Automatic Gain Control (AGC)This is a feature of many CCTV
cameras that helps keep the strength of the output signal constant,
even when the light level changes. In other words, it boosts the
signal strength at low light levels, and caps it at higher levels.
Auto Iris (AI)Cameras with an Auto Iris feature, have the
ability to compensate for large variations in light levels.
Particularly useful for cameras that need to compensate for changes
from bright sunlight to dark shadows. The auto iris circuitry is
normally linked to a motorised iris drive that physically opens and
shuts the iris on the lens. Closing a physical iris is a much better
way to protect a camera from being damaged by bright sunlight then
simply using electronics to reduce the signal strenth.
Alarm InputSome cameras and video servers have the ability
to accept alarm inputs. These are inputs from standard sensor
devices such as, Passive Infra-red (PIR) detectors, door contacts,
active beams etc. or relay outputs from intruder alarm panels,
or access control panels. Alarm input circuits can come in a
number of forms. Most commonly they are self powered, so will
operate with a passive switch style circuit. However, in some cases,
they expect to be provided with power (normally 12V) as the signal
of open or closed. In a few cases manufactures only supply TTL (very
low voltage) level inputs. In that case, they can only be safely
used with normal alarm sensors in conjunction with some additional
isolation circuitry. In this case, it is best to contact the
manufacturer for advice.
Ambient Light LevelThis refers to the amount of background
light in any given situation.
Analogue SignalMost CCTV cameras, although internally
working with digital components, produce a standard analogue signal.
This is where the image is represented by a variable voltage level
and frequency timings. In the UK and Europe, the standard for the
analogue signal output from CCTV cameras is called PAL. In the USA,
the standard used is called NTSC.
Angle of ViewThe table below gives an approximate value for
the angle of the field of view for lenses of various focal lengths.
30˚ is considered to be a normal view, telephoto (longer) lenses
have lower angles. Most CCTV cameras have one of the 3 sizes of imaging
devices listed below, 1/4", 1/3" or 1/2". As you can see, this makes
a big impact when choosing lenses.
Angle of View
ApertureThe aperture of a lens, is the area that allows the
light to pass into the lens. Lenses with large apertures allow more
light in than lenses with small apertures.
Aspect RatioThis is the ratio between the the height and
width of an image.
Automatic Level Control (ALC)This is a feature of some
cameras, that allows the camera to either bring out detail in bright
areas of the image, or bring out more detail in the darker areas,
depending on how it is set up. This is often useful where the view
has a window in the background, where the sunlight is causing the
image of the room to be much darker than required.
Automatic Terminating (Auto-terminating)Video signals are
normally transmitted along co-axial cable, which require a
terminating resistor at either end. If the signal is looped through
a piece of equipment that has an input and an output, then it should
be terminated if it is the final piece of equipment, but not
terminated if it is in between other equipment. Some appliances have
a manual switch for this setting, other equipment automatically
detects if termination is required and applies the additional
resistor as necessary.
Automatic White BalanceThis is a feature of some cameras
that automatically adjusts the colour settings to maintain the
quality of the white areas of the image.
Back FocusThis is the alignment of the rear of the lens to
the imaging device.
Back Light CompensationThis is a feature of cameras that
automatically adjusts the image to compensate for bright sunlight or
bright lights, to give more detail on the darker areas of the image.
For example to focus on the detail of a face of a person that has
the sunlight shining from behind.
Balanced SignalThis is a method of transmitting audio or
video signals over a pair of wires, often twisted pair cable. By
sending two equal, but opposite signals, this system minimises
external interference, and maintains signal quality.
BalunBalun stands for Balanced - Unbalanced. It is a device
used to interface between balanced lines and unbalanced lines. For
example, twisted pair to co-axial.
BNC ConnectorBNC is a bayonet style connector for coaxial
cable that is most commonly used for CCTV installations.
Charge Coupled Device (CCD)One of the two main types of
image sensing device used in cameras. It operates by converting
light energy into electrical charge.
ChrominanceThis refers to the part of the video signal that
contains the colour information.
C Mount Lens & CS Mount LensThere are two main types of
lens used in CCTV cameras. The C mount lens has a flange back
distance of 17.5mm. The CS mount lens has a flange back distance of
12.5mm. C mount lenses therefore have a longer focal distance. CS
mount became widely used, because it its more practical for many of
today's more compact cameras. Lenses are often supplied with a 5mm
spacer ring (sometimes called a C ring) that allows a C mount lens
to be used on a CS camera. Most modern cameras are CS.
Co-Axial CableThis refers to cable that has a central
conductor, surrounded by a shield sharing the same axis. The shield
can be made from a variety of materials including, braided copper,
or lapped foil. There are various standards for specific types of
co-axial cable. The cable used for normal CCTV installations is
Composite VideoA full video signal that combines picture
signal and synchronisation pulses.
CompressionDigital video pictures can be compressed with a
number of techniques. These include: JPEG and JPEG-2000 (for still
images), M-JPEG and MPEG (for moving pictures).
Digital Video Recorder (DVR)This is a device that records
video signal digitally. Normally this is on a large hard drive in
the machine. For this reason it is sometimes known as a hard disk
Direct Drive (DD)Some auto iris lenses require a DC signal
from the camera. These are known as direct drive lenses.
Depth of FieldDepth of field (also known as depth of focus)
refers to the range of distance from the camera that is in good
focus. The smaller the aperture of the lens, the longer the depth of
field. However, less light coming into the lens, means that the
image will be dimmer, so the camera needs to compensate by
increasing the gain. For example, the best (longest) depth of field
would be obtained on a bright, sunny day, with a very small aperture
on the camera. In this case the imaging device still gets a
reasonable amount of light, even with a small aperture. The worst
(shortest) depth of field would be in a dark room, with a large
aperture. To improve depth of field, the best approach is to
increase the amount of light on the subject, or use a more sensitive
Decibel (db)The decibel is a logarithmic scale. In the
context of CCTV it refers to voltage or signal strength. A 6db
increase is equivalent to doubling the signal strength.
Distribution AmplifierThis is a device that accepts one
video input and sends it out to 2 or more devices. This is often
used in a situation where the signal needs to be split between a
number of video monitors or recording devices. 2 or 3 devices can
often (but not always) be driven just by splitting the signal with a
"T" connector, but more than that, and invariably a distribution
amplifier is required to give an acceptable signal quality.
Digital Versatile Disk (DVD)Sometimes called digital video
disk. This is an optical disk the same size as a CD, used for
storing data of various formats. Including video, audio and computer
Electronic Iris (EI)This is an electronic implementation of
an auto iris. It uses electronics to simulate the effect of opening
and closing the iris, by increasing or decreasing the effective
shutter time of the camera.
External SyncAn external sync allows a piece of equipment
to take its video synchronisation from another unit, so that it can
align itself with the system as a whole.
Focal LengthThe distance between the centre of a lens, or
its secondary principal point and the imaging sensor. Lower lengths
give a greater field of view and less magnification. Longer lengths
give a narrower field of view and greater magnification. The table
below gives an approximate value for the angle of the field of view
for lenses of various focal lengths. 30˚ is considered to be a
normal view, telephoto (longer) lenses have lower angles. Most CCTV
cameras have one of the 3 sizes of imaging devices listed below,
1/4", 1/3" or 1/2". As you can see, this makes a big impact when
FrameThis refers to a full TV picture. The PAL signal
transmits 25 full frames per second.
Gamma CorrectionThis is an adjustment to ensure that
variations in light input, produce a corresponding output in light
GhostThis refers to a poor image, where the primary signal
is weak, allowing a secondary signal to interfere with the picture.
Hertz (Hz)Cycles per second.
ImpedanceThe total opposition offered by a device to the
flow of an alternating current. Measured in Ohms.
InterlacingPAL video signals transmit odd and even lines
alternately. This is a 2:1 interlace. The two sets of lines are
combined to form each single frame.
Internal SyncDevices with internal sync, have an internal
crystal to provide sync pulses, without needing reference from any
Infra Red (IR)Low frequency light below the visible
spectrum. This is often used for covert or semi-covert surveillance
to provide a light source for cameras to record images in dark or
zero light conditions.
IP Waterproof Rating (IP66 - IP68)IP ratings are a BSi
standard measurement for how waterproof something is. Many cameras
or camera housings are designed for outdoor use, and therefore need
to be waterproof to some degree. The details of the tests are
defined in BS EN 60529 : 1992. The IP number has two digits, and
optional letters after them. These have the following meaning:
0, 1, 2, 3, 4, 5, 6
Protection against ingress of
0, 1, 2, 3, 4, 5, 6, 7,
Protection against ingress of
|First Additional Letter
A, B, C,
A means protects against access
with back of hand.
B means protects against access
C means protects against access
D means protects against access
H, M, S, W
See BS EN
For further information or to get a copy of the standard, see the
BSi website at http://www.bsi-global.com/ or
IrisThis is a mechanical device that adjusts to vary the
amount of light passing through the lens of a camera.
JPEGJPEG is a standard for coding/compression of still
pictures. It is used in the CCTV systems to compress and store
individual frames of video. JPEG was developed by the Joint Photographic Experts Group.
Liquid Crystal Display (LCD)This is a technology used for
flat screen displays. Aside from being smaller and lighter, it also
has the advantage of using less power than traditional cathode ray
Line AmplifierThis device takes a weak video signal as its
input and outputs a new stronger version of the same signal. These
are useful/necessary for transmitting video over very long cable
runs. This is sometimes known as a video line corrector.
Line Fed CameraThis refers to the use of equipment that can
provide power to the camera and take the video signal along the same
cable. It normally involves placing an "encoder" unit at one end and
a "decoder" unit at the other. These units effectively combine and
then split the power from the video signal at either end of the
LoopingThis refers to connecting an additional device in
parallel with an existing video cable. For example, when driving a
video recorder as well as a monitor from the same video signal,
simply by T'ing off the cable.
LuxThis is a measure of the amount of light striking a
surface. i.e. the luminus flux density at a surface. One lux is one
lumen per square metre. Cameras for use in good lighting conditions,
or in daylight would normally be rated at 2 Lux or more. Cameras
with a Lux rating of 0.2 Lux or less would be considered low-light
cameras. It is not possible to get good colour definition in low
light levels, so in general low light cameras are always monochrome.
However, day/night cameras use electronics to switch from colour
during the daytime, to monochrome during night or low light
conditions. Many low light cameras are also infra-red sensitive, so
that infra-red illumination can be used. Particularly useful in zero
||Starlight - overcast night|
||Starlight - clear night|
|10,000 - 1,000,000
LuminanceThis refers to the part of a video signal that
carries the monochrome information. i.e. brightness information.
Matrix SwitcherThis is a device that allows any of its
camera inputs to be switched to one or more of its monitor outputs.
The outputs can of course also be video recorders.
Mechanical FocusThis refers to the process of manually
setting the focus on a lens.
MonitorThe device used to view video pictures. These
devices do not normally have television RF frequency receivers. They
normally have composite, or component video inputs.
MonochromeThis refers to a black and white image rather
than a colour one.
MPEGMPEG is a standard used for coding and compression of
moving images. It was developed by the Moving Pictures Experts
Group. It is now used widely for the compression of video
images. However MPEG isn't just one standard. They have developed
several standards for different uses. For example MPEG-2 is used for
DVD's and set top boxes. MPEG-4 was developed for multi-media
applications for fixed and mobile web applications.
MultiplexerThis is a device that takes inputs from 2 or
more video channels and combines them into one signal. This is often
done by using time division multiplexing, which interleaves frames
from each channel in such a way that they can be split out again.
Frequency division multiplexing uses different frequencies to
achieve the separation of the signals.
Network CameraThis refers to a camera that is designed to
record pictures and transmit them directly over a computer network
or dialup internet connection. Network cameras normally do not have
any analogue video outputs. The images are encoded directly in one
of the standard compression techniques, such as JPEG or MPEG.
NTSCThis is standard for TV signals developed by the
National Television Standards Committee in the USA. The UK and
Europe, use a similar, but different standard known as PAL.
PALThis is the standard for TV signals used in the UK. It
stands for Phase Alternating Line.
Pinhole LensThis is a type of lens with a very small
aperture. Normally used for covert applications, where it can easily
hide behind or within another object.
PixelA pixel refers to an individual area on the surface of
the imaging device, normally a CCD. It is made from photosensitive
material which converts light into electrical energy. In the context
of a display monitor, a pixel is also referred to as an individual
area on the surface of the screen which converts electrical energy
to visible light.
Peak to Peak (P-P)This refers to the measurement of voltage
of a signal between the most negative and most positive points.
RollThis is a fault condition on a video signal which
refers to loss of vertical synchronisation. It causes the picture to
continuously move up or down.
SECAMThis is the system used for TV signals in France.
SCARTThis is a standard used in Europe for a 20 pin
connector used to carry video and audio signals. It is most widely
used in domestic TV appliances.
ShutterThe shutter in a camera is the device that controls
the time period for which light is gathered by the collecting
device. Old cameras used mechanical shutters. However modern cameras
use electronic circuits to perform the same function with no moving
Signal to Noise Ratio (S/N Ratio)This is the ratio between
the signal strength and the noise levels on an audio or video
TerminationThis refers to a 75 Ohm terminator that is used
to terminate each end of a video line.
Television Lines (TVL)This is a measure of the resolution
of a video device. Higher number is higher resolution. 380 TVL is
considered medium resolution. 480 TVL or greater is considered high
Varifocal (Zoom)This refers to a type of lens that has the
facility to change the focal length. This allows adjustment of the
magnification and field of view of the camera.
Video Motion DetectionThis is a feature that detects motion
within a video signal. Normally this is used to trigger recording of
images. Advanced video motion detection systems have the facility to
adjust the sensitivity and object size that will trigger the system.
They also allow the image to be blocked out, such that only certain
areas of the image are taken into account when scanning for motion.
Video Server (Network Video Server)This is a device that
accepts inputs from CCTV cameras using the PAL, composite video
signal. It digitises the signal and then transmits images via a
network, dialup or GSM connection. Advanced video servers have
built-in video motion detection, and can buffer images while they
are being uploaded to a remote server, where they are recorded.
WaveletsThis is a compression technique used to give high
compression without degrading the image quality.