1 The significance of color control in print
Print is a visual product. In the past, people's evaluation of the quality of prints relied mainly on the feelings of the human eye. With the demand for standardized data, the color quantification of printed materials is an inevitable trend. This article provides the standardized data-quantizing values ​​of the four basic parameters that control the color for printing peer reference. That is: 1 solid density; 2 dot gain value; 3 print contrast (relative contrast K value); 4 superimposition percentage.
2 Contents of Print Control
1) Control of field density values
It is difficult to measure the thickness of the ink layer on the paper, and the thickness of the ink layer is generally characterized by the solid density of the printed matter. The field-reflective density of prints increases with the thickness of the ink layer of the print. For the general process, when the thickness of the ink layer increases to a certain value, the actual reflection density of the ink reaches a maximum value. Even if the thickness of the ink layer continues to increase, the solid reflection density no longer increases. Printing on blackplate paper with black, magenta, yellow and cyan ink, when the ink layer thickness reaches 1.1μm, the thickness of the ink layer is further increased, the field reflection density will no longer increase, and the measured maximum reflection density K is 1.80 to 2.00; C is 1.45 to 1.70; M is 1.25 to 1.50; Y is 0.9 to 1.05. Different papers, because the pigment permeability and ink absorption are different, although the ink layer thickness is the same, but the field reflection density is also different, such as the general coated paper can reach 1.5 or more; offset paper is 1.2 to 1.4; ordinary newsprint is below 1.0 ?
2) Control of dot gain value
Dots, also known as halftone dots. Y, M, C, and K plates are used for printing, and after overprinting, shades of different shades of the screen are used to represent shades of light on the screen. The actual printed dots are clustered into tiny geometric figures distributed on the screen. People rely on the visual difference in optics to observe the simulated tones formed by dot groups and give people a sense of reduction. If used in modern terms, it is a digital image that is transformed into a simulated hue.
Under normal printing pressure conditions, appropriate expansion of printing dots is unavoidable. The dot enlargement value refers to the difference between the dot area of ​​a part of a printed matter (indicated by FD) and the area of ​​the corresponding part of the original version (indicated by FF).
ZD=FD-FF
Density meter can be used to determine the density value of the solid part (color mark) and the density value of the dot part of the print (the paper density must be measured), and the dot area rate is automatically calculated.
In general, the original Internet site area rate is fixed (the value of FF can be measured naturally using a density meter), which determines the reproduction of a print tone and color. The actual dot area rate is affected by various factors in printing. The smaller the dot enlargement rate, the closer the actual dot area ratio on the printed sheet and the original dot area rate are, and the better the printing quality. Generally, the dot enlargement rate of 175 lpi prints should be controlled at about 15%.
3 Overprint Control
1) The significance of the trap rate
In color printing, the ink of the latter color adheres to the previous ink layer, which is called ink overprinting, also called transfer of ink. Colors on color prints are generally presented by juxtaposing or overlapping four-color dots. Therefore, the overprinting of the ink directly affects the tone reproducibility of the print.
Overprint rate refers to the performance of an ink printed on a paper and an ink printed on another ink. The more the latter ink adheres to the previous color ink, the higher the overprint ratio. The level of overprinting directly determines the quality of the printed matter.
2) Determination of superimposition rate
The superposition rate is often measured by the density method. In the following, the second ink is mainly used for superimposing on the first ink layer as the focus of discussion. The overprinting of three or four color inks can be similarly performed.
In the case where the thickness of the ink layer is not so large, the solid density of the ink increases as the thickness of the ink layer increases. Therefore, the ink's overprint rate can be obtained by measuring the solid density. The trap rate determined by the density method is:
fD=×100% (1a)
fD - the trap rate determined by the density method;
D2,1--the density of the second color ink after the second color ink is overprinted on the first color ink layer;
D2 - The density of the second color ink printed on the paper.
In addition, it can be seen from Figure 1 that D2,1=D1+2-D1 (2a)
D1+2-- Overprinted ink layer total density of two-color ink;
D1 - The density of the first color ink printed on the paper.
Substituting (2a) into (1a):
fD=×100% (2b)
In equation (2b), there are 3 density values ​​that need to be measured.
Similarly, if the three-color overprint, the first two colors can be seen as the first color, the last color is seen as the second color, can still be calculated by (2b). This method can be extended to four-color overprint. The higher the trap rate, the better.
4 Relative contrast control
The relative contrast value, also known as the print contrast value, refers to the difference between the density of the field and the dot density of about 70% divided by the density of the field, expressed as K:
K=1-
DR--the middle of the screen to the tone, the dot density of a dot area (usually taken in the dot area 70% or 75%);
DV - Printed field density value.
K is generally between 0.4 and 0.6. The larger the K value, the smaller the ratio of the dot density to the field density, the larger the printing contrast, and the smaller the dot expansion. Conversely, the smaller the K value, the more serious the dot expansion and the smaller the printing contrast. Figure 2 is the relationship between relative contrast and ink thickness.
When the ink layer thickness is δ0, the print contrast K value is the largest. When δ<δ0, there is no expansion of the dot edge, but the amount of ink is insufficient. When δ> δ0, the ink layer is too thick and edge extension occurs. Only at the maximum value of K, there is sufficient ink, and at the same time the minimum dot gain is the best ink supply point.
The level density of color reproduction is basically changed by a logarithmic curve, and the change of dot area rate is linear from a small dot to a 100% dot, and the dot area converted to a density is a logarithmic curve. Or the printing process is not necessarily a pure logarithmic curve. Therefore, controlling the value of K is not only an indicator that controls the middle tone of the image, but also controls the tone of the entire image.
(Author/Guo Linghua Chen Tianxi College of Vocational Technology, Shaanxi University of Science and Technology)