Printing Quality Control--Objective Evaluation (I)

Objective assessment of print quality

First, evaluation of tone (level) reproduction

Subjective evaluation of color printing products makes it difficult to have a uniform standard. To this end, it is necessary to use quality indicators for physical quantity measurement to establish objective evaluation criteria for the main quality content of printed materials.

The contrast range of transmission originals varies greatly, and color prints have to be reproduced as a screen with a substantially uniform density range. This density range is mostly lower than the density range of color originals. In the separation and printing process, compression adjustments must be made. 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 and copy the original tonal hierarchies, it is necessary to sum up the people's visual psychological needs, add the physical values ​​of the visual response, and then consider the distribution of the original levels to obtain the density reproduction curve of the print screen. The evolutionary data in the process of copying and reproduction is designed to design a tone reproduction curve of a specific manuscript.

1. The tone density of prints contrasts. The density of printed matter is so large that it can not only reproduce the level of some manuscripts or original scenes, but also make it possible to open up some contrasts between levels and levels, and the sense of three-dimensionality is also strong. In particular, the subtle contrasts can be enhanced and the clarity of the image hierarchy can also be enhanced. On the other hand, it also makes the color of the picture open, which makes the deeper saturated, the shallower brighter, and enhances the three-dimensional color.

However, the increase in the density of the printing ink layer is inconsistent with the dot gain, and it is impossible to achieve such a high density as a color transparency sheet. Only under the excellent conditions such as the adaptability of paper, ink, and equipment, and the precision of the printing press, can have lower dot gain value and higher ink layer density. Therefore, the objective technical standard for determining this quality index is to use a color reflection densitometer to measure the printed solid density of each printing ink layer and the printing density of 75% dot, and calculate the printing K value. When there is a higher printing K value (yellow 0.3 to 0.4, magenta and cyan 0.4 to 0.55, and black 0.5 to 0.6), their respective field densities High, and when the overprinting efficiency of each ink layer approaches 90%, the tone of the printed matter is also better. Such printed matter must be clearly structured, full and full of three-dimensional images.

In order to save printing ink, some people advocate reducing the density of the printing ink layer to reduce the dot gain. In this way, although the number of printed dot adjustments is reduced, the leveling of the dark tone end of the image is reduced, but the overall tone contrast is not the highest. , also can not reach the best reproduction of print tone.

2. Hierarchical copy reproduction curve. To objectively evaluate the reproduction quality of the gradation level reproduction of a document from a print screen, it is necessary to measure and draw a gradation reproduction reproduction curve of the print screen. There is a simple and practical measurement method: using the same series of transmissive and reflective densitometers, select the gray, gray, and gray levels of the same top, bottom, and middle tones on the original and printed image to measure their respective white light densities. In the coordinates shown in Figure 6-14, the density of each level point on the manuscript is taken as the basis, and the density of each level point corresponding to the reproduction on the printed matter is taken, and the points are connected to obtain the original tone level. The density curve is reproduced for the print level of the benchmark. Then, according to the manuscript content form, tone distribution status and reproduction requirements, as well as the density in the reproduction process, a comprehensive evaluation of the reproduction quality of the specific manuscript level reproduction is performed.

Because the contrast of the print is mostly lower than the original contrast, if the original order of the original copy is compressed at the time of copying, the density tone level distribution of the print screen is like the straight line A in FIG. 6-15. However, since there is a slight difference between the visual Munsell equivalent brightness value and the optical density class, the reproduction of the reproduction according to the straight line A, the light and dark changes of the dark tone of the reproduction are more obvious, and the compression is less on the visual sense. However, the change in the brightness of the bright tone is less, the perception is more compressed, and the main body of the middle tone has not been able to highlight it. To make up for this deficiency, we need to change to the reproduction of the curve B. The printed image can be satisfied with the visual experience, and the ideal and relatively faithful reproduction of the manuscript hierarchy relationship can be achieved. Emphasize.

The contents of each original image are different, and if they are all faithfully reproduced according to curve B, they may not always achieve the best reproduction effect. For example, the skin color of the character's picture is mostly in the middle and bright tone, and the curve C will give better copying effect; while the landscape picture, most of it is in the midst of the tone of the rich scenery, expressing the space by highlighting the contrast between high-profile and dark tone. Three-dimensional, more use of curve D, can get the best copy of the screen. All of these have also added the impression of people's perception of objective scenery under the condition that the original tone levels are relatively standard, and different subjective reproduction requirements have been added to different screens. However, they still basically belong to the standards that are faithful to manuscripts. Copy reproduction.

However, the majority of original color films are not standard, and their tone levels are distributed, often with varying degrees of defects. The recipients of color prints do not look at the manuscripts, but only require the print screens to be close to satisfying the general visual and psychological art needs. This can not faithfully reproduce and reproduce the insufficiency of the original image on the print screen. To do this, we must first measure the hierarchical density of the manuscript, draw a rough distribution curve of the manuscript, and judge the defects of the distribution of the tonality of the original manuscript according to people's common subjective visual psychological feelings. At the same time, depending on the content of the manuscript (such as scenery, people, and still life), or according to the customer's adjustment and replication requirements, different tone levels can be adjusted to reproduce and reproduce the curve, so that the level of print distribution, in line with people's natural The general impression of the scenery and the visual and psychological art requirements. This is the main adjustment task for the electronic color separation layer copying.

For example, for an overexposed color film, where the middle tone and light tone level are thin, the reproduction curve of the high-middle-level reproduction should be enhanced as shown by the curve B in Figure 6-16; the under-exposed color film has obvious high-profile levels and the main tone density is high. Most of them fall in the dark tone, dark tone level and level, should be used in the curve C; and the color film shot back light, high and dark tone levels flat, the middle tone level is small and contrast, you need to use the picture Copy reproduction curve like curve D. Different kinds of different manuscripts each require different replication reproduction curves.

For the comprehensive evaluation of the gradation reproduction of the print screen, a gradation reproduction curve such as that shown in FIG. 6-14 is also made, and then compared with the originally designed original gradation reproduction and reproduction curve. If the curves of the two are close to each other and the tone level of the print screen is satisfactory, this is the best reproduction of the print. If there is a difference between the two curves, the printed image will not generally be satisfactory. This can be started from the level of the manuscript copy design, color separation screen adjustment, along the opposite direction of the tone cycle diagram derivation, check the deviation of the data generated by the tone of each step of the copy process, you can find the objective process factors affect the reproduction of the level reproduction In order to strictly control or modify the data. In this way, the standards for product quality evaluation, quality control, specification data, and copy design are organically combined, and the scale for verifying the quality of color printing products comes out. Most of the evaluation criteria are measured by objective technical data. In the process design of reproduction and reproduction, necessary subjective factors still need to be added to make an accurate and comprehensive decision.

In short, the evaluation of printed image tone reproduction, if only from the perspective of print quality inspection, is to measure the density of printed solids in each color ink layer; measure and calculate the overprint rate of each color ink layer; Large or measured calculations print K values; check the transfer quality of the ink dot; measure and trace the density reproduction curve of the print 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.