Targets with highly reflective surfaces can bounce enough light back to the sensor to effectively pass unseen in front of it.
Edited by Robert Repas
The reflective properties of plastic make it a notorious offender in this regard. The problem even extends to those objects covered in shrink wrap.
Polarizing filters help block false readings from shiny objects. Polarized retroflective sensors transmit light along one polarization axis. The light is then reflected back to the sensor from a corner-cube reflector.
Corner-cube reflectors have two distinct reflective properties. First, light always reflects back to the source regardless of the angle at which it enters the reflector. Second, it rotates polarized light 90° due to multiple reflections inside the reflector. The receiver’s polarizing filter aligns with the polarization of the reflected light.
Light reflected from glossy or highly reflective surfaces keeps the same axis of polarization. Without the 90° rotation of the cornercube reflector, the light is blocked by the receiver’s polarizing filter and the sensor signals that the target has passed.
However, some shrinkwrapped products still create problems because plastic has depolarization properties. While the amount of light reflected is not as efficient as a corner-cube reflector, at short distances a sensor can be tricked into thinking it sees the reflector. The result is a false reading. The likelihood of this problem grows exponentially when the plastic is layered, or if moisture is present in the shrink wrap.
This problem is addressed by using retroreflective sensors with foreground suppression. Optical apertures added to the transmitter and receiver elements tighten the light spot so the receiver detects only light reflected at the proper angle and axis rotation. This creates an area in front of the sensor that mechanically blocks any false readings to ensure error-free detection of highly reflective, depolarizing targets.
Pepperl+Fuchs (www.am.pepperl-fuchs.com) supplied information for this column.