OUTDOOR PROTECTION By James Babb
Advanced Detection
Methods for Perimeter
Photo Beams
P e
erimeter protection and intrusion detec-
t
ti
tion can be unwieldy, requiring special
c
consideration to prevent the occurrence
o
of false alarms, especially in light of poten-
ti
tially noisy environments.
Recent studies and programs have called
attention to issues with regards to being able to have
the confidence to dispatch personnel on alarm notifi-
false alarm reduction programs of the FARA type but
also for improved technology, which is what this ar-
ticle addresses.
Photo beam perimeters continue to be the first line
of protection for an alarm system and detection at the
protected premises. This article is designed to take a
look at the technology behind detectors and several
different beam approaches, which include:
cations. There is an alarming (no pun intended) trend Detection of light--There are detectors devised
for virtually every wavelength of light known to ex-
The trick of any detection scheme is ist, deep UV through far reaches of the infrared spec-
trum. Detectors simply receive photons in the region
to maximize the signal and minimize of interest (outside the region in the form of noise) and
the noise (S/N ratio). convert the photons to electrons. The electrons fill up
a bucket which is based upon an R/C time constant
toward a verified response approach—not to dispatch (Resistance/Capacitance), which then dumps the
emergency personnel until and unless a confirmation bucket at a fixed interval.
of the alarm condition is obtained. There are even or- There are many sources of noise that can affect the
ganizations such as the False Alarm Reduction Asso- detected ratio of Signal to Noise (S/N), ranging from
ciation (FARA) with programs designed to exchange Shot and Johnson noise of the circuit itself, through
information, provide a clearinghouse for agencies and external noise sources such as ambient light (DC)