INSTALLATION INSTRUCTION FOR PIR-75
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Annex 2:
Instructions for Terminating Screens and Ground Surface
Introduction
The detection range of a PIR detector is not limited but a function of size, speed and temperature contrast
of a target against its background. Alignment of the detectors should be carried out in such a way that the
field of view is terminated by a natural or artificial background at the end of the range. Reflections from
inside or outside the field of view have to be avoided as well as possible.
Definition of Field of View
The numbers stated in the technical manual for the vertical and horizontal dimensions are defined – per the
usual standards – as the half power points representing 50% of the relative sensitivity at that distance for a
typical human target. Absolute signal amplitudes depend on temperature contrast, speed and size of a
target and can be up to 20 times (and more for large vehicles) higher than the alarm threshold. For a strong
signal the dimensions will appear to be increased compared to the nominal values. This needs to be
considered when dimensioning a terminating screen.
Dimensioning a Terminating Screen
From the field of view definition it becomes clear that the width of a terminating screen has to be larger
than the actual figures given in the technical manual, this particularly when a street crosses the field of view
behind the screen.
A practical factor for the width of a screen is
1.5 times the width Y
of the field of view at the distance
from the detector.
The considerations for the vertical dimension are similar although a certain installation flexibility is given by
tilting the unit slightly downward with the risk of losing sensitivity in the uppermost part of the curtain.
Therefore it is recommended to extend the screen also vertically in the same way using the same factor of
1.5 times the elevation Z
.
Example
:
Screen at 80 m for a curtain height of 3 m with a PIR-75 (see Table 1)
Width Y
2.6 m
Screen Width
3.9 m
Elevation Z
1.0 m
Screen Height Above Axis
1.5 m
Total screen height = curtain height - elevation Z + screen height above axis
This results in
minimum
dimensions of
3.9 m wide
by
3.5 m high
(3.0 m - 1.0 m + 1.5 m) for the
terminating screen.
0
20
40
60
80
1 m
2 m
3 m
4 m
Z
Axis
Typical side view of a terminating screen in 80 m distance from the detector. The dimension „Z“ is app. 1.0
m corresponding to the value of the table in section 5.
