Glossary for detailled explanation - take your pick:
Objects within a certain switching distance are detected by an optical sensor with background suppression without influence of any reflective backgrounds and almost independently of the objects's surface reflection. The background suppression is created by the overlapping of the beams of sender and receiver.
Within the blind zone, there is no reproducible object recognition possible. The blind zone is the area between active surface and minimum switching distance in which an object is recognised. Foreign objects within the blind zone lead to a malfunction of the sensor.