Photoelectric Sensors

Photoelectric sensors are devices that use visible or infrared light to detect the presence of an object or its distance from the sensor. Photoelectric sensors consist of an emitter that sends the light and a receiver that receives the light. When the emitted light is interrupted or reflected by an object, it alters the amount of light that arrives at the receiver. The receiver detects the change and sends it as an electrical output. The sensing configuration is important to understand when selecting a photoelectric sensor. The three most common are: through-beam, diffuse-reflective and retroreflective.

Through-beam sensors require the emitter and receiver to be installed opposite each other to enable the light from the emitter to be received by the receiver. When an object passes between them, the light is interrupted and reduces the amount of light that enters the receiver. The reduction in  the intensity of light is what detects the presence of the object. Through-beam sensors have the longest sensing range and are very effective in dusty, smoky and oily environments. Because the object’s physical presence is what is detected, it does not matter if the object is dark, shiny or of varying colors.

Diffuse-reflective sensors contain the emitter and receiver within the same housing. When light from the emitter encounters an object, the light is diffused, sending some light back to the receiver. The object reflects the light with more intensity back to the receiver. This increase in light intensity is used to detect the object. Diffuse-reflective sensors are easy to install, however, they are less accurate in dirty environments and are affected by the texture, color and characteristics of the object.

Retroreflective sensors contain the emitter and receiver in the same housing and also utilize a retroreflector. A light beam enters a corner cube prism and is reflected from the three surfaces, returning the light beam to its source. A retroreflective sensor is similar to a through-beam sensor in that it detects an object when the object interrupts the light beam, however, it is slightly less accurate. Retroreflective sensors are a good option when wiring connections are only possible on one side of a device.