Glossary of Common Terms relating to Digital Video Surveillance
 

ABERRATION - Any inherent deficiency of a lens or optical system which is responsible for imperfections in shape or sharpness of the image.

ANGLE OF VIEW - May be expressed in Diagonal, Horizontal or, Vertical. Smaller focal lengths give a wider angle of view

APERTURE

The opening of a lens that controls the amount of light reaching the
surface of the pickup device. The size of the aperture is controlled by the
iris adjustment. By increasing the F-stop number (F/1.4, F/1.8, F/2.8
etc.) less light is permitted to pass to the pickup device.

APERTURE SCALE - The aperture scale is referred to as a F-number. The international aperture scale is: F1, F1.4, F2, F2.8, F4, F4.6, F8, F11, F16, etc.

ASPHERICAL LENS - A lens which one or more of the elements have a non-spherical surface. Aspherical surfaces are shaped to reduce the spherical and other aberrations.

AUTOMATIC GAIN CONTROL (AGC)

A section in an electronic circuit that has feedback and regulates a
certain voltage level to fall within predetermined margins.

AUTO IRIS

An automatic method of varying the size of a lens aperture in response to
changes in scene illumination.

AUTO-IRIS LENS - A lens with a electronically controlled iris. This allows the lens to maintain one light level throughout varying light conditions. 

BACK FOCUS - Back focus is a term used to describe the relationship of the distance of the lens to the image device. This distance is critical to maintain the proper depth of field through changing focal lengths and varying light conditions. The correct back focus is normally achieved by adjusting the image pick-up device on the camera itself.

BANDWIDTH

The amount of data that may be sent over a wire (commonly called a network) is referred to as a band-with.  At any one time this data is measured in Kilobits, Megabits or Gigabits.  As a short hand the first letter of each name is used.  An example would be that Kilobits is shortened to “K”. This measurement refers to the number of “on and off signals” that can be sent per second. The average household modem used to connect to the Internet operates at 28.8Kbps, which is 28,800,000 bits per second. Few, if any companies operate on a household modem and we can apply this measure to the standard office network. The average network operates at 10 Megabits, which is 10,000,000.00 (ten million) bits per second. That is a big number and a lot of data traveling at one time. This number is generally enough for 50 to a 100 average users or 10 to 15 people playing Real-Time Radio over the Internet.

BACKLIGHT COMPENSATION

When filming in directions with backlighting such as entrance-ways or
windows where the backlighting causes the subject to appear as a
silhouette, this function automatically adjusts to the most appropriate
aperture.

CAMERA FORMAT - The approximate size of a camera image pickup device. This measurement is derived from the diagonal line of a chip or the diameter of the tube. Currently there are five format sizes in the CCTV industry 1", 2/3", ½", 1/3" and, ¼"

CHARGE-COUPLED DEVICE (CCD)

The new age imaging device, replacing the old tubes. When first invented
in the 1970's, it was initially intended to be used as a memory device.
Most often used in cameras, but also in telecine, fax machines, scanners,
etc.

CLOSED CIRCUIT TELEVISION (CCTV)

Television system intended for only a limited number of viewers, as
opposed to broadcast TV. A CCTV Camera is a unit containing an imaging
device that produces a video signal in the basic bandwidth.

C-MOUNT

The first standard for CCTV lens screw mounting. It is defined with the
thread of 1"(2.54mm) in diameter and 32 threads/inch, and the back
flange-to-CCD distance of 17.526mm (0.69"). The C-mount description
applies to both lenses and cameras. C-mount lenses can be both, C-mount
and CS-mount cameras, only in the later case an adaptor is required.

CS-MOUNT

A newer standard for lens mounting. It uses the same physical thread as
the C-mount, but the back flange-to-CCD distance is reduced to 12.5mm
in order to have the lenses made smaller, more compact and less
expensive. CS-mount lenses can only be used on CS-mount cameras.

DC TYPE AUTO-IRIS - Auto-iris lenses where the iris is controlled by the circuitry of the camera.

DEPTH OF FIELD - The regions in front of and behind the focused distance where the image remains in focus. With a greater the depth of field, more of the scene near to far is in focus. Lens aperture and scene lighting will greatly influence the D.O.F.
 

F-NUMBER

In lenses with adjustable iris, the maximum iris opening is expressed as a
ratioffocal length of the lens)/(maximum diameter of aperture). This maximum
iris will be engraved on the front ring of the lens. the brightness of the image formed by the lens, controlled by the iris. The smaller the F-number the brighter the image.

F-STOP - A term used to indicate the speed of a lens. The smaller the F-number the greater amount of light passes through the lens.

FIELD OF VIEW - The horizontal or vertical scene size at a given length from the camera to the subject.

FOCAL LENGTH - The distance from the center of the lens to a plane at which point a sharp image of an object viewed at an infinite position. The focal length determines the size of the image and angle of FOV seen by the camera through the lens. This is the center of the lens to the image pickup device.

HUNTING - An industry term used to describe a auto-iris lenses inability to stabilize under certain light conditions.
 

GAIN

Any increase or decrease in strength of an electrical signal. Gain is measured in
terms of decibels or number times of magnification.

IRIS - A mechanical diaphragm which can be controlled manually or automatically to adjust the lens aperture. 

LENS FORMAT - The approximate size of a lens projected image. In most cases the lens will project a image slightly greater than the designated image size to insure the pickup device is completely covered. It is recommended that camera and lenses are the same format size. A lens a larger format size can be used on a smaller format camera, however a smaller format lens should never be used with a larger format camera. 

LENS SPEED - Refers to the lens aperture or its ability to transmit light. This is measured in F-stops.  

LEVEL CONTROL - Used to set the auto-iris circuit to a video level desired by the user. Turning the level potentiometer towards the HIGH position will open the iris allowing more light to pass through the lens, towards the LOW will close the iris allowing less light to pass through the lens. 

MANUAL IRIS LENS - A lens with a manual adjustment to set the iris opening (aperture) to a fixed position. This type lens is generally used in fixed lighting conditions.

MINIMUM OBJECT DISTANCE (M.O.D.) - The closest distance a given lens will be able to focus upon a object. Generally the smaller the focal length the shorter the M.O.D. This distance can be altered with use of extension tubes. 

MINIMUM SUBJECT ILLUMINATION

This is minimum illumination required for recognizing the subject. This is
generally 2 to 6 lux for color cameras, and 0.1 tux for black and white cameras.
Also, minimum subject illumination changes according to the F value of
combined lenses. With F1.2 lenses and F1.4 lenses, combined with an F1.2 lens
minimum subject illumination is reduced, so that imaging is possible In dark
locations.

PINHOLE LENS

A fixed focal length lens, for viewing through a very small aperture, used in
discrete surveillance situations. The lens normally has no focusing control but
offers a choice of iris functions.

PRE-POSITION LENSES - Pre-position lenses are specially designed lenses with extra mechanical/electrical components to allow for computer interfacing. This function allows the lens (when used with the appropriate controller) to feedback to the controller information relevant to zoom and focus propositioning allowing the controller to quickly scan to a preselected scene, arriving in focus at the proper zoom point without operator intervention. 

RESOLUTION

A measure of the ability of a camera or television system to reproduce detail.
The number of picture elements that can be reproduced with good definition.

SIGNAL-TO NOISE RATIO (S/N)

An S/N ratio can be given for the luminance signal, chrominance signal and
audio signal. The S/N ratio is the ratio of noise to actual total signal, and it
shows how much higher the signal level is than the level of noise. It is expressed
in decibels (dB). The bigger the value is, the crisper and clearer the picture and
sound will be during playback. An S/N ratio is calculated with the logarithm of
the normal signal and the noise RMS value.
 

SYNC

This is the timing used by the security camera when imaging (synchronizing).
There are sync systems that synchronize with the internal sync signal circuit
(internal sync.), or that synchronize with a sync signal input to the security
camera from outside (external sync.), or those that use the cycle of the
camera's AC power to synchronize (line lock). When switching between several
security cameras for surveillance using one monitor, synchronizing the cameras
is necessary. For this, the line lock and external sync systems are used.

WHITE-BALANCE (COLOR CAMERAS ONLY)

Sometimes, the color of clothes seen under lighting inside appears green when
viewed in sunlight outside. This is due to variation in the color temperature of
the light source. The human eye is highly adaptable to variation in the color
temperature of light sources so that even when the light source changes, white
appears as white. CCD adaptability is poor, and to reproduce unbalanced white
when the light source changes, correction of the image signal is necessary.