Справочник Пользователя для Pelco MD2001-X
Pelco Manual C616M-B (11/97)
11
Image interruption by snow, rain, and fog is obviously more frequent when the
camera is a distance from the scene being viewed. All these factors affect the total
distance that can be covered, the lens focal length and the usefulness of motion
detectors on outdoor scenes. They must be judged by the installer after determin-
ing the camera operating environment.
camera is a distance from the scene being viewed. All these factors affect the total
distance that can be covered, the lens focal length and the usefulness of motion
detectors on outdoor scenes. They must be judged by the installer after determin-
ing the camera operating environment.
A short focal length (wide angle) lens is used when 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
when using wide angle lenses.
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
when using wide angle lenses.
The field-of-view and magnification of a focal length lens also depends on the im-
age format (size) of the camera tube. Tables and formulas are often available from
the camera or lens supplier to help determine what lens is best.
age format (size) of the camera tube. Tables and formulas are often available from
the camera or lens supplier to help determine what lens is best.
These factors can affect the number of cameras needed in an installation. They
should be determined by calculation or experimentation during the initial design of
the video system.
should be determined by calculation or experimentation during the initial design of
the video system.
4.7.2 Lens Aperture and Lens Control
The aperture (f-number) of a lens is a measure of the maximum lens opening avail-
able to pass light to the camera. It is also a measure of the relative performance of
a camera and 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/1.4).
able to pass light to the camera. It is also a measure of the relative performance of
a camera and 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/1.4).
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 fre-
quency noise, increases the probability of false alerts and may necessitate a re-
duction of the sensitivity setting. Reduced video signal directly reduces the appar-
ent sensitivity of the motion detector. These factors limit the degree of scene dark-
ness at which the motion detector will operate.
and reduced video signal increases. Increased picture noise, especially low fre-
quency noise, increases the probability of false alerts and may necessitate a re-
duction of the sensitivity setting. Reduced video signal directly reduces the appar-
ent sensitivity of the motion detector. These factors limit the degree of scene dark-
ness at which the motion detector will operate.
The performance of various camera and lens combinations at different light levels
may be obtained from the manufacturer’s data. For optimum performance, the
MD2001 requires a moderately noise-free video signal at industry standard voltage
levels. The camera and lens combination should be selected more conservatively
than is normal in installations not using motion detectors.
may be obtained from the manufacturer’s data. For optimum performance, the
MD2001 requires a moderately noise-free video signal at industry standard voltage
levels. The camera and lens combination should be selected more conservatively
than is normal in installations not using motion detectors.
Camera placement must be such that normal movements in the scene do not cause
the lens auto-iris to open or close rapidly. This can usually be accomplished by
mounting the camera high enough to obtain a neutral background scene.
the lens auto-iris to open or close rapidly. This can usually be accomplished by
mounting the camera high enough to obtain a neutral background scene.
The automatic iris lens should be adjusted for “average”, rather than “peak”, re-
sponse to avoid overreaction of the iris to brief light changes. When adjustable, the
time delay of the auto-iris should be set as long as possible.
sponse to avoid overreaction of the iris to brief light changes. When adjustable, the
time delay of the auto-iris should be set as long as possible.
Some auto-iris lenses are subject to “hunting” (a tendency to continuously open
and close a small amount while searching for the optimum iris aperture). This effect
is often visible as a slowly pulsating overall brightness of the monitor image. The
MD2001 will tolerate a small amount of lens hunting. Excessive hunting usually
indicates a problem with the iris control mechanism and should be corrected.
and close a small amount while searching for the optimum iris aperture). This effect
is often visible as a slowly pulsating overall brightness of the monitor image. The
MD2001 will tolerate a small amount of lens hunting. Excessive hunting usually
indicates a problem with the iris control mechanism and should be corrected.