Photoelectric presence sensors utilize photoelectric emitters and receivers to detect presence, absence, or distance of target objects. They can be categorized into three main categories, through beam, retroreflective, and proximity.  Through beam photoelectric sensors are configured with the emitter and detector opposite the path of the target and sense presence when the beam is broken.  Retroreflective photoelectric sensors are configured with the emitter and detector in the same housing and rely on a reflector to bounce the beam back across the path of the target. This type may be polarized to minimize false reflections.  Proximity photoelectric sensors have the emitter and detector in the same housing and rely upon reflection from the surface of the target. This mode can include presence sensing and distance measurement via analog output. The proximity category can be further broken down into five sub-modes: diffuse, divergent, convergent, fixed-field and adjustable field.  With a diffuse sensor presence is detected when any portion of the diffuse reflected signal bounces back from the detected object.  Divergent beam sensors are short-range diffuse-type sensors without any collimating lenses.   Convergent, fixed focus, or fixed distance optics (such as lenses) are used to focus the emitter beam at a fixed distance from the sensor.  Fixed-field sensors are designed to have a distance limit beyond which they will not detect objects, no matter how reflective.  Adjustable field sensors utilize a cutoff distance beyond which a target will not be detected, even if it is more reflective than the target.  Some photoelectric sensors can be set for multiple different optical sensing modes.  Reflective properties of the target and environment are important considerations in the choice and use of photoelectric sensors.