IMO proximity switches consist of three different types: Inductive that only detect metals; Capacitive that can detect almost any object; and Ultrasonic that can detect objects which can reflect sound beyond human capability to hear. This is a modulated sound created by the sensor.
IMO range of Inductive sensors come in many shape and sizes, the common ones being cylindrical, ranging from 3mm in diameter to M30 body styles, with block types also available. All options are available in DC, with M12, M18 and M30 also being available in AC. Capacitive sensors are available in M18 and M30 body styles, with M18 being available in DC only and M30 in both DC and AC. Ultrasonic sensors are available in M18 body style and DC only.