1. An electro optical locator device for determining the position of an object on a conveyor comprising first and second light sources fixed in space above the said conveyor and emitting light in sheet form in first and second planes defining a dihedral angle with its apex substantially on the conveyor transverse to the movement thereof and each plane being at an acute angle in relation to the conveyor for projecting onto the conveyor a single line of light transverse to the conveyor motion, wherein an object carried by the conveyor intercepts the light above the surface of the conveyor at positions laterally spaced from the apex of the dihedral angle so that the single line of light is projected onto the conveyor only outside the boundaries of the said object, a linear photosensor array of photo elements responsive to light in a plane perpendicular to the conveyor aligned with the apex of the dihedral angle for viewing the single line of light projected onto the conveyor outside an object boundary, the illumination of individual photo elements being dependent on the position of the boundaries of the object with respect to the conveyor and means responsive to the respective photosensor elements effective to determine the pattern of illumination of said photosensor array at a particular conveyor position to determine the position of the boundaries of the object relative to the conveyor.