Light curtain
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A light curtain is a device which uses an array of photoelectric sensors to detect the presence of an object. Light curtains are commonly used with industrial equipment to 1) ensure machine safety at a point of operation control, area access control, or perimeter access, or 2) detect object size in a plane. A light curtain will typically consist of a 'stick' of infrared transmitters and another 'stick' with a corresponding set of receivers. If the plane of detection between the pair is blocked an electrical safety circuit will remove power to ensure safety.
When used for industrial safety a light curtain is referred to as an electro-sensitive protective equipment and would be described in IEC 61496.