Edited by Ropert Repas
But the same type of sensor takes on another role with a technique known as background evaluation.
Background evaluation uses background reflection to determine the presence of the target. The light source, typically an LED, is aimed at a fixed point called the background reference. Many times the background reference is a floor or wall. The receiver has two light-detecting elements. One element senses light reflected by targets far away while the other looks for near targets.
A lens directs the reflected light to either the near or far receiver element based upon the angle at which the light is reflected. The distance between the sensor and the target determines the angle of the reflected light. Targets near the sensor reflect light at a larger angle, so the lens directs the light to the near receiver element. Since the background reference is farther away, the reflected light has a smaller angle to the receiver. The smaller angle makes the lens focus the reflected light onto the far element. A comparator in the sensor determines if more light is seen at the far or near receiver elements, and thus can determine if the reflected light is from the reference or a closer target.
Background evaluation comes into play when the target either reflects the light away from the sensor or does not reflect enough light to be monitored by the near receiver elements. When that happens, neither the far nor near receiver elements see the reflecting surface. However, background evaluation notes the loss of the far reference and thus triggers a "target present" signal even without detecting a near target.
Pepperl+Fuchs (am.pepperl-fuchs.com) provided information for this column.