Abstract: The inventive sensor illuminates the powder layer (40) by means of sensing segments (30, 31, 32), said powder layer scattering the measuring beams. Flaws in the powder layer lead to increased reflection such that a reflected measuring beam is reflected into the segments (30, 31, 31). The reflected beams are guided to a control unit (21) by the fiber lines. Said control unit recognizes flaws according to fluctuations in the intensity of reflected beams and generates a corresponding control signal.