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 1461 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 lens, 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 alerts.
When such a lens is used, the camera placement must be such
that normal movements in the scene do not cause the auto-iris
to open or close rapidly. This can
be accomplished by
ensuring that there are no light areas of the scene which could
be blocked by permissible movements. Mounting the 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 reactions of the iris to brief light changes.
-Some auto-iris lenses are subject to “hunting” (a tendency to
continuously open and close a
amount while searching
for the optimum iris aperture). This effect is often visible in
the image as a slowly pulsating overall brightness of the
monitor image: The 1461 will tolerate 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 the 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
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.
B
A target may move so slowly that its motion is not perceived
by the 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 Alerts and Misses
Common causes of excessive false alerts are: sensitivity
control set too high, loose or defective coaxial video
connectors or cables, spurious moving objects or lights in the
camera field of view, camera ALC setting too high, unnoticed
shadows, flickering fluorescent lights, etc.
Common causes of excessive misses are: sensitivity control set
too low, object in motion is too fast or too slow, object in
motion is too small to effect sufficient video change, etc.
If false alerts and misses continue to be a problem, the entire
installation should be reexamined. Installation changes to
improve performance may be possible. Such changes may
include: relocation of the camera, change of lens, change of
lighting, or addition at different locations of other cameras and
motion detectors to cover a wider variety of motions.
Summary of Contents for 1461
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