Photoelectric Sensors

A retroreflective sensor contains both the emitter and receiver elements in the same housing. It uses a reflector to bounce the emitted light back to the receiver. Similar to an opposed-mode sensor, it senses objects when they interrupt or "break" the effective beam. Because retroreflective sensing is a beam-break mode, it is generally not dependent upon the reflectivity of the object to be detected. 

However, it can be tricked by shiny objects. For those targets, a polarized retroreflective sensor should be used to prevent proxing. Proxing is when an object with a shiny surface returns enough light to the sensor to mimic the photoelectric beam coming back from the reflector and causes the object to not be detected. In a polarized retroreflective sensor, the emitter sends light waves through a filter that aligns them on the same plane. These light waves bounce off the reflector, and return to a vertically polarized filter on the receiver. When this polarized light reaches a shiny target, the light is reflected back to the sensor on the same plane as it was emitted and is blocked by the filter, signaling a broken beam. When the polarized light hits the reflector, however, it is scattered into unpolarized light with light waves on both the horizontal and vertical planes. Some of this light will pass through the receiver’s filter and the sensor will detect the reflector and know the beam is unbroken. 

A retroreflective-mode sensor offers a convenient alternative to opposed mode when space is limited, or if electrical connections are only possible one side of the installation. Retroreflective-mode sensors offer relatively long ranges. 

• Second-highest excess gain mode 
• Polarized model available to prevent the beam from proxing off shiny objects 
• Coaxial optics available for clear objects and precision 

Background-suppression (BGS) sensors are a diffuse-type sensor with a defined limit to their sensing range, ignoring any objects that lie beyond that range. There are two types of background-suppression sensors: fixed-field and adjustable-field. Both types use triangulation to determine the cutoff distance which allows the sensor to ignore anything beyond that point. The available excess gain inside the fixed sensing field is usually high, allowing sensing of less-reflective surfaces. A background-suppression sensor can often detect a dark target on a white background, as long as the background is past the sensor’s cutoff.

• Detects objects out to a set sensing distance
• Ignores background objects
• Very low color sensitivity