SYSTEM CONSIDERATIONS
The performance of various camera-lens combinations at
different light levels may be obtained from the manufacturer’s
data. For optimum performance, the
requires a
moderately noise-free video signal at industry standard voltage
levels. The camera-lens combination should be selected more
conservatively than is normal in installation not using motion
detectors.
Iris Control Method Some auto-iris lenses exhibit
characteristics requiring special consideration when used with
video motion detectors. With these lenses. large light or dark
objects, moving in the field of view, near but outside the
sensitive window area of the motion detector, may open or
close the lens rapidly enough to cause undesired alarms.
When such a lens is used, place the camera in such a way that
normal movements in the scene do not cause the auto-iris to
open or close rapidly. This can usually be accomplished by
ensuring that there are no light areas of the scene which could
be blocked by permissible movements. Mounting
camera
high enough to obtain a neutral scene background will usually
suffice.
The automatic iris lens should be adjusted for “average”, rather
than “peak” response to avoid over-reaction of the lens to very
small bright objects. When adjustable, the time delay of the
auto-iris should be set as long as possible, to avoid short
transient
of the iris to
light
Some auto-iris lenses are
to “hunting” (a tendency to
continuously open and close a small amount while searching
for the optimum iris aperture).
effect is often visible in
the image as a slowly pulsating overall brightness of the
monitor image. The
tolerates a small amount of
lens hunting. Excessive hunting usually indicates a problem
with the iris control mechanism which should be corrected.
Detectable Targets
The detectability of a moving object is determined by its size,
contrast with the background, and speed of motion. Once an
object appears larger than approximately 3% of the monitor
screen dimension, or 0.1% of the screen area, its detectability
will not increase greatly. Therefore, the best trade-off of area
coverage against sensitivity occurs when
lens and camera
position are selected to give this size for objects of interest.
When maximum detection probability is desired a slightly
longer focal length lens may be used.
In order to detect an object it must have some visual contrast
with the scene background. Contrast may be enhanced by
improved lighting and by positioning the camera so that the
targets appear against a background of contrasting color. For
most targets. a light background provides the highest contrast.
A target may move so slowly that its motion is not perceived
by
unit. A longer focal length lens magnifies both size and
apparent speed of motion.
A target may move through the scene so rapidly that its
presence is missed by the camera, and thus by the system.
This condition is very unusual but may often be corrected by
camera positioning and lens selection.
In any given installation, these factors should be determined
during installation planning. If necessary, experiments under
actual operating conditions should be conducted.
False Alarms and Misses
Common causes of excessive false alarms are: sensitivity and
retention settings too high, loose or defective coaxial video
connectors or cables, spurious moving objects or lights in the
camera
of view, camera
setting too high, unnoticed
shadows, flickering fluorescent lights: etc.
Common causes of excessive misses are: sensitivity and
retention settings too low and adjacency too high, object in
motion is too fast or too slow, object in motion is too small to
effect sufficient video change, etc.
If
alarms and misses
to he a problem, reexamine
the entire installation. Installation changes to improve
performance may be possible. Such changes may include:
relocation of the camera, change of lens, change of lighting, or
addition of other cameras and motion detectors at different
locations to cover a wider variety of motions.
Further discussion of common scene problems that contribute
to false alarms and misses is provided in the section on Scene
Management.
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