SYSTEM CONSIDERATIONS
SYSTEM CONSIDERATIONS
Motion detectors operate on the principle that any change in
the scene from a camera is caused by some moving object.
They monitor the content of a camera scene within a sensitized
area and detect differences in the scene content in that area
from one TV frame to the next. Moving objects in the
sensitized area in the field of view cause video changes in that
area which, depending on the setting of the sensitivity, cause
alerts. However, particularly in outdoor installations, normal
motions such as blowing leaves, small animals, birds, or clouds
may cause unwanted alerts. 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 the video signal by improper cabling or poor
grounding will also cause changes in the scene content and
produce false alerts. The sensitivity adjustment establishes the
amount of change in scene content that is required within the
sensitized area before the unit produces an alert. Reducing the
sensitivity will reduce unwanted or false alerts. However,
sensitivity to desired alert conditions is then also reduced.
Other important system considerations for achieving optimum
motion detector performance are discussed below:
Camera Selection
The 1461 may be used with both
2:l
interlace and random
interlace cameras. No external sync drive from the camera is
.:
required. These features are unusual in motion detector
systems.
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 will give a lower level of
performance than 2: 1 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 will require a slightly lower sensitivity setting to
minimize false alerts.
Lens Selection
Video motion detector systems are affected primarily by three
lens parameters: Focal length, Aperture, and Iris Control
Method.
Focal Leneth
-
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 more 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. They 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
difficult for the motion detector to sense. There is, therefore, a
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
tube. Tables or formulas are usually available from the camera
or lens supplier.
These factors can affect the number of cameras needed in a
given installation. They should be determined by calculation
or experiment during the initial design of the video system.
Lens Aperture
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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 drops, the possibility of both
increased picture noise and reduced video signal increases.
Increased picture noise, especially low frequency noise,
increases the probability of false alerts and may necessitate a
reduction of the sensitivity setting. 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.
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