1. A method for calibrating an image scanner signal processing circuit by determining a scanner signal correction factor required to compensate for nonuniformity among imaging pixels of a scanning array, the method comprising the steps of: sequentially setting the signal processing circuit at first and second correction factors for all imaging pixels of the scanner array; sequentially scanning first and second reference sources representing high and low scanner output signal values to derive first and second scanner signal samples from the first and second reference sources, respectively, for the first correction factor and to derive third and fourth scanner signal samples from the first and second reference sources, respectively, for the second correction factor; and applying a target correction factor to the signal processing circuit for each individual imaging pixel determined from (a) a desired difference between samples obtained for the individual imaging pixel by scanning the first and second reference sources, (b) the first correction factor for the individual imaging pixel, (c) the second correction factor for the individual imaging pixel, and (d) the first through fourth signal samples for the given imaging pixel.