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directed to a retroreflector and returns to the detector. The switching
and output occur when an object breaks the beam.
Since the light travels in two directions (hence twice the distance),
reflex controls will not sense as far as through-beam sensors. How-
ever, reflex controls offer a powerful sensing system that is easy to
mount and does not require that electrical wire be run on both sides
of the sensing area. The main limitation of these sensors is that a shiny
surface on the target object can trigger false detection.

3.1.4 Polarized Reflex Detection
Polarized reflection controls use a polarizing filter over the source
and detector that conditions the light such that the photoelectric con-
trol sees only light returned from the reflector (Fig. 3.9). A polarized
reflex sensor is used in applications where shiny surfaces such as
metal or shrink-wrapped boxes may false-trigger the control.
Polarized reflex sensing is achieved by combining some unique
properties of polarizers and retroreflectors. These properties are (1)
polarizers pass light that is aligned along only one plane and (2) corner-
cube reflectors depolarize light as it travels through the face of the
retroreflector (Fig. 3.10).
Light from the source is aligned by a polarizer. When this light
reflects off the retroreflector, it is depolarized. The returning light
passes through another polarizing filter in front of the detector. The
detector’s polarizer is oriented at 90° to the source’s polarizer. Only
the light that has been rotated by the corner cube retroreflector can
pass through the detector’s polarizer. Light that bounces off other
shiny objects, and has not been rotated 90°, cannot pass through the
detector’s polarizer, and will not trigger the control.
Polarized reflex sensors will not work with reflective tape con-
taining glass beads. Also, shiny objects wrapped with clear plastic

FIGURE 3.9 Polarization refl ection controls.