Abstract: This invention relates to a system of determining which of a plurality of molds produced a particular container. A concentric ring code is molded into the bottom of each container as it is produced. The containers are then passed by a reading station, where light whose intensity is proportional to the angle of incidence is projected onto the bottom of the container. Readings are taken of the variation of intensity of light reflected to a particular point in order to determine the position of rings on the container. Suitable electronics then decode the ring position to determine the container code, thus allowing identification of the mold which produced each container.