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      Ink measurement in offset printing

Print constrast
  In order to determine the normal ink film thickness it is necessary to have a criterion, which permits an unbiased evaluation. This criterion is known as print contrast. It is computed using the a formula. Normal ink film thickness is obtained, where the print contrast reaches its maximum. 

Characteristic curves of printing
  The characteristic curve of printing shows, to what extent the printed dot differs in size from that on the film respectively press and depends on many factors such as the D0 the type of press, (single-color or multi-color, paper, blanket, impression pressure and others). It serves as means of communication between repro house and production printing. 
  The characteristic curve of printing is established with the aid of a screen grey-scale wedge. The screen density of the individual step is known. The Murray-Davies formula permits the computation of the screen density in print. The values are entered in coordinate system and the curve is plotted. This plot is a convex curve, since a 100% congruence, corresponding to the film, is not possible; it is prevented by light trapping.
The characteristic curves of printing are also important for matching proof print and production print.

Ink receptivity
  Poor ink receptivity when printing wet on wet results in severe ink variations, when colors in the area of 70-95% dot percentages are overprinted. In such cases the results achieved on a proofing press, printed dry, cannot be achieved in the production run.

Dot gain
  The difference between dot percentages on film and those on the production print is called dot gain.   

Two methods are commonly used to determine it
     1.        The computation, using the Murray-Davies-formula
      2.        Coarse screen fine screen comparison, by System Brunner 

  The dot gain for the individual tonal values, established according to Murray-Davies, is entered in the coordinate system and displayed as characteristic printing curve. 

The System Brunner uses isometric diagrams to represent the dot gain.

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