Lighting Details
Formats/Resolution
Focal Length/Field Of View
FAQ's
Tips/Guides
Need more information on video
surveillance? Can't decide what camera
will best fit your application? Below
are several excerpts from a research
report from the
National
Criminal Justice Reference Service
that may assist you in your decisions.
CCTV Basics
The first thing you
should ask yourself before purchasing a
security surveillance system is: What do
I need this system to do? Do you need
detection of an incident only,
or do you need to identify the
object in question. Keep your answer in
mind when reading this document.
The second thing you
should ask yourself before purchasing a
system is: What is my application? The
most common applications are: security
applications, safety applications, and
management applications.
Many elements must be considered
before designing your surveillance
system.
Without proper equipment and studying
all these variables, you may be deceived
on what really is causing poor imaging.
For example, what might look like a poor
camera, may be a poor monitor. You can't
simply take a high resolution camera and
expect its high performance to be
visible on a poor monitor or display.
Each component in the system will affect
the overall performance. Your overall
quality is only as good as the weakest
component in your system.
Scene and Lighting
The scene refers to the objects or area
to be observed as well as the
environment in which it will be
observed. One important aspect that must
be considered is the environment. The
environment contains multiple colors,
materials, reflective surfaces, and
varying degrees of light within the
picture. To select the proper equipment
you must determine the amount of light
present during peak times of operation.
The amount of light on the scene
determines everything from picture
clarity to focus. What is the minimum
light that will be available? Will it be
more cost efficient to go with a better
night-view camera or adding more
artificial lighting to the scene?
For more detailed information on
lighting please click here
The Cameras
A camera's performance largely depends
on the amount of light present, as well
as the imager used. When the level of
light changes dramatically, usually a
camera equipped with automatic iris
control can help ensure consistent image
quality. Auto iris enables a camera to
open or close its lens accordingly to
the varying levels of light, limiting or
increasing the amount of light passing
onto the sensor. Cameras are available
in several imaging formats expressed as
1/2, 1/3, 1/4 inches. These are the
sizes of the imager used. Generally
speaking, you should match the camera's
format with the len's format (ex. 1/3"
sensor with 1/3" lens). It is crucial to
understand a camera's format,
resolution, and corresponding lens focal
length when determining what camera will
best suit your needs.
For a more technical discussion of
formats, resolution, and pixels please
click here
The Lens
The lens plays a large role in a
system's design. The primary function of
the lens is to collect light from a
scene, focus the image to produce a
sharp image on the camera's imager.
Selection of a lens is critical. The
lens directly affects the size, shape,
and sharpness of the image to be
displayed on the imager. Factors such as
distance to the scene, focal length,
desired field of view, lighting, and
format affect the size and clarity of
the image.
The field of view (FOV)
is the actual picture size
(height/width) produced by a specific
lens. If the view is not suitable,
consider a different lens to increase or
decrease the field of view.
Camera lenses are
divided into two major categories: fixed
and varifocal (manual zoom). A fixed
lens obviously has a fixed focal length,
while a varifocal lens enables the user
to change its focal length to produce a
zooming effect (narrowing the FOV).
Focal length is the distance from the
optical center of the lens to the focal
point near the back of the lens. This
focal length distance is displayed on
the lens (in millimeters). A lens with a
focal length of 8mm on a 1/3" camera
produces a field of view similar to the
view produced by the human eye. A
wide-angle lens has a short focal
length, while a telephoto lens has a
long focal length. In order to change
the field of view, you must change the
lens.
The ability of a lens to
gather light depends on the relationship
between the lens opening (aperture) and
focal length. This relationship is
symbolized as the letter F, also know as
F-stop. The lower the F-stop number, the
larger the aperture, thus the greater
ability to pass light through the lens
to the camera's imaging device. For
example, a lens with an F-stop of F1.2
can gather much more light than a lens
with an F-stop of F4.0. A lens with a
low F-stop number is sometimes referred
as a "fast" lens.
Depth of field is
another consideration when determining
the proper lens. Depth of field is the
area in focus ahead of and behind the
main object. When you focus on a
particular object there is an amount of
area behind the object and in front of
the object that will still be in focus,
although not as sharp. Depth of field
increases or decreases based on the
length of the lens, the len's aperture,
and distance from the camera to the
subject.
- Lens length
Shorter lens (ex. wide angle) =
longer depth of field
Longer lens (ex. 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
If depth of field is important
consider increasing artificial lighting
or install cameras with normal angle
lenses.
Camera lenses generally come with a
C-mount of CS-mount and must be matched
appropriately to the camera's mounting
requirements. The difference is the
distance from the lens' actual optics
and the camera's imaging device. The
C-mount lens is 17.5mm from the imaging
device, whereas the CS-mount lens is
12.5mm away. A C-mount lens can be used
on a CS-mount camera only if a 5mm
spacer ring is added (this is why some
cameras allow C or CS mount lenses, they
just include a spacer). Though, a
CS-mount lens cannot be used on any
C-mount camera.
For more technical information on
lenses, focal length, and field of view
click here
Video
Transmission Methods
There are many transmission methods that
exist today. The purpose of the
transmission medium is to carry the
video signal from the camera to the
monitor or other device. The most common
mediums include: coaxial cable, fiber
optic, CAT5 cabling, phone lines,
microwave, and radio frequency. The
choice of determining which medium to
use depends on many factors including
distance, environment, cost, and
facility layout.
Coaxial cable is the most popular in
the CCTV industry. The cable, preferably
copper, is shielded to minimize
interference from any nearby electronic
devices or electrical wires. It is the
most economical solution for short run
applications. Usually no longer than 300
feet. This type of cable is used for
direct connections with no special
conversions.
Fiber optic transmission technologies
convert an electronic analog signal into
a digital signal using a series of light
pulses or lasers. The medium that
carries these light signals come either
in plastic or glass rods. Fiber optic
transmission is unaffected from almost
any type of interference. Fiber optics
have a large signal capacity (bandwidth)
and have no possibly for spark. Fiber
optic cabling offers a cost-effective
method for sending large amounts of data
over long distances (miles). Special
conversions and devices are needed to
facilitate this type of media
transmission.
Telephone line is a standard twisted
pair of wires that can transmit signal
up to 1 kilometer. It is possible to use
standard telephone lines for video
transmissions with the use of
specialized transmission and receiver
equipment.
If already in place, microwave can be
a very efficient and cost-effective
method of delivering black & white or
color 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
monitor locations cannot be established
or is prohibitively expensive.
Microwave transmission is regulated by
the FCC, and a license is required.
Radio Frequency (RF) is a reliable, but
short distance, line-of-sight video
transmission technology. It is becoming
increasingly popular where hardwiring
methods are either impossible or
impractical, and has been used
successfully to reduce cabling costs
even within large buildings.
Environmental conditions or other RF in
the area can affect it.
The Monitor
The monitor receives the transmitted
electronic video signal from the camera
and paints it across a cathode ray tube
(CRT) to display an image to a viewer.
Although similar in function to a TV
set, 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 television sets.
Peripherals
There are many devices on the market
today to bring multiple video signals
and channel them through one device,
either enabling multiple channel viewing
on a single display, sequencing multiple
channels on a single display, recording
capabilities, and many other features.
The most common include quad processors
or splitters, digital video recorders,
and time lapse VCRs.
Quad processors allow up to four
cameras to be displayed on a single
screen. Each camera will occupy a
quarter of the screen and a single
camera can be selected to display full
screen. If a VCR or recording device is
attached to the quad, then on playback
you will see all four screens. Some quad
processors have additional features,
such as motion detection that triggers a
recording device to record only when
motion has occurred within the images.
Overview of Commercial DVRs
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Commercial
DVRs operate under the same
principles as their residential
counterparts, with the main
difference being (1) the DVRs
storage capacity, (2) its
features (which will depend upon
the software), and (3) the DVRs
ability to record multiple video
inputs. While a residential DVR
will only have a single video
input (the cable or satellite
signal), commercial DVRs are
used to record video from
multiple security cameras, and
so they will have anywhere from
4 to 32 video inputs. To process
so many video signals requires a
specialized piece of hardware,
which is called the 'video
capture card'. The DVR also
requires specialized software to
manage the multiple video
signals, and to provide the
interface for viewing,
searching, copying, and
otherwise interacting with the
video. These 2 components, the
'video capture card' and the
specialized DVR software, are
the unique components that make
the DVR something more than a
common computer.
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VCRs have
traditionally been used in video
surveillance, and they have
become an integral part of
almost every security
department. There have always
been 2 main deficiencies of VCRs
however. The first is the fact
that a VCR can only input a
single video signal. To get
around this, devices called 'multipexers'
and 'quads' were invented. A
multiplexer can take in up to 16
video signals and a quad can
take in 4 (there are also other
slight differences between quads
and multiplexers), but since the
2 devices are nearly identical,
lets just look at multiplexers.
The multiplexer combines the 16
camera signals into a single
video signal, and that signal is
sent to the VCR. The resulting
video signal, or video output,
looks like a 4 by 4 grid. So if
you are looking at a monitor
with a muliplexed signal coming
into it, you will see all 16
video signals (or however many
cameras you have, between 1 and
16), but each video signal will
only take up a small portion of
the screen. The limitations here
should be obvious, it becomes
difficult to see the video in
such a small area, and any
copying off or searching of
video becomes complicated
because all of the video is
recorded onto the same VHS tape.
Commercial DVRs 'fix' these
problems, by recording each
camera signal independently from
the other camera signals. This
means that even though you may
have 16 cameras all being
recorded to the same DVR, each
camera can be searched, copied
off, and otherwise interacted
with independently. Also, each
video signal can be set up
differently, with its own
lighting, color, motion, alarm,
and resolution settings.
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The second
main limitation of the VCR
involves the recording media, or
VCR tape. Each tape can record
only hours worth of video, and
then it must be replaced or
recorded over. The only way
around this is to use a
'time-lapse' enabled VCRs, which
only record images once every
few seconds, and can be combined
with alarms so that the
recording time can be measured
in days rather than hours. These
VCRs have there own limitations
however, including the fact that
they are severely limited on
image quality, and are only
useful in very specialized
situations. If using any type of
VCR, some amount of employee
time must be dedicated to
interacting with the system, to
either change tapes, or to
search through tapes for past
incidents. With a DVR, the video
footage is recorded to a hard
drive, or to multiple hard
drives. With today's large hard
drive capacity, it is easy to
record weeks, months, and
sometimes even years of video
footage onto a single DVR (see
Table 1 for DVR recording
times). In addition to longer
recording times, recorded video
from any camera, and from any
date and time, can be viewed
within seconds. With many of
today's higher end DVRs, the
video can also be accessed
remotely via the Internet.
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So are there
any downsides or limitations to
DVRs? The short answer is 'yes'.
The main limitation involves the
quality at which you record
video, which is determined by 2
things, image resolution and FPS
(frames per second). With a VCR,
the image quality is
pre-determined by the VCR,
usually corresponding to between
240 and 400 lines of television
resolution, and from between 25
to 30 FPS (except of course with
a time-lapse VCR, which may only
record 1 FPS or less). With a
DVR the image quality is
adjustable, with typical
resolution settings of either
medium (204x352 pixels) or high
(408x704 pixels), and anywhere
from between 1 to 30 FPS. While
a DVR can actually record video
that is of much better quality
than a VCR, there is a trade-off
between the video quality and
the amount of time that the DVR
will be able to record. The
resolution settings of VCRs and
DVRs are also a little hard to
compare, with all of the
differences between analog and
digital factored in, but a good
rule of thumb is that a VCRs
resolution will lie somewhere
between the medium and high
resolution of a DVR. Another
limitation of the DVR is the
maximum number of frames per
second that it can process
overall (which is determined by
the 'video capture card'). This
number is then divided by the
total number of camera signals.
For example, a DVR with a
maximum FPS of 120 can record 4
cameras at 30 fps, 8 cameras at
15fps, and 16 cameras at 7fps.
For most commercial applications
a setting of 3 FPS to 5 FPS per
camera is usually acceptable
(think of taking 5 pictures for
every second), whereas for more
critical video, say a casino
blackjack table, a setting of 15
FPS to 20 FPS may be required.
The FPS settings must also be
balanced with the amount of time
that needs be recorded, if a DVR
can record 1 month of video with
10 cameras set at 8 FPS, then
that same DVR can record for 2
months if all 10 cameras are set
to 4 FPS. Of course the FPS of
each camera can be set
independently from the others,
so that more critical cameras
can be set to a higher FPS if
desired. The following table may
give you a better idea of how
FPS, resolution, and hard drive
capacity are all related.
Table 1. DVR
recording capacities
*The following
are rough approximations of DVR
storage times
| # cameras |
FPS |
resolution |
HardDrive capacity |
storage time |
| 4 |
3 |
normal |
100GB |
1 month |
| 4 |
5 |
normal |
200GB |
1 month |
| 4 |
30 |
normal |
900GB |
1 month |
| 8 |
3 |
normal |
400GB |
2 months |
| 8 |
10 |
normal |
400GB |
3 weeks |
| 8 |
3 |
high |
400GB |
3 weeks |
| 8 |
5 |
normal |
800GB |
2 months |
| 16 |
4 |
normal |
800GB |
2 to 3 months |
| 16 |
2 |
normal |
800GB |
4 to 6 months |
| 16 |
4 |
high |
800GB |
1 month |
| 16 |
10 |
normal |
1.2TB |
1 to 2 months |
| 16 |
1 |
normal |
800GB |
1 year |
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| There are
several different standards for
video compression, which factor
into the amount of time that a
DVR can record. The steadfast
rule is, the greater the
compression, the greater the
loss of image quality. The table
above is approximated using the
MPEG-2 compression standard,
which is the most widely used.
With the new MPEG-4 and High End
JPEG compression used today,
recording times will differ
greatly from the above chart. |
