Patent US 5,581,636

A concentric ring target detector receives input pixel data from an optical scanning device. The pixel data represents images on a conveyor belt and moving objects bearing information-encoded labels which are advanced by the conveyor belt. Input pixel data is alternately applied to a pair of FIFO blocks, one of such blocks receiving odd and even pixels from scan line N, the other of the blocks receiving pixels from scan line N-1. Pixels from the FIFO blocks are coupled through a pixel switch to a multi-tap first-in first-out block. Each FIFO block applies data to the pixel switch at one-half the rate at which it was received by the FIFO block to maintain constant throughput without the FIFO's overflowing or emptying. The pixel switch ensures that the pixels from scan lines N and N-1 remain in serial order. Pixels from scan line N and N-1 are shifted through a multi-tap FIFO. The multi-tap FIFO has a plurality of taps that enable reading of each pixel written to the multi-tap FIFO as the pixels are shifted through the FIFO. Various one of the tap outputs are coupled to a pair of template correlators. The image frame represented by the pixels applied to the template correlators are compared to templates in the correlators. The templates represent an image of a concentric ring acquisition target disposed on an information-encoded label. If either template correlator recognizes a predetermined correlation threshold, a detection signal is applied to a detection gate causing a detection signal line to activate at a point where the coordinates of the center of the concentric rings disposed on the information encoded label can be determined.