3.4. Effective tone scale range

Optical density range ΔD of a print is in general defined by the difference between densities of ink solid D_s and paper D_p as:

ΔD = D_s - D_p 3.7

However, this parameter doesn’t consider the specific of tone rendering by halftone dot area variation. The fewer but more adequate range meaning gives the difference ΔD_eff of densities D_{bl min} and D_{dot min}, i.e. of the areas with minimal blank spaces and dots reliably printed over a sheet within a run:

ΔD_eff = D_{bl min} - D_{dot min} . 3.8

Figure 3.8. Print characteristic location within the effective ΔDeff (1) and formal ΔD (3) ranges; non working (2) and non reproducible (4, 5) intervals; 6 and 7 – original densities corresponding to banding occurrence in stationary image areas

Tone rendition control by dot area variation is possible just in this interval. Marked in figure 3.8 gaps (D_s - D_{bl min}) and (D_{dot min}- D_p) aren’t usually included in the reproduced range to avoid operating the infinitely small and therefore non controllable tone values in the highlights and darkest print areas. “Black” and “white” points of an original are tied to the print densities D_{bl min} and D_{dot min} to make gradation control manageable. In other cases, at assigning the tone value of an ink layer (S = 100%) to “black” and that of a paper (S = 0%) to “white” the small contrast detail and pattern are lost in the dark and light areas of original.

If, for example, the ink solid is assigned for the darkest parts of a black suit it may occur impossible to reproduce on a print its other a bit lighter details such as lapels for their brightness can relate to unreal, non-printable blanks of just 1.5%. Their creating on a print can be impossible due to dot gain, filling and other, lower concerned, reasons. This case is indicated as the upper “non-working” portion 2 of graph 3 in figure 3.8.

Difference between formal ΔD and effective ΔD_eff density ranges can be quite essential. In relief 60 Lpi news printing it was, for example, about 0.3, so the “white” on illustration was rather dark as compared for clean margins of paper not at all qualified by high whiteness.

Assigning the null and hundred percent values to ultimate gradations of an original is also fraught by the false contours in stationary image areas of continuous tone variation. With white and black assignment according graph 3 of figure 3.8 the acute jumps have place on continuous tone transition at locations 6 and 7 corresponding to the “real”, printable tiny dot and blank space. Their absolute sizes can differ even in a given type of technology and comprise the certain equivalent of a printing process noise level being dependant on a substrate, plate, ink properties, press parameters and their interaction specific.