Some theories in traditional printing quality control
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The traditional printing replication theory believes that the density of the field is the key to determine the tonal range of the copied image. Therefore, the focus of controlling the quality of the printed matter is to control the parameters such as the density of the field, the dot gain and the relative contrast K value. These parameters are printed on the printed sheet. The film thickness is closely related, and controlling the thickness of the ink film is the key.
Field density and density meter
Density is the base 10 logarithm of the reciprocal of the reflection (transmission) rate. The solid density refers to the area coverage of the printed sheet access point is 100%, that is, the density value of the portion of the printed sheet completely covered by the ink layer. Field density is a reflection of the combined effects of printing pressure, ink supply, ink strength and paper absorbency. The maximum solid density of ink is limited by the printing method and paper. Different types of paper have different maximum solid density.
Table 1 below is the field density range recommended by the GRETAG color control system. The system makes more detailed requirements on the field density range depending on the paper being printed.
In the quality evaluation of color prints, the value of the field density value is an important basis for objectively evaluating and controlling the quality of printed matter. It has an important influence on the tone and color.
(1) The density of the ground has a great influence on the tone reproduction. It is an index that directly affects the darkest tone of the image, and has an effect on the entire tone reproduction curve. The density contrast of printing manuscripts is large, but due to the limitations of printing materials and printing technology, the density range of printed matter is much lower than the density range of the original. Therefore, the original tone must be compressed and adjusted. When the density is high, the density of the printed matter With a large contrast range, it is possible to reproduce some of the originals' levels, so that the contrast between the levels can be opened and the three-dimensionality can be enhanced. However, if the density of the field is too large, it will easily cause excessive dot gain, which will make the level of the dark tone level and affect the tone reproduction. Therefore, controlling the density of the field plays an important role in the tone reproduction.
(2) Field density also affects the reproduction of color. The solid density not only affects the reproduction of the inter-colors obtained by superimposing the originals and any two primary colors, but also affects the color gamut range of the reproduction, and also affects the printing gray balance of the three primary colors, which affects the color reproduction effect of the color printing. In practice, the thickness of the ink layer is controlled by controlling the density of the solid to a certain range, thereby controlling the reproduction of the color, and the density of the ground affects the saturation of the color and the reproduction of the overprinted color.
In the traditional printing control and quality evaluation, there are many places where density values are used. The most commonly used measurement tool is the density meter, which is cheaper and therefore more popular. Technically, the densitometer can measure the specific density values of yellow, magenta, and cyan inks by means of red, green, and blue color filters and optical systems. Because these data can guide the generation of managers to normal screening, determine the amount of ink, exposure, ink and water balance, etc., and thus control the quality of color printing. Despite this, the weakness of density detection is also obvious. The density meter can measure the density of the ground and the integral density of the dots. The measurement bar can also measure the dot enlargement and relative contrast, but the density has no direct correspondence with the Munsell value. For example, the magnitude of density is not a linear relationship with lightness, but a curve relationship. In the evaluation of color, the response value of the color filter is far from the observation of the human eye. Therefore, the density meter cannot simulate the observation force of the human eye, and the density value does not reflect the visual effect of the human eye, and does not reflect the chromaticity of the specific color (such as the LAB value of the color and the XYZ tristimulus value).
Network expansion
The dot gain value refers to the difference between the dot coverage of a part of the printed matter and the dot coverage corresponding to the original (negative or positive) film. The effect of dot gain on print quality is very important. Changes in the size of the dot will cause the tone to affect the change of color and sharpness. The increase of the control dot is an important factor to ensure the quality of the printed matter. There are two main types of dot gain during printing: geometric dot gain and optical dot gain. The increase of the geometric dot is the phenomenon that the dot size expands under the action of the printing pressure, and the increase of the optical dot is determined by the double reflection characteristic of the light itself.
The dot gain value is the result of the interaction of the geometric dot and the optical dot. For high-quality prints with high quality requirements, it is mainly printed on coated paper with high smoothness, and the dot gain value should be controlled within a small range to meet the printing quality requirements.
(1) The effect of dot gain on the tone level is obvious. In order to reproduce the dark level on the original, the quotation process is to convert the continuously adjusted image into a mesh image composed of dots by the screening method, and the dot is the image. The basic printing unit, the tone level is reproduced by the dot area ratio, and the excessive dot increase will cause the contrast between the middle and the dark to be reduced, and the level loss will affect the correct reproduction of the image tone reproduction.
(2) For color prints, color reproduction is very important, and among the three attributes of color, hue, brightness, and saturation, hue is the most obvious attribute. In the lithographic offset multicolor printing process, the dot gain has the greatest influence on the reproduction hue change. If a certain color dot is excessively enlarged, the gray balance cannot be achieved, and the color of the picture will be color-shifted. If the dots of the three primary colors are excessively increased, Will make the color darker and turbid.
In traditional printing, the most common method of controlling print quality is to control the Tone Value Increase (Tone Value Increase) of each CMYK color version. Due to many factors such as printing pressure and printing speed, the expansion of outlets is inevitable, but it must be controlled within the allowable range. In principle, the expansion of the Internet access point should be as small as possible. For general printed matter, the 50% dot gain of the printing industry standard recommended by China is 10 to 25%.
TVI is easy to measure, most handheld densitometers can calculate TVI; changes in solid density or paper density do not cause computational difficulties; TVI is important to determine optimal performance; after reaching the specified in-situ ink density value, Check the TVI to ensure that each unit is printing normally, otherwise curve corrections may be introduced to correct physical or chemical anomalies that should have been corrected on the press; during printing, whenever color problems cannot be resolved with simple ink adjustments, measurements are taken. The TVI value of each ink zone indicates whether the plate has a printing error.
However, the TVI curve has its own disadvantages: the TVI curve only focuses on the change of the dot of the monochrome version, and there is no problem when it is separately controlled, but once several colors are printed together, the hue of the ink changes, two or three colors. Changes in gray balance caused by ink overprinting, etc. cannot be controlled by TVI; even the same CMYTVI curve may produce different gray balances; TVI is based on density and is not directly related to chromaticity data; TVI controls the size of dots, but The dot size does not control the visual appearance. It does not specify the visual criteria for critical gray balance, image contrast, and image color saturation; the TVI curve does not control the tone.
Relative contrast
The relative contrast K value can be expressed by the formula: K = (DV - DR) / DV. Where DV is the solid density value of the color ink, DR is 75% or 80% of the density of the dot block, and the dot density of 75% is often used in China, while in Europe, the dot density of 80% is used. The K value varies from 0 to 1. When the K value is large, the difference between the solid density and the dot density is large, that is, the 75% and 80% dot gain is small, the sample tone is pulled open, the level is clear, and the color saturation is high; The value of K is small, indicating that 75% or 80% of the dots are enlarged, the density in the field is small, the tone of the sample is not open, the level is blurred, and the color saturation is low. When the K value is maximum, the solid density at this time is the best, and the gradation shift is also in an optimal state. Therefore, the correct K value is of great significance for improving the proof and quality of the sample.