Inverse equation

d_min = 1/5L 3.11

allows, from the other hand, for rough judgment the certain screen ruling value and process facilities conformity in providing the desired image definition and sharpness. For printing with the halftone frequency of 175 Lpi (80 lines/cm) it’s, for example, required the ink, substrate, plate, press… allowing for providing the minimal dot of 25 μm.

Screen rulings L and process characterizing d_min values, which submit to such rule and quite correlate to industry practice, are given in table 1.

Table 1. Screen ruling L and process characterizing d_min values which submit the rule L = 1/5d_min correlating to industry practice of a “white point” 4% and contrast priority over print sharpness and definition

Screen ruling, Lpi		Sizes and tone values of min print elements
		assumed by practice		ISO recommended
		Dmin, μm	Smin, %	Dmin, μm	Smin, %
newspapers	75 100	80 50	4	43
					3
magazines	120 150	40 33		35 28
commercial printing	175 200	29 25		23 28 ?
					5

Meanings from ISO 12647 don’t, however, follow such logic. While recommending the “white” point of 3% for rulings of 60 - 175 Lpi, it prescribes 5% for 200 Lpi, i.e. advises to lower the contrast for “art” printing by making this point 2% darker. In spite of the use of higher quality substrates and more accurate processes, the characterizing them minimal dot becomes, as it’s seen from the table, at such condition the same as at 150 Lpi, i.e. 28 against 23 μm. It doesn’t correspond to industry practices where even smaller minimal dots are sometimes used for the specific or most prestigious jobs on coated glossy stock.

Another example of ignoring the logic of screen ruling choice is often met in promotion of processes, consumables and equipment potential by stating the facility of operating with a tiny dot of 1% or 2%. Such dot availability (if properly measured) allows for twice or one and half greater ruling which results, in turn, in four or two times larger volume of pictorial data.

This way grounded relationship of process facilities and screen ruling value doesn’t only displays the current situation but is also actual for the correct comparison of traditional and modified halftoning techniques. Among other conditions, it can be lawful if the both are focused on the process of a certain own noise level, i.e. using the same size minimal dots. Conclusions on relative images advantages or disadvantages turned out to be unfounded if, for example, the minimal dot corresponds to norm of 4% on print of a first type but to 1% on another [3.18 -3.20].

Another example of problem caused by the unfounded screen ruling setting gives the flexography, widely demanded in recent years for color packaging. There was always a problem to get the tiny dots in relief printing and that’s why the ruling, even with hard metallic plates, had rarely exceeded 135Lpi. Frequencies of 150 -175 Lpi turned out to be significantly overstated in the considered context for elastic plates since didn’t provide a small printable dots in the highlights. The lack of such dots resulted in tone “jump” in the highlights as indicated by section 4 and point 6 in figure 3.8. There were the attempts of the hybrid screening use to control the lightest or darkest gradations by reduce the amount of a minimal available dots or blanks as shown in figure 3.11.

Figure 3.11. Conventional (a) and hybrid screen (b) with the reducing of utmost dots and blanks amount to provide the additional highlight and darkest tone levels

However, the reduce of such dots amount increases the unit pressure at nip causing the bending of a dot stem which, in turn, results in the excessive inking of substrate. More or less appropriate solution was later found by the preserving within a thinned out area of several supporting plate dots of a greater than minimal size.

Providing on a print the extremely small dots comprises the fundamental criterion of halftone printing optimization in relation of the ink supply and pressure. Any deviation from this way found press settings for compensating, for example, the prepress tone or color rendering imperfections results in underuse of technology facilities because of the inevitable reduce of effective density range.

Ruling value propriety is checked by the process own noise level estimation and, namely, by the measurement of ultimately minimal dots reliably and steadily provided within a run over press sheet. However, there is used in wide practice to set the press parameters for given type of technology following to recommended, “standard” densities or colorimetric values of the ink solids as far as the growth or reduce of ink layer thickness inevitably results in the blanks filling or tiny dots loss.³ These densities are, however, just secondary, mediated indications on providing the effective range of tone values.

It’s also important to notice that the originals content can be essentially different. In some pictures, for example, gravures the fine detail and contours may prevail and be most significant while in other ones such fine drawing is expressed insignificantly, but the correct tone rendition urgently required for the vast continuous transitions. It’s evident that the somewhat greater ruling, to certain degree ignoring tone rendering, would be preferable for the first of them than predetermined by the above method of choice. Conversely, the screen frequency can be to some extent lowered to increase the number of reproduced levels of a “soft” original.

It may seem, relying on facilities of digital processing, that the screen ruling can be nowadays varied within an edition or even for different images on a single page. But it should be noticed that the tone and color transition characteristics of all printing system also differ with the ruling change. However, there is no yet information on the color profiles variation within an edition and, moreover, for different illustrations on a page.

There is also rightful the approach to ruling value and adequate it printing technology choice at condition of reproduction the predetermined spatial frequencies of an original. If, for example, the goal is put of definite imaging the 2 lines/mm grid of original, there is required the screen of 100 Lpi and the technology level, which guarantees the effective range at such ruling.