The Datalogic SR21 is a photoelectric slot sensor designed for automation engineers and machine builders working in packaging and labelling applications. Its 2 mm slot width and 12-bit (4096-step) resolution enable precise detection of labels, register marks, and semi-transparent materials at switching frequencies up to 25 kHz with a response time of 20 µs max. Two model variants address distinct tasks: the SR21-IR uses infrared 880 nm emission for label and hole detection on continuous surfaces, while the SR21-RG employs dual red/green emission for colour print register mark detection on transparent films. An IP65-rated ZAMA housing with glass lenses withstands demanding industrial environments, and the AUTO-SET push-button — supporting both static and dynamic configuration modes — simplifies commissioning on fast-moving production lines. Power supply accepts 10 … 30 Vdc with reverse polarity protection, and antivalent NPN and PNP outputs on separate pins provide flexible integration.