Foreground suppression vs. background suppression.
Background suppression:
Background suppression, objects within a set switching distance are detected, without being impeded by the reflective background, and nearly independent of color and surface of the object.
Background suppression is achieved by cutting the beam from the emitter to the receiver. To do so, it divides the visible field into an active area and the background. In addition, by dividing the receiver into at least two adjacent areas (e. g. by using a dual diode or a PSD element) and means of a geometric arrangement (triangulation), the actual position of the object within the sensing range can be determined.
Through this, the object and the background can be distinguished accurately. Diffuse sensors with background suppression are characterized by low gray value shift and hysteresis.
Foreground Suppression:
Photoelectric proximity sensors with foreground suppression are able to detect objects at defined sensing range. All objects between the sensing range (set to the background) and the sensor are detected. To ensure that these sensors function reliably, the background (for example, a conveyor belt) needs to be relatively light and should not vary in height. Photoelectric proximity sensors with foreground suppression are particularly well-suited for detecting dark and very shiny objects.
Know more about Balluff Photoelectric sensors