Summary issues
Tone values are reproduced in autotype method by relative variation of print element and blank space areas.
Being rather conditional the terms AM and FM screening do not largely reflect the substance of gray scale reproduction in printing.
Halftone dots are not noticed and perceived inseparably in spite of their placement frequency doesn’t go beyond the borders of visual response.
At print viewing or measurement the part of blank space illumination diffused within the substrate is captured by the edge of an ink layer causing the apparent enlargement of a halftone dot – optical tone value increase.
Factual and apparent dot areas of a halftone tint can be calculated from the measured reflectance factors or optical densities averaged over the aperture of an instrument.
Tone values are controllable just within their effective range bounded by the sizes of minimal dots and blanks steadily printable within a run over a press sheet.
Their absolute dimensions are defined by the technology own noise level which, in turn, depends on the properties and interaction specific of an ink, plate and substrate.
There is no dot gain for these dots.
Such dots providing is the basic optimization criterion of setting the printing system components interaction while the density of solids comprises just the secondary, derivative parameter.
At given minimal dot and blank sizes the raise of screen ruling increases the sharpness and definition, reduces the halftone structure acuteness but degrades the image contrast and tone rendition due to shortening of an effective range.
With the given size of minimal dot the halftone print contrast is inversely proportional to the square of screen ruling.
Tone value 3 -5% for the whole range of used for many years screen frequencies indicates the priority of ensuring the contrast of an image in relation of its other quality indexes.
Optimal value of the screen ruling lies near the value, which is back to the five dimensions of minimal halftone dot.
Tests
3.1 Abbreviation Lpi (lines per inch) is used in characterizing:
a) strokes density of the imagesetter;
b) scanner resolution;
c) halftone frequency;
d) acuity of vision.
3.2 Formal density range is estimated in printing by the optical densities difference of:
a) ink layers on the OK and production sheets,
b) print areas with the minimal available dots and blanks;
c) the ink solid and clean paper;
d) ink layers on the proof and OK sheet.
3.3 According to Sheberstov-Murray – Devis model the tone value of 100% gives the optical density of:
a) clean paper;
b) print area with minimal blank space;
c) ink solid;
d) print area with minimal dot.
3.4 Optical dot gain occurs due to:
a) excessive pressure and ink filling;
b) partial absorption by the paper of light passed through the ink layer;
c) partial scattering of illumination within the paper of a blank space in direction of the dot;
d) dot edge brightness increase by the light scattered in its direction within the blank.
3.5 Effective density range is estimated in halftone printing by the optical densities difference of:
a) ink layers on the OK and production sheets,
b) print areas with the minimal available dots and blanks;
c) the ink solid and clean paper;
d) ink layers on the proof and OK sheet.
3.6 Noise level of the printing process is in general characterized by:
a) surface properties of a print substrate;
b) plate surface condition;
c) uniformity of solid ink layer;
d) size of the minimal available print element.
3.7 For a given printing technology the halftone frequency is in general set from condition of:
a) invisibility of the print screen structure;
b) priority of the contrast with respect to other print quality parameters;
c) reproducing the specified spatial frequencies of an original.
3.8 Confirmed by practice the quantitative sign of contrast priority is comprised in the:
a) ink solid density value recommended by standard;
b) equality of the screen period to five minimal dot sizes in various printing technologies;
c) dot gain value prescribed by standard;
d) colorimetric value required by customer.
3.9 Image contrast and effective density range can be expanded in a given printing conditions by means of the:
a) use of a scanner with the greater dynamic range;
b) reduce of screen ruling;
c) selection of the tone rendition curve of greater gradient;
d) increase of halftone dots frequency.
1 Due to this reason the term “photographic” quality is sometimes used to accentuate the diluted dot structure of, for example, some ink jet prints.
2 Ink transfer dependence of substrate surface properties is mostly concerned in research and handbooks in relation to the solid ink layer as if to “painting” the paper by some rubber roll without, to the pity, taking into account the specific of halftoning and its effective density range.
3 ISO Technical Committee 130 “Graphic Technology” doesn’t formulate the criteria or methods for halftone printing optimization. Values recommended by ISO 12647 are empirical, taken from the industry practice for some reference substrates and processes.