Proximity sensors simply detect the presence or absence of objects using electromagnetic fields, light or sound. The major technologies for proximity sensors are inductive, capacitive, photoelectric and ultrasonic. Each has unique operating principles and capabilities while each are particularly suited to certain applications and environments. Inductive devices are non-contact units designed to detect the presence or absence of a ferrous material. Capacitive devices are capable of detecting both metallic and non-metallic targets such as powders, granulates, liquids and solids. Photoelectric devices offer the largest variety of problem solving choices in the proximity sensor market. Ultrasonic devices use sound waves to detect the presence or absence of objects and are thereby not affected by color or transparency. Proximity sensors are used in today's automated workplace.