1. A method of generating a darkness value for each of a multiplicity of bubble targets printed on a substrate as an outline and an alphanumeric symbol within the outline, by sampling a digital scan file of pixels having respective gray scale values and analyzing the gray scale patterns of pixels using a programmed digital computer, comprising: defining a sensing area of pixels within the outline; in the sensing area, comparing pixel gray scale values to filter out at least some of the pixels associated with the symbol; applying evaluation criteria to the pattern of gray scale pixel values in the filtered sensing area; and from the applied evaluation criteria, generating a darkness value for the target.