
which is likely given the cameras reduced sensitivity; only
visible lighting can be used.
Clearly, it would be appropriate if the infrared
filter was in the optical path during daylight to give good
colour rendition. It would also be best if there was no filter
in place at night so that the camera is as sensitive as a normal
monochrome camera, therefore it would be sensitive to infrared
light, enabling overt or semi-covert surveillance.
Finally, there are Day-Night cameras, for
instance the VCP501 Zoom camera from Videcon, which physically
move an infrared cut filter in and out of the optical path to
give true colour rendition in daylight, and low light and infrared
sensitivity at night.
Because of there different wavelengths, infrared
and visible light focuses at different points behind the lens.
It is often a problem achieving a sharp image in both day and
night modes. When using a standard auto-iris lens on a Day-Night
camera, always perform the back focussing at the lowest light
levels when depth of field is at a minimum.
The Concept Pro VCP501 is equipped with an
auto-focus lens and will automatically give as sharp an image
as is possible with which ever type of light is being used,
visible or infrared.
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Standard CCTV camera and lenses cannot be
placed outdoors without some protection from the elements.
They must be placed in an environmental housing
to keep them dry and maintain the safe operating temperature
range of the equipment.
Standard environmental housings for CCTV come
complete with heater and thermostat.
The thermostat will usually switch at about
20 degrees Celsius. Be careful when working in housing; even
on a reasonably warm day the heater maybe on and if so will
be extremely hot.
The heater is located at the bottom of the
glass or perspex window at the front of the housing. It is designed
to keep an upward draught of warm air flowing over the glass
to prevent moisture condensation.When
selecting camera housings ensure that there is enough room to
comfortably accommodate all the necessary equipment including
power supplies and telemetry receiver etc.
Remember it may also be necessary to fit a
video isolation transformer at commissioning to eliminate hum
bars from the picture. To prevent the need to fit another box,
it is convenient if there is initially enough room in the camera
housing to fit it.
Integrated camera and lenses, often referred
to as bullet cameras are sealed to prevent ingress of moisture.
The sealing is so good that they can be permanently immersed
considerable depths if required.
These do not need a heater. It is not possible
to make adjustments to these cameras because they are permanently
sealed.
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Ingress Protection (IP) is an indication
of the protection offered by a housing or enclosure to the ingress
of particles and water.
The first number indicates particle
protection and the second, protection against water ingress.
IP5x gives protection against dust
with limited ingress permitted.
IP6x gives total protection against
dust.
IP5x gives protection against low-pressure
jets with limited ingress allowed.
IP6x gives protection against high-pressure
jets with limited ingress allowed.
IP7x gives protection from total
immersion in water up to 1 metre deep.
IP65 is generally accepted as the
minimum standard considered as weatherproof and suitable for outdoor
use. To maintain protection, particularly against water, the cable
entries into the receiver box must be done using IP55 or greater
rated cable (stuffing) glands.
One common reason for product
failure and repair is that the cable entries have been created
either through the top or side of the box. Water as a result is
sucked into box as it warms and cools over a period of days. The
water level rises slowly in the box until it reaches the live
electrical connections inside.
Always ensure connections
are made through the bottom of the box.
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Light levels are normally measured in Lux. This
is a measure of the light energy falling on to an area of 1m2
of surface per second. Typical light levels are:
Environment |
Lux |
Full Summer Sunlight |
50,000 |
Dull Daylight |
10,000 |
Shop/Office |
500 |
Dawn/Dusk |
1-10 |
Main Road Lighting |
30 |
Side Street Lighting |
0.5-3 |
1/4" Moonlight |
0.1 |
When deciding which camera to use for a given
lighting condition, do not choose one that will only just give
a picture.
Try to give the camera approximately 10 times
its quoted minimum scene illumination. Cameras and auto-iris lenses
will be able to deal with any excess light. The major problem
is when they do not have enough light to make a picture.
Note that the majority of all sensitivity figures
for cameras are quoted with the Automatic Gain Control (AGC) turned
on. This is extra gain, which the camera applies to the video
signal to improve the brightness of the picture. When AGC is applied
it creates more noise and a snowy picture.
If in doubt always give more than the required
amount of light. This will improve the picture in a number of
ways. The picture will be brighter and less noisy, since less
AGC gain is required. The sharpness of the image may improve if
the iris on the lens is opened, increasing the depth of field.
The f-number or aperture ratio of the lens can
directly affect the picture obtained in low-light conditions.
For example, with the iris fully opened, a f1.0 lens can let twice
the amount of light into the camera than a f1.4 lens.
Lenses with apertures greater than f1.2 are
generally referred to as aspherical. These lenses can be used
on most cameras to improve night-time pictures.
The sensitivity of covert cameras with pin-hole
lenses is often quoted as 0.1 Lux @ f1.4. This seems to indicate
that the camera will work in ¼ moonlight.
The pin-hole lens will have a usual aperture
ratio of f4 and need approximately 1 lux to produce a picture.
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