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Fiber Optics in Sensors and Contr ol Systems

FIGURE 3.10 Corner-cube refl ector. 139

shrink-wrap will potentially false-trigger a polarized reflex control,
since under certain conditions these act as a corner-cube reflector.
The polarized reflex detection sensor offers the following advan-
tages:
• It is not confused by the first surface reflections from target
objects.
• It has a high dark/light contrast ratio.
• It is easily installed and aligned. One side of the sensing zone
only need be wired.
It also has certain limitations:

• Its operating range is half that of a nonpolarized sensor since
much of the signal is lost in the polarizing filters.
• The sensor can be fooled by shiny objects wrapped with
shrink-wrap material.

3.1.5 Proximity (Diffuse-Reflection) Detection
Proximity detection is similar to reflex detection, because the light
source and detector elements are mounted on the same side (Fig. 3.11).
In this application, the sensors detect light that is bounced off the
target object, rather than the breaking of the beam. The detection zone
is controlled by the type, texture, and composition of the target
object’s surface.
Focused proximity sensors are a special type of proximity sensor
where the source and detector are focused to a point in front of the
sensor (Fig. 3.12). Focused proximity sensors can detect extremely
small objects, or look into holes or cavities in special applications.