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An opto-electronic seed spacing evaluation system that measured time intervals between seeds and detected front-to-back location of seed drop events relative to the planter was used to rapidly determine planter seed spacing uniformity in the laboratory. The seed detection sensor for the opto-electronic system consisted of a rectangular photogate with 24 phototransistors receiving light beams from 24 LEDs opposite to them. The system also included circuitry to interface the photogate with a digital I/O board in a personal computer. The opto-electronic system was tested with three planter configurations. During the tests, the photogate was positioned beneath the seed drop tube in a position representing the bottom of the furrow, and directly above the belt of a grease belt test stand. Seed spacings obtained with the opto-electronic system were compared with measurements of the same seed spacings obtained from the grease belt test stand. The information on the front-to-back location of seed drop events relative to the planter significantly improved the electronic seed spacing measurements in all cases. Seed spacing measurements obtained using the opto-electronic system determining time intervals between seeds and front-to-back locations of seed drop events relative to the planter were strongly correlated (average r = 0.951) with the same seed spacing measurements obtained using the grease belt test stand. The opto-electronic system can be used instead of a grease belt test stand to rapidly obtain quantitative evaluations of planter seed spacing uniformity in the laboratory.