The QS30 series self-contained fixed-field powerful visible red diffuse mode Sensor with far-limit cut-off (a type of background suppression). Their high excess gain and fixed-field technology allow them to detect objects of low reflectivity that are directly in front of another surface, while ignoring the surface in the background. The cut-off distance is fixed. Backgrounds and background objects must always be placed beyond the cut-off distance. Retroreflective, polarized retroreflective and diffuse models have potentiometer on back of housing for easy sensor range adjustment.