Rotating light source system for detecting an obstruction in a space, for use in a mobile storage system

An object detection system for use in detecting the presence of an object within a space, such as in an aisle of a mobile storage system bounded by at least one mobile storage unit, includes a pair of pivotable light emitter/receiver unit located outwardly of each side of the space and a series of reflectors located outwardly of each side of the space. Each emitter/receiver unit includes a pivotable emitter which emits a beam of light which passes across and through the space and impinges upon the reflectors located outwardly of the opposite side of the space. A receiver associated with each emitter/receiver unit receives reflected light from the reflectors. The presence of an object within the space prevents the light beam from reaching one or more of the reflectors, and the absence of one or more reflected light beams is detected such that operation of the mobile storage unit is prevented until the object exits or is removed from the space.