American Dynamics 1461

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

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.