PURPOSE:To compute centers of gravity of beams R, G, and B accurately, by computing the centers of the beams, that is, the centers of gravity, from all the brightness data crossing sampling lines. CONSTITUTION:Responding to the phosphor stripes R, G, and B arranged in order with specific pitches horizontally to a cathode-ray tube screen, at least three data sampling lines 2R, 2G, and 2B are established vertically respectively, and measuring patterns for R, G, and B are moved at a time horizontally in specific pitches. Each brightness data in the vertical direction in the measuring pattern area on R, G, and B at each data sampling line in every horizontal movement pitch is memorized in order. This brightness data measurement is continued until the beams pass through the sampling lines perfectly. Depending on these brightness data, each center of R, G, and B is computed, and the convergence condition is measured depending on the data from these centers.
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