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 4.2.5 Photoelectric Sensors. These sensors, also called "beam" sensors,
transmit a beam of visible or invisible light to a receiver. If the beam is
interrupted or broken, an alarm is produced. Since visible beams are
bypassed easily, only sensors which use invisible light (normally in the
infrared spectrum at a frequency of about 1.0 micron) are considered
suitable for DoD and USN applications. If the beam is continuous, the
receiver may be "captured" by the substitution of another light source, thus
permitting bypass. One should use, therefore, only infrared sensors which
"pulse" the beam at a certain frequency and make substitution difficult.
Modifying the frequency of the pulsed beam will cause an alarm. Generally,
infrared transmission is achieved by filtering a visible light source which
reduces the effective range of the beam. Pulsing is achieved either by use
of propeller blades in front of the light sources or by an oscillator
connected to both transmitter and receiver. Because of the mechanical
problems which occur with the propeller motor, etc., the latter method is
preferred. Also, the pulse modulation is more accurate with the second
method, allowing the receiver to discriminate on the basis of phase as well
as frequency and provide a higher level of security. Photoelectric sensors
can be used outdoors as well for specific applications. The following
considerations for applications and installation apply to photoelectric
sensors:
a) Alignment is critical. The greater the distance between
transmitter and receiver, the more easily misalignment can occur. Mounting
on unstable surfaces (e.g., walls which vibrate) also can cause misalignment
or nuisance alarms.
b) The use of reflectors is a cost-effective way of extending the
sensor coverage. Care must be exercised, however, in the number of
reflectors used because this system's detection capability is reduced by
each additional reflector introduced.
c) Long, narrow spaces such as corridors are cost-effective
applications. Such applications are often called "traps" to back up
building perimeter intrusion point sensors.
d) Light beams do not penetrate physical objects and therefore
cannot protect masked areas.
e) The vulnerability to bypass can be reduced by employment of
multiple beams at various heights.
f)
Smoke, steam, or other air particles can degrade performance.
4.2.6 CCTV - Motion Detection. Also called video motion detection, this
sensor uses successive images from a closed-circuit television (CCTV) camera
to detect motion. A microprocessor digitizes the image signal from a CCTV
camera and, at a set interval (1-2 seconds), repeats the process with a
second
13.02-50
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