Clear objects pose a challenge for all photoelectric sensors. The sensor is typically set for maximum sensitivity and operates with high margin, or excess gain, which means the unblocked light beam is many times stronger than that required to energize the sensor's output. The result is a very small change in signal. If the light beam is polarized, the direction and extent of polarization will be altered to a greater or lesser degree depending on the optical properties of the material. This alteration could depolarize the light beam and reflect a strong detectable signal.
Most plastics used for containers and packaging are optically active.
The main use of polarizing filters in photoelectric sensors is to enhance the sensor's ability to ignore reflections from shiny surfaces such as bottles or plastic wrap. Away from the center, where the bottle surfaces are not perpendicular to the light beam, the light loss due to reflection is greater. Additionally, clear bottles are not always uniform in their ability to pass light. Certain areas of the bottle may pass the light with a different shift in beam direction than other areas. This lensing effect changes the characteristics of the application, often increasing the signal instead of reducing it.