Analysis of Related Problems in the Evaluation of Color Image Printing Quality

Differences in the evaluation of color image print quality often occur. In actual work, users, experts, and operators have often encountered different evaluations of the same printed matter. This is because there are two evaluation standards for the reproduction quality of color images, namely artistic standards and technical standards. When evaluating color image prints, people sometimes use natural scenery as the standard, and sometimes use the original as the standard. Therefore, the inconsistency in the evaluation of quality of printed products is first and foremost caused by inconsistent artistic standards. When studying the print quality of color images, we must first separate the artistic standards and technical standards. Only the quality evaluation of prints based on artistic standards cannot truly and comprehensively reflect the quality characteristics of the entire printed matter. It must focus on the technical point of view and use the "faithfulness of manuscript printing" as the evaluation criteria. This article is about the methods, content, manuscripts, and other issues related to the evaluation of color image print quality.

A simple analysis.

First, the evaluation method of color image printing quality
1. Subjective evaluation. It is based on the original manuscript of the reproduction, the evaluator makes an evaluation based on the psychological impression of his knowledge, technical knowledge, aesthetic point of view, and hobbies against the printed sample or printed matter. Such evaluations vary from person to person and it is unlikely that a unified conclusion will be reached. For example, depending on the location and the surrounding environment, in particular, the visual difference caused when the illumination conditions of the copy (compared to the original) is observed. The subjective evaluation cannot fully reflect the quality characteristics of the printed matter, but it is the final arbiter of the quality of the printed matter. At the present stage, the method of appraising the printing quality is mostly based on subjective evaluation. However, subjective evaluation factors should be objectively interpreted to make it scientific and consistent with objective evaluation.

2. objective comment. Based on the determination of physical properties of prints, quantitative analysis of prints is performed through instruments or tools, and objective evaluations are made in conjunction with print quality standards. Some printing companies have now adopted this evaluation method throughout the process design and production process, and randomly and automatically control the printing quality. Such as the electronic pre-proofing in the process of plate making, the printing plate in the process of printing plate inspection device, the water supply during the printing process, the ink remote control device.

3. Overview. It is based on the objective evaluation method, plus the subjective evaluation and verification of various factors, that is, the combination of subjective psychological impressions and objective data analysis, thus making evaluation criteria more scientific management. The main point is to determine the quality characteristics of various physical quantities that constitute the image based on the restoration of the original. Thus, the test data can be integrated and confirmed to make it the basis for controlling print quality.

Second, the evaluation of color image printing quality content <br>Print quality evaluation content, mainly depending on which of the above evaluation methods are used to determine, usually identified as the main evaluation content: tone reproduction; color reproduction; clarity; inhomogeneity Repeat rate - average quality.

1. Tonal reproduction. For the transmission characteristics of the images with the change of brightness and darkness of the image, they are represented by tone-replicating curves.
2. Color reproduction. For the composition of the color, it is measured with a densitometer or X, Y, Z of the CIE colorimetric system.
3. The clarity of the image. For clarity or subtle levels of image contours, use test methods or stars.
4. Printing inhomogeneity. The unevenness of the image caused by ink sticks, ink spots, ink film irregularities, and paper failures that occur during the printing process of an image is expressed by measurements such as a densitometer.
5. Print repetition rate. In order to maintain the stability of printing quality, the quality of the printed matter is required to reach a high repetition rate, and the average quality value is determined through automatic control in production, and expressed by a statistical method.
The above five points are the main points of color image printing quality management. Both subjective and objective evaluations are the main content. However, in the subjective evaluation, these evaluation contents are only the nature of the difference, there is no quantitative data relationship; and objective evaluation, is the use of appropriate physical quantities for quantitative analysis, combined with data and subjective evaluation.

Third, the basic factors of color image printing quality evaluation - network expansion and control <br> lithographic printing mainly through the network of multiple transmission, and finally after comprehensive imaging. The print quality of color images depends on the tone level, color, and sharpness of the reproduction. These are expressed through dots. Dots are the smallest sensory units that form the basis of an image. In color printing, the amount of ink is determined, which acts as an image outline for organizing colors. Therefore, the dot quality, size, and ink layer thickness directly affect print quality.

1. Network expansion phenomenon. There are many changes in outlets, and the expansion of outlets is the main characteristic of the transmission of outlets. The dot enlargement is that the dot area of ​​a certain part of the printed material is enlarged compared with the dot area of ​​the corresponding part on the original printing. In the lithography process, dot enlargement is an unavoidable process phenomenon. There are many reasons for the proliferation of outlets, including printing pressure, blankets, ink thickness, and paper.
2. The calculation of dot gain values. The dot gain value is the difference between the clear dot area printed on the paper and the dot area of ​​the corresponding portion on the original screen. Its calculation formula is:
FD=1-10-DR/1-10-DV×100%
ZD=FD-FD
In the formula:
FR - the dot area of ​​the measured part of the print;
DR - the dot integral density of the measured part of the printed matter;
DV - density in the field on printed matter;
FF--the dot area of ​​the corresponding part on the original;
ZD - outlet expansion value.

3. Dot gain value control. In order to guarantee the quality of the same system print, the same system control conditions must be used to control the change of printing, proofing and printing. Shaiban is the bridge connecting plate making and printing. The gap between proofing and printing due to different dot gains can be adjusted through the printing process. Therefore, we must master the changing rules of the dots on the printing plate and use the control bar to effectively control the dots. Variety. In addition, we must try our best to unify the raw materials used in the prototyping and printing process to prevent the gap between the two networks from widening. When printing, the precision of overprinting, the thickness of the drum lining, and the hardness of the underlaying material should be adjusted and fixed in the best state. Based on the ideal printing pressure, slight adjustments are made to suit the smoothness of the paper, and the ink viscosity, fluidity, and The balance of ink and ink is adjusted to the best printing applicability, and the best effect of ink dot transfer is achieved and the amount of expansion is minimized.

Under normal circumstances, the dot enlargement value should be controlled at a rate of 10% to 15%, and the field density value of the ink should be controlled within ±0.05. 2% of the small dots can be printed, and 95% of the dots are not. Stick to death. When controlling the dot gain value, it is generally based on the dot change of 50%, because the 50% dot density increase after printing is equivalent to the 50% dot gain, ie if the 50% dot density is increased by 0.10 during printing, The dot gain value is 10%.

Fourth, the color image printing quality evaluation of the manuscript requirements <br>The manuscript used for printing is usually divided into three kinds of translucent manuscripts, reflection drafts and the second manuscript, to correctly evaluate the color image printing quality, regardless of the type of manuscript All of the following basic requirements should be met:
1. High definition. It is the basis for realizing the tone, color, texture, three-dimensionality, perspective, etc. of recorded scenes.
2. Density contrast. The density contrast of the manuscript must be normal, and the manuscript contrast is generally between l.8 and 4.0. The original picture has bright, medium, and dark tone levels. The tone level distribution is normal and the texture is realistic (if there are rich shades of tone, intermediate tone There are few levels and you must unclude the tone levels and compress the intermediate tone levels during scanning.
3. Color saturation. Check the original under the standard light source (5000K), the original color saturation should be large. The color saturation of the original, the copy effect is good.
4. Image particles. The granularity of the original affects the color, level, and texture. Fine grained originals facilitate gradation and tone reproduction.



Source: Print World