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The
scene and light
The scene refers to the objects or area to be observed and the total
environment in which the exist. A scene often contains different
colors, surfaces and materials that reflect varying levels of light. To
select proper equipment, it is necessary to determine the minimum
lighting level (day or night) that will arrive from the scene to the
camera lens. A scene or target area can be illuminated by natural or
artificial light sources. Natural sources include the sun, the moon,
and the stars. Artificial sources include incandescent, sodium,
florescent, infrared, and other man-made lights. An axiom in CCTV
security applications is: The better the light, the better the picture.
Colour
vs. Black and White
Colour cameras generally require higher levels of lighting than their
monochromatic (B/W) counterparts. Colour produces a more natural,
richer image than black and white and may keep the operator's interest
for longer periods of time. It also makes it easier to detect subjects.
For example, with a color system a viewer can easily distinguish a red
car from a green one, while on a black and white system both cars would
appear a similar shade of grey. In retail applications, a colour system
can help security personnel identify shoplifters and their clothing
more easily and convincingly. Colour accuracy is extremely important in
gambling casinos, where hundreds of pounds can ride on the ability to
recognize the difference between maroon chips and red ones. While the
use of colour cameras is growing, black and white cameras continue to
offer some distinct advantages. Black and white cameras are better
suited for extremely low light situations. The ability to capture good
quality images in low light situations increase the cost of both black
and white and colour cameras. Before purchasing cameras, you should
consider the crossover points between lighting costs and camera costs.
It's possible that low-light cameras will cost less than paying to
increase the lighting level of a parking lot.
The Camera
Cameras
convert the visible scene captured by a lens into an electric signal
and transmit that signal to a monitor for viewing. Several
considerations should be taken into account when choosing the proper
camera/lens for any video system:
The purpose of the video system (detection, assessment, identification
etc...)The overall sensitivity of the camera needed based upon the
actual applicationThe amount of varying levels of light available at
the sceneThe environment in which the camera will operate
(indoors/outdoors)The field of view (FOV) required by the
applicationThe lensCostCamera performance depends largely upon the
reflected light at the scene and the quality of the cameras imager
Where the level of available light can change dramatically, a camera
equipped with automatic iris control can help assure consistent image
quality. Automatic iris control enables cameras to open or close an
auto iris lens to adjust the amount of light passing through the lens.
On a bright, sunny day an auto iris lens camera will close the lens'
iris to prevent strong light from reaching into the camera's imager. At
night, the camera will open the iris to allow greater amounts of light
into the camera.
Cameras are available in various "formats" expressed as 1/2, 1/3, or
1/4 inches. These measurements represent the overall usable size of the
cameras imager. In general, you should match the cameras format to the
lens format. For example, a half inch camera should be paired with a
half inch lens. This only applies to camera that need a separate lens
(i.e. C-Mount types) as opposed to cameras with a built in lens.
Fixed and PTZ Cameras
CCTV
cameras can be fixed or have pan, tilt, and zoom (PTZ) capabilities.
Fixed cameras are mounted on a fixed bracket and cannot move in
response to operator commands. PTZ cameras are motor driven and can pan
left or right, tilt up or down, and zoom in and out. A camera housing
protects the camera and lens from vandalism and the environment. It
also can enhance the appearance of the camera installation and conceal
the equipment from the casual observer. All outdoor cameras require a
housing of some type. The National Electrical Manufacturers Association
(NEMA) rates housings on their ability to withstand environmental
conditions. Protection from cold, heat, dust, dirt, or other elements
is needed to ensure optimal performance and extend the life of the
camera.
Dome Cameras
Many
PTZ cameras today are disguised in dark coloured Plexiglas housings
called domes. Dome cameras are found in practically every major
department store and in many industrial/commercial locations such as
hospitals, colleges and government facilities. They are particularly
popular wherever aesthetics are valued. Dome cameras offer three
primary benefits:
Deterrence - Domes make it virtually impossible for suspects to
determine where the camera is pointed.Economy - Domes equipped with a
camera, lens, and PTZ capabilities can be augmented with dummy
camerasAesthetics - Domes conceals all the internal equipment in a nice
clean shell.Smoked Plexiglas bubbles have the same effect as wearing
sunglasses. It reduces the amount of light reaching the lens affecting
the colour accuracy picked up by the camera.
The Lens - optics
Lenses
(Optics) play an important role in the design of a CCTV system. Their
primary function is to collect reflected light from a scene and focus a
clear, sharp image on the camera's imager. Typically the more light
that passes through the lens, the better the quality of the picture.
Selection of a lens is especially critical because it directly affects
the size, shape, and sharpness of the image to be displayed on the
imager. Factors such as distance from the scene, focal length, desired
field of view, lighting and format affects the size and clarity of the
image on cameras imager.
Field of view
The
field of view (FOV) is the actually picture size (height and width)
produced by the specific lens. If the field of view is not suitable,
you may consider using a different lens (wide angle, telephoto, etc...)
to increase or decrease the field of view. Camera lenses can be divided
into two basic types.: fixed focal and varifocal (sometimes known as
zoom). A fixed focal lens has a constant focal length, while the
varifocal lens can change its focal length. Focal length is simply the
distance from the optical center of the lens to a focal point near the
back of the lens. This distance is written on the lens and expressed in
millimeters (mm). Fixed focal length lenses are available in various
wide, medium, and narrow fields of view. A lens with a "normal" focal
length produces a picture that approximates the field of view of the
human eye. A wide-angle lens has a short focal length, while a
telephoto lens has a long focal length. When you select a fixed lens
for a particular view, bear in mind that if you want to change the
field of view, you must change the lens.
When both wide scenes and close-up scenes are needed, a varifocal lens
is best. A zoom lens is an assembly of lens elements that move the
change the focal length from a wide angle to a telephoto while
maintaining focus on the camera's imager. This permits you to change
the field of view between narrow, medium, and wide angles.
F-stop
The ability of a lens to gather light depends on the relationship
between the lens opening (aperture) and the focal length. This
relationship is symbolized by the letter f, which is commonly referred
to as the "F-stop," and can be found printed on the side or front of
the lens. The lower the F-Stop number, the larger the maximum lens
aperture and the greater the lens' ability to pass light to the camera
imager and the better it can view a low light scene. For example, a
lens with an F-stop of f/1.2 can gather a great deal more light than a
lens with an F-stop of f/4.0. A lens with a low F-stop number is
sometimes also called a "fast lens".
Depth of Feild
Another
consideration when determining the proper lens is depth of field. Depth
of field is the area in focus before and behind a subject. This means
that when you focus precisely on a subject a certain distance in front
of and behind the subject also will be in focus, although not as sharp.
Depth of field increase or decreases based on the Iris.
Lens Length
short lens (wide angle lens) longer depth of field
long lens (telephoto) shorter depth of field
Aperture
wide aperture (low F-Stop) shorter depth of field
narrow aperture (high F-stop) longer depth of field
Distance to object
Short distance, shorter depth of field,
long distance, longer depth of field
Purchasing
and planning decisions should take these factors into account since
depth of field can affect image quality (and may jeopardize the ability
to identify and prosecute subjects). If depth of field is important,
you may want to explore options such as increasing artificial lighting
or installing cameras with normal lenses rather than telephoto lenses,
etc...
Lens Mounts
Camera
lenses generally come with either a C-mount or CS-mount and must be
matched appropriately to the camera's mounting requirements. The
difference between the two mounts is the distance of the lens options
from the camera's imager. The C-Mount lens is 17.5mm from the imager;
the CS-Mount lens is 12.5 mm from the imager. Follow these guidelines
when purchasing equipment:
A C-Mount lens can be used on a CS-mount camera only if a 5mm spacer is
added.
A CS-Mount lens cannot be used on a C-Mount camera.
Video Transmission Methods
The
purpose of the transmission medium is to carry the video signal from
the camera to the monitor. Today, many video transmission methods
exist: coaxial cable, fiber optic, phone lines, microwave, and radio
frequency. Due to varying application technologies in use within the
same CCTV system. The choice of transmission mediums depends on factors
such as distance, environments, cost and facility layout. In addition
nearly all methods of transmission suffer from various forms of
interference or loss. The essence of good design is to minimize this
impact.
Examples
of current video transmission include:
Coaxial cable
A coaxial cable is one that provides a continuous physical connection -
or closed circuit - between the camera and the monitor. The cable is
shielded to minimize interference from any nearby electronic devices or
electrical wires. Copper braided coaxial cable is recommended to
maximize conductivity and minimize potential interference. For
traditional CCTV systems, as well as many applications today, this is
the most common economical method of signal transmission over
relatively short distances (few hundred feet).
Fibre Optics
Fiber optic technology changes an electronic video signal into pulsed
or laser light and injects (transmits) it into one end of a glass rod
(the fiber optic cable). At the other end, a receiver translates the
pulsed light back into an electronic signal capable of being displayed
on a monitor. The transmission is unaffected by any kind of
interference, water in conduit or high voltage being run in the same
conduit. Fiber optic capable have a large signal capacity (bandwidth)
and no possibility of a spark from a broken fiber. Hence, there is no
fire hazard to a facility even in the most flammable environment. Fiber
optics offers a cost-effective method of sending large transmissions
over long distances.
Telephone Line
A telephone line is a standard twisted pair of wires that can transmit
the image for distances up to one kilometer without signal boosting.
The dedicated line connects the transmitter (camera end) with a
receiver (monitor end). Through the use of specialized transmission and
receiver equipment, it is possible to use standard telephone lines for
video signal transmission (RSM).
Microwave
If already in place, microwave can be a very efficient and
cost-effective method of delivering black and white or colour video.
Microwave turns the video and data signals into high radio frequency
signals and transmits them from one point to another via free air and
space. A receiver then converts the transmission back into the video
and data signals and displays the scene on a monitor. Good quality
transmission can be achieved over a line of sight path. Microwave
technology offers a large bandwidth to carry video, however it can be
affected by environmental conditions. It is a practical option when a
wire path between the camera and the monitor locations cannot be
established or is prohibitively expensive. Microwave transmissions are
regulated by the FCC and a license is required.
Radio Frequency
Radio frequency (RF) is a reliable, but short distance, line of sight
video transmission technology. It is becoming increasingly popular
where hardwiring methods are easier impossible or impractical, and has
been used successfully to reduce cabling costs even within large
buildings. Environment conditions or other RF in the area can affect
it.
The Monitor
The
monitor receives the transmitted electronic video signal from the
camera from the paints it across a cathode ray tube (CRT) to display an
image to a viewer. Although similar in function to a TV, a CCTV monitor
provides higher lines of resolution (better picture quality) and
accepts only video signals rather than RF/antenna signals.
Lines of resolution refers to the total number of horizontal lines the
camera or monitor is able to reproduce. The more lines on a screen, the
better or sharper the video picture will appear. CCTV monitors can
provide up to 1000 lines of resolution compared to an average of 300
lines provided by some TV sets.
Several factors can affect the monitoring function: Size of the
monitor, its positioning and angle relative to the viewer, and the
quality (resolution) of the monitor itself. In all cases, sufficient
growth must be factored into any console design. It's also important to
note that all monitors generate heat. Whether on a table or enclosed in
a console, be sure to provide adequate ventilation and
air-conditioning.
Most CCTV systems use both dedicated monitors and call-up (switchable)
monitors. A dedicated monitor displays the video from only one camera.
A call-up or switchable monitor enables the operator to call or switch
different or multiple cameras into view. Generally call-up/switchable
monitors are larger than dedicated monitors and give operators the
ability to view multiple images simultaneously (multiplexed) as well as
scrutinize the camera image more closely.
There are many different monitor sizes available. When choosing the
proper size of monitor, you must first determine the distance of the
monitor in relationship to the user. Also determine the quantity of
cameras to be displayed on a given monitor simultaneously
(multiplexed).
The peripherals
As the number of cameras and monitors increase, simple system design
eventually gives way to more complex designs that require peripheral
components. The peripheral components may include switchers, VCRs,
Multiplexors, quads, video printers and time date generators.
Switchers
A
video switcher enables different cameras to be switched to call-up
monitors. In a smaller, cost-conscious applications, a manual switcher
allows users to select the camera they want to see by pressing the
appropriate button. The most popular type of switcher, a sequential
switcher, contains circuitry that will switch one camera to another
automatically. The operator can set the length of time (dwell time)
that a scene remains on the monitor before sequencing automatically to
the next camera. This allows operators to keep tabs on numerous cameras
with only one monitor, but also creates a drawback know as "switcher
dilemma".
To illustrate switcher dilemma, imagine a system with eight cameras,
each programmed to switch after dwelling on the monitor for 5 seconds.
In this scenario, a considerable gap will occur between the time the
first image is displayed and the time the eighth image is displayed.
The situation worsens when recoding the video for review at a later
time. On playback, you may see a door opening on camera 1, then
suddenly the video switches to camera 2, followed by camera 3 and so
on. By the time camera 1 appears again, the door is closed and you are
left wondering who came through the door while camera 2 through 8 were
flashing sequentially on the monitor.
Matrix Switcher
A
matrix switcher is a more complex design enabling the user to switch
any video signal to any call-up monitor in a large-scale system. They
normally incorporate PTZ control and other features such as preset and
alarm inputs and outputs.
Multiplexers
Unlike
conventional recording systems, a video Multiplexors collects
full-screen pictures from up to 16 cameras and displays them
simultaneously on a monitor. Operators have the option of displaying
any camera full screen or multiple cameras in reduced sizes.
Multiplexors also can record all cameras in the system onto a single
videotape. The cameras are recorded sequentially at a high rate of
speed. As mentioned earlier, a standard video signal is comprised of 30
separate frames each second. In a video system containing 15 cameras,
the Multiplexors selects two frames from each camera and records them
to a single videotape. The result is an effective frame rate of 2
frames per second, instead of the standard 30.
Most Multiplexors today contain motion detection features that enables
the system to record more frames of video from cameras showing motion
than from those not showing any motion. The Multiplexor does this by
reallocating frames from one camera to another as needed. The net
result is higher quality recordings of scenes that are more likely to
be important to security personnel.
When a time lapse VCR is used with a multiplexor, the recording mode
should be as short as possible to reduce the number of seconds required
to record all the cameras. This is why it is a great advantage to use
hi-density or virtual real-time recorders when using Multiplexors.
Virtual real-time VCRs record 4 times the frames per second of
conventional time-lapse VCRs.
One of the strongest advantages of using Multiplexors is that during
playback the multiplexor decodes the tape allowing investigators to
display only selected frames with the same address. This pullout
feature saves investigators hours of time reviewing recorded actions.
Another advantage is that during playback, any desired camera can be
displayed full screen. (duplexing)
Quads
The
main feature of a quad is the ability to compress images from four
separate cameras and simultaneously display them on a single monitor
screen. When four cameras are displayed, each occupies a quarter of the
screen. A single camera can be selected and displayed full screen as
well. Unlike Multiplexors recording, quad recordings yield only what
appears on the monitor at the time of recording. If the VCR is
recording in quad mode, then the playback is in quad mode.
Recording CCTV
Most systems now use Digital Video recording direct to hard drive. Long
recording periods can be achieved by using the right video compression
the following types are used:
- MPEG4
- JPEG2000
- Wavelet
- MPEG
- H.265
- MJPEG
Recording times can be increased with the combination of Video motion
detection , selectable frames rates, and the right video compression.
Archived images are normally saved to CDR or DVD complete with
watermark to prevent unauthorised tampering of the evidence.
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