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Thus the two complementary technologies provide an inherent immunity to
false alarms. Dual technology is ideal for use in hostile environments. Since
both must trip simultaneously to cause an alarm, installation is easier and
requires less discipline.
PIR ZONE DIMENSIONS
Regardless of the lens pattern selected, or the number of PIR beams
produced by the lens, the relative proportions of beam width, beam height,
beam separation, and zone width always remain the same and are illustrated
in Fig. 1. Note that beam and zone dimensions at any distance may be
calculated by the following formulas.
Beam Width "w" (in.) = Distance "d" (ft.) x 0.4
Beam Separation "s" (in.) = Distance "d" (ft.) x 0.4
Beam Height "h" = 2 x Beam Width "w"
Zone Width "z" = 3 x Beam Width "w"
Examples: At 10', a zone is 8" high x 12" wide.; at 30', it is 2' high x 3' wide.
DETECTION PATTERNS
Figure 2 illustrates maximum PIR and microwave detection patterns super-
imposed on each other. PIR detection patterns are adjustable, within limits,
both vertically and horizontally (see ADJUSTING THE COVERAGE AREA
for detailed adjustment procedures).
The microwave detection patterns shown are for free space. In practice,
when confined by walls and ceilings, reflected waves tend to flood the area,
providing volumetric coverage. Furthermore, when used in long, narrow
corridors, the effective range may be extended by as much as a factor of 2
due to the guiding effect that the corridor has on microwave energy. Micro-
wave detection range is adjustable. Reducing range, however, will reduce all
other dimensions proportionately.
Insensitive Areas
The insensitive area is that area directly beneath the unit within which an
intruder is undetectable. It is shown as a shaded area in the side-view
detection pattern that follows, and assumes that an intruder will be at least 5'
tall and that the protected area is not accessible by crawling or crouching.
Fig. 1. Relative Zone
The size of the insensitive area will increase as the mounting height increases
Proportions.
(see Fig. 2b). Nevertheless, it may be necessary to increase the mounting
height in order to avoid an object that could obstruct the coverage pattern.
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