1. Image and imaging system parameters correlation

As far as above mentioned parameters characterize the image itself they are sometimes referred as the, so called, first order parameters while to the second order ones are related the properties of a technical system, process or device providing the corresponding first order parameter as a factor of image quality [2.7].

In this concept the brightness and reflection of an original correlate to the amplitude of a signal at photo sensor output in analogue scanner or to the number of its quantization levels in digital reproduction. The optical density and lightness are corresponded to said signal after its non linear transform.

Optical density range and image contrast are defined by the signal dynamic range or bit depth used in encoding. Print definition and sharpness are correspondingly related to resolution and step response of a system while the copy tonal content is defined by the Tone Reproduction Curve (TRC) of a signal amplitude transformer.

Such differentiation assists in adequate use of terminology. Resolution or TRC don’t, for example, sound correct when applied to the image itself instead of some imaging system, its component or process. In this context the direct characterizing the image quality by parameters of test wedges and patterns of printing process control may also look confusing.

4. Picture quality control

   1. Evaluation of an image data, means and reproduction results

There are variety of measuring devices in use for objective evaluation the parameters of originals, intermediate images and final prints. They are available in portative implementation comfortable for the use in production environment. Their indications are calibrated in units adopted for the graphic industry and, as well, are computer compatible allowing for the direct input of, for example, the ink spectral parameters in technologic data bases of print quality control.

Along with the optical densities the digital densitometer can show the multiple of the other useful for control data: print element area, optical and physical dot gain, print contrast, ink trap…as well as the data showing the spectral purity of process inks. High resolution slide scanner can with the microscopic accuracy provide the dot density profiles of a halftone transparency. Color coordinates and differences are measured by colorimeters for various CIE metrics according to target viewing conditions.

Electronic reproduction and desktop publishing applications operate the similar parameters for the optically captured originals after their proper interpretation by the special hardware or program.

The identity of measurement techniques and metrics in different print production locations to those used in creating and processing the original copy in photography, computer graphics and prepress is also important.

Visual comparison and subjective quality evaluation of color copies is provided in normalized viewing conditions in the light booths equipped by several standard sources of illumination. To assure the truthfulness of checking the reflection originals, color proofs, OK sheets and pressrun prints are supplied by the metamer labels – color of light indicators.

Various graphic test elements are widely used for the control of above mentioned second order parameters of reproduction process and equipment. Patterns and textures of different frequency, orientation, geometry and contrast are useful for resolution estimation. Reproduction of multistep tone wedge can illustrate the TRC and system response to the smallest steps of an input gradation. Continuous tone degrade tint, vignette demonstrates the smoothness of tone and color rendition as well as detects the false pattern (moiré) appearance.

Tests with the standard realistic images, patterns and wedges placed over a whole press sheet are used for the general adjustment of a printing process and equipment while in production runs the smaller ones are located in per ferial sheet area.

The number of important parameters is directly controlled by such instruments like ruler, lens, microscope…

2. Parameters conformity and standardization

In informative and aesthetic relation the printed image is mostly addressed to psychovisual perception of a viewer. It’s until now to the certain degree produced with the use of intuitive, empirical approaches and subjective judgments. Inborn or acquired artistic skills were for a long time considered for the sign of professional fitness to printers’ gild. Such issues had become less critical just in the last century due to appear of the objective, quantitative estimation for properties of raw materials, intermediate and final results of printing as well as with the development of a normative base and standardization.

Creative component of color illustrating had, as result, completely shifted from the print house to the author, editorial, design, publisher… stages. In some cases, for example, with the use of “digital” press the printer being earlier the key figure of a process gets the position of an operator of some automated complex. Printing stage completely lost the function of illustration properties control and readdressed it to the prepress image file preparation. That makes the tasks of this stage normalization and standardization especially actual in relation to the halftone image generation specific. However, the criteria of printing process optimization aren’t completely investigated because of the necessity to take into account the vast variety of factors.

Electronic mass media can’t exist without the robust standardization of signal parameters, file formats or of a pickup camera and display properties. Color printing is, to the contrary, more or less successfully provided for a multiple process inks combinations, kinds of substrates, printing methods, machines… This significantly sophisticates the standardization in graphic technology in relation for, as example, non ambiguous reproduction of color.

Radical changes of the late had shifted the accents in scientific descriptions and research tasks actuality. Not so far ago were the times when the number of scientifically approved proposals on print quality improvement couldn’t be realized due to the lack of means to control the ink amount thereon. Modern computerized prepress is able to operate it with the discreteness, which corresponds to 25 – 100 square microns of an ink layer area, thus making, to the contrary, actual the question of direction and degree of said amount changing to achieve the best result.

Along with the normalization and stability providing, this fact makes also actual the methods of finding the optimal process settings. They should to take into account the specifics of originals, plate making and printing stages as well as the mentioned control facilities of prepress. Finding of the optimal regime, as will be shown lower, is often performed in compromise conditions of the contradictory satisfying the quality properties having, in spite of their equal importance, rather different nature and metric.

Standardization became especially demanded with the transition from “closed” to “open” electronic publishing of 1990ies. All the prepress operations, including the halftone transparencies output, were earlier performed in a single process inside the print house or repro centre and therefore were focused on more or less familiar properties of all stages. It was also possible to specify settings and functional transformation parameters from job to job as well as to periodically provide the total, through (input – output) calibration (fingerprinting) for the whole process or its separate operations. There was used, for example, to directly confront in look-up table the color values provided by given press and the input signals of a scanner.

At coming to PC or Mac based, open prepress environment there were acutely aroused the questions of color values non ambiguous interpretation by various software applications, computer platforms and per ferial devices (scanners, monitors, proofers, printers…). However, all the efforts International Color Consortium (ICC) on creating the norms, measuring and software means of Color Management Systems (CMS) through Color Connecting Spaces (CCS) wouldn’t be effective without the strictly talked over characteristics of material part of technology embracing the plate making and printing stages as well as the materials and equipment used therein.

This circumstance had widely initiated the development of a new industry standards and renewing of the existing ones for objective, quantitative defining the technologic regimes and parameters.

ISO 12647 standard “Graphic technology - Process control for manufacture of half-tone colour separations, proofs and productions prints” was adopted in1996 in Part 1 concerned of “Parameters and measurement methods”. Its other parts recommending the particular values conformably to various kinds of printing were later on re-edited by ISO Technical Committee “Graphic Technology” (TC 130) and, being translated on some other languages, had adapted for the state standards in several countries [2.8, 2.9].

Such standards comprise the general means of professional communication within a print-publishing environment and provide the proper conformity of particular technology stages. Nevertheless, the complete following to standard values guarantees the achievement of some averaged results but not the extremely possible ones for given production conditions. It’s partly explained by these values being empirically got from the industry experience and for some exemplary cases. So, by the great part they haven’t the definite theoretical background in philosophy of finding the optimal meanings. There are no, for example, methodology for choice of such important parameters as the screen ruling, dot form, press settings, etc. So, this kind of issues is especially accentuated in disclosure of the following chapters.

Key summary issues

Image processing can be concerned in such principally different directions as pattern recognition, computer graphics and picture transfer.

The distinctive signs of these directions are: presence or absence of an image at input or output; volume of a source and resulting data and others.

Basic direction in prepress processing is the reproduction of an original presented electronically or on a material substrate.

The goal of reproduction is in producing some intermediate image in digital form, on a film or a plate with its properties providing the best print quality in a press run.

Color of a visual object is essentially and differently changed by the various photographic and electronic means in its initial presentation including such as the graphic original.

Instead of relation to a visually appeared object of surround world the reproduction task would be technically more convenient discerned being referred to its replica – graphic original in electronic or material form.

Complete reproduction system is inherent in three basic functions (stages):

• capturing (analysis) with producing the image signal;

• signal functional transforms;

• making the copy (synthesis).

In relation to a source scene or object the printing comprises just a part of some hybrid system integrating the several complete ones.

Image signal functional transforms can be conditionally subdivided with taking into account their effecting on such copy properties as:

• gradation and color;

• definition, sharpness, geometric accuracy of fine details and contours;

• geometry and size;

• image structure.

Quantitative image property parameters are preferred for the objective judgment of its quality.

Individual element is characterized by brightness, lightness, hue, reflection, optical density…while the whole image by contrast, density range, definition, sharpness, tonal content…

Each of an image parameters is respondent by the certain characteristic of reproduction system – second order parameter.

Print quality control is effective in prepress at the condition of further technology stages optimization (by these stages own criteria), normalization and stability.

Tests

2.1 Bringing the data of an original to the volume accommodated by print is provided at:

a) reproduction stage;

b) proof printing;

c) plate making;

d) press make ready;

e) press run.

2.2 Data volume of a halftone print is in the greater degree limited due to:

a) original image;

b) scanning system;

c) signal storage;

d) screening;

e) plate and press resolutions.

2.3 Low spatial frequency transforms have minimal effect on the image:

a) tonal content;

b) color;

c) sharpness and definition;

d) tonal continuity.

2.4 High spatial frequency transforms have minimal effect on the:

a) fine detail and counter geometry;

b) tone rendition;

c) sharpness;

d) definition.

2.5 The group of format transforms doesn’t include the image:

a) magnification;

b) rotation;

c) cropping;

d) color separating;

e) trapping.

2.6 Perception of the light intensity from a viewed print is more directly related to:

a) optical density;

b) brightness;

c) lightness;

d) reflection;

e) contrast.

2.7 Adjacent patches of the visually uniform tonal step wedge differ from each other by an equal discrete of:

a) absorbance;

b) reflection;

c) brightness;

d) lightness;

e) optical density.

2.8 Optical density comprises the logarithm of:

a) absorbance;

b) opacity;

c) reflection;

d) transmittance.

2.9 Optical density comprises 2 units if the reflected (transmitted) part of illumination is equal:

а) 1/2;

b) 1/10;

c) 1/20;

d) 1/50;

e) 1/100.

2.10 Image contrast is estimated as the ratio of:

a) incident and reflected luminous floods;

b) maximal and minimal reflections;

c) maximal and minimal optical densities:

d) color difference - delta E.

2.11Visually percept image contrast is in greater degree defined by the image:

a) brightness range;

b) sharpness;

c) tonal content;

d) definition;

e) average brightness.

2.12 Halftone print definition is estimated by:

a) image data volume;

b) number of picture elements;

c) number of lines discerned at it unit length;

d) printer resolution.

2.13 Image sharpness is directly defined by;

a) minimal width of a reproduced line;

b) inverse value of an edge fringe width;

c) light scattering function;

d) modulation transfer function.

2.14 Transient response characteristic of a reproduction system influences the following image property:

a) definition;

b) sharpness;

c) tonal content;

d) contrast.

2.15 Bit depth of encoding influences on:

a) definition;

b) sharpness;

c) density range;

c) tone rendition smoothness.

2.16 The following attribute isn’t considered as the image parameter:

a) definition;

b) sharpness;

c) resolution;

d) contrast.

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