These were the best ways to precisely sense an object’s true leading edge, especially when the leading edge could change, such as with various types of pallets, parts being ejected from a die, stacks of newspapers, bent rods, cartons of different height and/or shape, or other items.

A new class of photoelectric sensors called retroreflective area sensors now solves this application requirement with improved performance over the old methods. Retroreflective area sensors utilize multiple transmitter beams and multiple receiver elements in a single sensor housing to create a continuous height detection field. 

The sensor housing is directed at a corner-cube reflector. When no object is present, the light travels from the transmitters in the sensor housing to the reflector, and then is reflected back to the receivers in the sensor. After the sensor is installed and aimed at its reflector, a simple teach, usually pressing a pushbutton, is done so the sensor learns the conditions of target absence. The sensor detects an object when it enters the sensing area between the sensor and the reflector.

Unlike an array of single beam sensors, which have conical light beam patterns and are typically much smaller, retroreflective area sensors have a continuous height rectangular light beam pattern, enabling them to detect the same size object consistently throughout the entire sensing area. And because a retroreflective area sensor offers a larger sensing area than a single-beam sensor, an object’s true leading edge is detected, even if it has no defined shape or position.

Since retroreflective area sensors only involve a single housing, both the material and installation costs are well under that of both a thru-beam light grid and comparable arrays of single-beam sensors. And they are mechanically and electrically the same as a single beam device. Retroreflective area sensors offer a smarter sensing alternative to single-beam arrays or thru-beam light grids with enhanced performance and with a lower cost.

Jeff Allison is Product Manager at Pepperl+Fuchs ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ).
www.pepperl-fuchs.us