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SYSTEM
CONSIDERATIONS
Motion detectors operate on the principle that any change in
the camera scene is caused by a moving object. Motion
detectors monitor the content of a camera scene within a
sensitized area and detect differences in the scene content from
one video frame to the next. Moving objects within the
sensitized area cause video changes in that area which,
depending on the sensitivity settings, cause alarms.
However, particularly in outdoor installations, normal motions
such as blowing leaves, small animals, birds, or clouds may
cause unwanted alarms. Other factors, such as instability in
the camera, rapid changes in light level caused by an auto-iris,
jitter caused by camera interlace, or electrical noise signals
superimposed on me video signal by improper cabling or poor
grounding will also cause changes in the scene content and
produce false alarms. The sensitivity adjustments establish the
amount of change in scene content required within the
sensitized area before the unit produces an alarm. Reducing
the sensitivity reduces unwanted or false alarms. However,
sensitivity to desired alarm conditions is then also reduced.
Other important system considerations for achieving optimum
motion detector performance are discussed below.
Camera Selection
AD1562 DuoTect-C may be used with both 2:1 interlace
The
and random interlace cameras. No external sync drive from
the camera is required.
Nevertheless, camera selection is very important. The selected
cameras should be electrically and mechanically stable, low-
noise, high quality units for best performance.
In general. random interlace cameras give a lower level of
performance than 2:l interlace cameras. Small disturbances of
the image are inherent with random scanning. These
disturbances may be interpreted by the motion detector as
object motions. Consequently, a unit using random interlace
cameras requires slightly lower sensitivity settings to minimize
false alarms.
Lens Selection
Video motion detector systems are affected primarily by three
lens parameters: Focal length, Aperture. and Iris Control
Method.
Focal Lenrth - The focal length of a lens directly affects the
apparent size of objects as seen on the monitor and by the
motion detector. Focal length also affects the overall field of
view of the camera. A long focal length (telephoto) lens
magnifies objects in the scene but covers a smaller area. A
short focal length (wide-angle) lens covers a large overall
scene, but shrinks the apparent size of objects in the scene.
Motion detectors can be affected by lenses of either too short
or too long a focal length.
A long focal length (telephoto) lens is often used outdoors to
view distant areas. These lenses also magnify the effects of
camera motion. When used, the camera must be stably
mounted. Avoid unsteady towers or poles. Telephoto lenses
also magnify the disturbances caused by atmospheric effects,
such as the image "swimming" seen when hot air currents
cause air density changes in the path between the camera and
the scene,
Image interruption by snow, rain, and fog is obviously more
frequent when the camera is very distant from the scene being
viewed. All these factors limit the total distance that can be
covered, the lens focal length and the utility of motion
detectors on outdoor scenes. These factors must be judged by
the installer after determining the operating environment for
each camera.
A short focal length (wide angle) lens is used where the
camera is close to the scene, and a greater field of view is
needed. Objects in the scene appear smaller on the monitor
and to the motion detector. Very small objects are more
limit to the coverage advantage of wide angle lenses.
The field of view and magnification of a given focal length
lens also depends on the image format (size) of the camera.
Tables or formulas are usually available from the camera or
lens supplier.
The above factors can affect the number of cameras needed in
a given installation. This should be determined by calculation
or experiment during the initial design of the video system.
Lens Auerture - The aperture (f-number) of a lens is a measure
of the maximum lens opening available to pass light to the
camera. It is thus also a measure of the relative performance
of a camera/lens combination at low light levels. Low light
level performance is affected by the camera type. Large
apertures are indicated by small f-numbers (for example,
f/l .4).
As the light level on a scene is reduced, the possibility of both
increased picture noise and reduced video signal increases.
Increased picture noise, especially low frequency noise,
increases the probability of false alarms and may necessitate a
reduction of the sensitivity settings. Reduced video signal
directly reduces the apparent sensitivity of the motion detector.
These factors ultimately limit the degree of scene darkness at
which the motion detector will operate.
A
SYSTEM
CONSIDERATIONS
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