Tone Reproduction in Offset Lithography

Abstract

The basic assumptions of Tritton and Wilson’s theory of the ideal screen negative for photolithography are examined. These assumptions do not hold rigidly because of changes in dot size from negative to positive and from positive to paper print and because of the effect of light scatter within the paper. A relationship between the tones of a screen negative made on modern “lith” materials and those of a contact positive prepared from it is established. A similar relationship exists between the contact positive and the deep etch offset paper print prepared from it. On the basis of these results the characteristic curve of an ideal screen negative for photolithography is computed and compared with the Tritton and Wilson curve and curves for real screen negatives. The performance of available cross-line and contact screens is examined: both fall far short of the requirements for satisfactory tone reproduction. The highlight reproduction of both types is similar; in the shadows the contact screens give the worse reproduction. The shadow contrast with magenta contact screens may be more than five times the highlight contrast. The use of filters for controlling contrast in contact screens is unsatisfactory since the shadow contrast is affected much more than the highlight contrast, and a further drawback is the great variability of these screens from one batch to another. It is concluded that the ideal screen for photolithography has not yet been prepared.