Character Legibility and Digital Facsimile Resolution

Abstract

Character legibility was measured as a function of facsimile device parameters (pitch and sampling distance, and scan orientation) and of source document parameters (character height and case). The results indicate (I) very slight average difference in legibility (less than 1%) due to scan direction; (2) a more substantial difference (about 9%) between upper and lower case; and (3) legibility directly proportional to pitch and sampling distances, and to character height. For individual characters in standard typescript or similar material, 97.5% legibility apparently requires resolution on the order of 110 × 110 elements to the inch (scanning aperture nine mils square). Two secondary comparisons established the fact that individual character legibility (a) remained virtually unchanged when continuous scanning (one-mil resolution) was substituted for the best (five-mil) sampling distance, and (b) varied less than 2% when extra test documents were generated to check the repeatability of the experiment.