Factors affecting the quality of printed images and evaluation criteria

Monochrome halftone images and multicolor halftone images. Both the text and the line-drawing manuscript reproduced images belong to the line image, which transmits the image information through the shape and layout of the image information face. This image usually has a high enough color intensity but no hierarchy, and it is generally expected that it contrasts with the background to form a clear contrast.

The second method of transmitting information on prints is to use a mesh density level to produce images. Mesh prints should have a good "uniform mesh" characteristic and expected density distribution in order to achieve the desired effect of the image.

The third method uses color changes to convey more information in addition to the density level. This ability is obtained by overprinting several different colors of halftone images. Satisfactory printing colors can be obtained with trichromatic inks, the purpose of using black ink is just to get better quality. The quality parameters of such prints are much more complex than the previous ones, and it also involves complex color balances and other issues.

Subjective evaluation method

1 multidimensional scaling

The multidimensional scale is a scale technique based on mathematical statistics. In the case of pair-wise comparison of differences between samples or determination of sample satisfaction, multidimensional scaling methods can be used to analyze and identify the main parameters used in people's assessment. When evaluating printed samples in this way, the relative importance of the main parameters of the printing quality can be determined;

Make the subjective evaluation and the objective evaluation or the nature of the paper intrinsically related; also get the reliability of each print quality assessment, each appraisal staff (such as printing, papermaking experts, readers, advertising staff, etc.) and the evaluation team Information such as the consistency of the evaluation.

Multidimensional scaling technology was proposed by Tokosson. Its content is: If there is a perceived difference between two elements, then the difference can be expressed in a geometric distance. If this difference is recorded on a scale, the scale on the scale shows this distance, and then the distance can be used to create more than one-dimensional geometric models that reflect the relationships between the samples.

An important feature of multidimensional scaling technology is that it can be used to multidimensionally scale the subjective psychological factors of the judges. The role of each parameter in an evaluation can be represented by a desired vector.

2 pair comparison method

People have subjective characteristics when judging the quality of printed matter. Different people will make completely different conclusions. This kind of objective inconsistency cannot be regarded as bias or randomness and ignored. Even if there is a reference that can be used as a benchmark for comparison, there will be inconsistencies in the evaluation results, and the inconsistencies in the subjective evaluation cannot be regarded as deviations and randomness, either.

In many cases, there is no reference that can be used as a benchmark. At this time, only internal comparisons can be made between the objects being evaluated. There are two commonly used methods: (1) arrange the judged samples in a certain order; (2) put a set of judged samples in each sample Samples are compared with other evaluated samples one by one, scored on the basis of comparison, and judged according to points. This is the pairwise comparison method. It belongs to a subjective evaluation method.

When implementing pairwise comparison methods, note that:

(1) Ensure that when the judges compare two samples at the same time, do not have irrelevant factors to interfere with the comparison.

(2) The printed products to be compared should be in the same standard lighting condition. The background should be neutral. The room used for comparison should ensure that the assessors can focus on their work and can not interrupt the evaluation or interfere with the judges.

(3) The samples to be compared should be provided to the judges in random order so that the comparison process can be conducted without any prejudice.

(4) The judge must be simply and clearly explained on what basis the judgement should be made. There must be no ambiguity.

Such subjective comparison experiments are only internal comparisons among the samples. The comparison results are only relevant to the samples being compared. Whether the results of the pairwise comparison are reliable or not can be determined by examining the "reliability of the judgement" of the judges, which is measured by calculating the reliability coefficient.

Objective assessment method

Evaluation of 1 tone (level) reproduction

The contrast range of transmission originals is very different, and color prints have to be reproduced as a basic-to-intensity picture of the density range. This density range is mostly lower than the density range of color manuscripts. In color separation and plate making, compression adjustment must be done. What kind of redistributive adjustments are taken at each tone level depends, on the one hand, on the distribution of the tonality levels of the manuscript, and at the same time it relates to human visual perception. Among them, both the Munsell brightness value of the visual response and the subjective factors of people's visual psychological requirements. To adjust the copying of original tonal hierarchies, it is necessary to sum up people's visual psychological needs, add the physical values ​​of visual response, and then consider the distribution of the original level distribution to obtain the reproduction curve of the density tone level of the print screen, and then include the copy. With the evolution data in the reproduction process, a tone reproduction curve of a specific manuscript is designed.

The evaluation of the reproduction of printed image tone, if only from the perspective of print quality inspection, is to measure the density of printed solids in each color ink layer; to measure and calculate the overprint rate of each ink layer; to measure the increase in printed dot pitch or Measure and calculate the printing K value; Check the transfer quality of the ink dot; Measure and draw the density and gradation reproduction curve of the printed matter to the original. Through the measurement of these objective technical data, and then compared with the quality specification standards formulated by this department, the quality grade of a specific color printing product can be determined.

2 Evaluation of color reproduction

There are three different concepts for reproduction of colors. The first is the color reproduction in the physical sense, which requires that the spectral distribution of the reproduced color to the original color at each color point is exactly the same. The printed matter is for visual viewing. It is difficult to achieve the same-spectrum color reproduction in the physical sense, and it is not necessary. The second is the reproduction of the sense of color, so that the printed reproduction image is the same as or close to the color point of the original, ie, the effect of different spectrum and the same color. This is the measure standard of objectively evaluating the color reproduction. The third is the psychological sense of color reproduction, that is, the color of the print reproduction may be somewhat different from the original color, but it may meet the visual psychological satisfaction in the color effect. Here, subjective evaluation factors are added.

Due to the lack of color performance characteristics of printed materials (paper, ink, etc.), imperfections in separation and reproduction of color separation equipment, and lack of color in the print reproduction method itself, the actual printing technology cannot faithfully restore the original document. All the colors of the original scene, even if their reproducible parts do not reach the level of faithful restoration, can only be relatively close. This

It brings some difficulties to the objective evaluation of the quality of print reproduction.

It can only be compared from the results of colorimetric measurements on the degree of closeness of the print to the original or original scene. Between similarity and dissimilarity, people's psychological psychological requirements of color vision are reconstructed to make a comprehensive and comprehensive evaluation of print color reproduction.

If the degree of closeness of the printing color to the reproduction of the original color is set to set an objective technical measurement standard, it shall include: measurement inspection of the color reproduction range of the printing ink, measurement and verification of printing gray balance reproduction, and absolute accuracy of the color reproduction of the original color Measurement tests, as well as measurement calculations relative to the degree of reproduction.

3 Evaluation of sharpness reproduction

The clarity of color prints is an important quality indicator for image reproduction. In addition to the special artistic conception of image representation, there should always be a certain level (main body or background) of each picture that is clear.

Reprinted from: Print Quality and Standardization