Datasensor S51 series offers the most cost-effective solution in M18 photoelectric sensors which is today improved thanks to the increase of the operating distances. The diffuse proximity model presents a 10 cm fixed operating distance with wide emission spectrum and the another model with 1 - 40 cm trimmer-adjustable operating distance. Datalogic standard retroreflective models have an operating distance up to 4 m whilst the polarised retroreflective models used for reliable detection of reflective objects are fitted with sensitivity adjustment and have a 3.5 m operating distance. The emitter and receiver models, used for longer operating distances, reach 18 meters. The output mode can be inverted using the dark/light selection input wire provided.