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

A method of detecting the presence of an object in a space 26 i.e. such as an aisle of a mobile storage system 10 comprises positioning a light beam emitter 30a, 30b at a first location outside of the space; rotating the light beam emitter across the space and sensing the passage of the light beam (140a, 140b figure 2) across the space, wherein the presence of an object (A figure 2) within the space obstructs the light beam and indicates the presence of an object within the space. At least one of the first and second sides may be defined by a movable storage unit where movement of the unit is prevented by the presence of an object within the space. A first light beam emitter 30a may be mounted outside the first side of the space 12a and a second light beam emitter 30b may be mounted outside the second side of the space 12b. A series of reflectors 32a, 32b and receivers (130 figure 9a) may be positioned at the first and second sides of the space where each receiver is positioned adjacent to one of the emitters. The emitter and receiver may be a combination unit (98 figure 6) mounted within a housing (84 figure 6) that is rotatable.