The relative importance of spatial image characteristics in the reproduction of alphanumeric characters has been examined, and it is concluded that significant anisotropy exists. At low distortion levels, horizontal image degradation is somewhat less objectionable than vertical degradation, but at higher levels vertical distortion is significantly less objectionable. The upper and lower portions of characters are found to be much more important for legibility than are the central portions. Upper portions of characters carry more legibility information than the lower portions. The implications of these findings on image transmission systems are discussed.
C. J. Bartleson, "Anisotropy in Alphanumeric Reproduction Tolerances," Appl. Opt. 6, 341-345 (1967)