is also disclosure of a vertically progressing arrangement of height detecting photodetectors positioned above and upstream from the trigger photodetector the height detecting photodetectors perform a height classification at the instant of activation of the trigger photodetector the system includes a plurality of recognition circuits for recognizing different classes of containers and the height classification is used for selecting those recognition circuits to be employed for recognition of the particular container being observed application_number US 4246879 A description in preferred embodiment a series of containers 11 are transported through an observation station 10 as illustrated in fig1 containers 11 are carried by a conveyor 12 and are aligned in space progression against a side board 13 preferred apparatus for aligning containers 11 against sideboard 13 is disclosed in u s patent application ser no 042 469 filed on even date herewith and assigned to the assignee hereof in the general case conveyor 12 may carry containers grouped in carrying cartons as well as individual containers as illustrated it has been found that containers positioned within cartons are best recognized as taught in ser no 924 885 and reference may be made to that application for a description of such container recognition apparatus the scanning system of this invention is adapted to.