Print tone reproduction

The reproduction of color images of printed materials requires not only the reproduction of colors, but also the density of shades, which is also called tone level or tone level. Usually expressed in tone density (tone value).

Density is a characteristic measure of the absorption of light by an object, and is a parameter used to measure the amount of light absorption by an object, that is, the ratio of the amount of incident light to the amount of reflected light or the amount of transmitted light. Can be divided into reflection density or transmission density.

(1) Relationship between dots and tone

The definition of reflection density is: the logarithm of the reciprocal of the reflectance. That is D = lg (D is the reflection density, R is the reflectivity)

The printed matter is displayed by the dot coverage (the ratio of the dot coverage area to the total area), that is, the change of the dot percentage shows a gradation. Assuming that the white part of the printed product is 100% reflective and the dot part is 100% absorbed, the reflectivity of a certain area of ​​the printed product can be expressed by the following formula:
R = 1-aa is the dot coverage rate, which can be seen from Table 1-5. When the dot coverage rate increases, obviously the tone density is Dt =

Table 1-5 Relationship between dot coverage and tone density

Outlet coverage

10%

20%

30%

40%

50%

60%

70%

80%

90%

99%

Tone density

0.045

0.097

0.155

0.222

0.301

0.398

0.523

0.699

1.000

2.000

In the image information restoration, the tone is the change rule of the order of the light and dark. Tonal density is a measure of the brightness or tone of a printed product. The tone density increases in a logarithmic relationship. The outlet coverage rate increased from 0% to 50%. The tone density only increased by 0.3, while the dot coverage increased from 90% to 99%, and the tone density increased from 1.0 to 2.0. Obviously, from the bright tone to the middle tone, even if the dot coverage changes greatly, there is no obvious change in tone density. In the dark part, the coverage of outlets fluctuated slightly. There will be a great change in tone density. This non-uniformity shows that there are some small changes in the dots in the high and middle tones. The tone reproduction of the printed matter has little effect, and small changes in the dark tone have a greater impact on tone reproduction. Therefore, when printing, the control ratio of dark tone is high and the difficulty of middle tone is high.

[Related Links]

Often when printing bright portraits and dark tone prints such as portraits of people and bedroom furniture, people usually find the light tone slightly dirty, but the dark tone part is dirty (the dots are pasted to cause waste) but it is difficult to find.

During printing, the dot coverage does not necessarily change continuously. Taking square dots as an example, discontinuous mutations appear in the following three parts.

Bright tone part: 0 ~ 5%
Middle tone: 50% before and after dark tone: 97% ~ 100%
For small dots, due to the presence of the wetting fluid, less than 3% of the dots are often lost.

In the dark part, due to the spread of the ink layer, more than 97% of the dots cannot be reproduced. In the midtone section, when the dots change from unconnected to the surrounding dots connected, the dot coverage changes suddenly in this area.

The reproduction of the tone of the printed matter depends on the change of the dot coverage, and the unevenness of the dot coverage necessarily leads to the discontinuity of the tone of the printed matter. This is inevitable under current printing conditions. However, it is possible to reduce the amplitude of the tone change or change the area of ​​the tone change. Using different dot shapes can achieve the above purpose.

The tone of the printed matter is achieved by the dot. The change of dot coverage is the first condition for the gradation of printed matter. In addition, the shape of the dots can change the discontinuous area and amplitude of the tone. The number of dots and lines can determine the number of light and dark levels of the printed matter. The angle of the network line can reflect the effect of the tone reduction after the color overlapping of the printed matter.

When the original is to be copied, first of all, according to the characteristics of the original, the correct number of dot lines, the angle of the line and the shape of the dot should be selected, and the tone copy curve should be determined according to the printing conditions and the characteristics of the original. When printing, it is mainly to control the change of dot shape, control the solidity of the dots, and ensure the opening of the picture layer; control the smoothness of the dots, maintain the clarity of the picture, control the expansion of the dots, and ensure the high, medium and low tones Reproduce correctly. Obviously, the conditions for the reproduction of the tone of the printed matter must start with the control of the above basic elements.

(2) The relationship between tone length and printing

The tone length of the original version refers to the difference between the maximum dot coverage and the minimum dot coverage. The determination of the length of the original tone is mainly based on the dot coverage of the highlights and dark tones that can be presented by the printed matter and the dot expansion value during printing. For example, the dark tone of the printed matter can show 98% dot coverage, and the highlight area can reproduce 30% dots. The dot expansion value of the dark tone area is estimated to be 15%, and the coverage of the deepest dot on the original version is 98% -15% = 83%. For this reason, the tone length of the original version is 83% -2% = 81%

The length of the original tone directly affects the quality of the printed matter. If the level of the original version is long, the coverage of the deepest part of the original version plus the printed dot expansion value exceeds the dot coverage that can be displayed by the dark tone of the printed product, which will cause the dot coverage of the area to be deeper, making the entire screen darker. . If the tone of the original is too short, the coverage of the deepest dot on the original plus the expanded value of the printed dot cannot reach the dot coverage required by the printed matter. At this time, by increasing the ink layer, a larger dot expansion value or visual density is forcibly achieved, and as a result, the entire picture layer is lost, giving a stiff feeling.

It can be seen that in order to ensure product quality, the dot expansion must be relatively stable during printing. There are certain operating specifications for the adjustment of printing pressure, the use of ink auxiliary agents, and the balance of water and ink. It makes the plate making have higher accuracy and stability when determining the length adjustment of the original plate. [next]

(3) Gray balance and background color removal

The method of achieving neutral gray through three primary colors overprinting is called gray balance. Specifically, under certain printing suitability (under certain conditions of paper, ink, plate, proofing or printing conditions), the three primary colors of yellow, magenta, and cyan are overprinted from light to dark, according to a certain dot ratio. , The whole process of obtaining different achromatic colors (white, light gray, gray, dark gray, black) is called gray balance of printing. Its significance is to indirectly control all the tones on the entire screen by controlling the gray part of the screen. It is a measure of whether color separation and color printing are correct or not. It is a principle of common observance and implementation of data and standardized production in each process of the entire process of replication.

Factors affecting the gray balance in the printing process:

(1) The raw materials used for printing mainly refer to ink, paper, etc .;
(2) Printing conditions, including machine accuracy, printing pressure, wetting fluid, etc .;
(3) Chromatic sequence;
(4) Ink layer thickness;
(5) Environmental temperature and humidity.

The above factors have different effects on the gray balance of printing. However, the gray balance generally exists as follows: in any gray area, the dot coverage of the cyan version is the largest, and the dot coverage of the yellow and magenta is basically close. The influence of various factors will cause the gray balance to differ in value.

The so-called background color removal is the process of reducing the proportion of the three primary colors and correspondingly increasing the proportion of black when the three primary colors are used to restore gray and black in the four-color reproduction. The purpose is to:

(1) Improve printability and ink drying;

(2) Reduce ink cost;

(3) Compensate for the color cast of the dark tone part;

(4) Stable reproduction of gray (neutral gray) printing.

Because the background color is removed by using the achromatic structure process in color printing and copying, the black plate in four-color printing is particularly important. However, in actual work, there are often many print operators who are not fully aware of the importance of gray balance and black plate, resulting in problems with print quality.

[Case 1-1]

A factory used a four-door domestic monochrome machine to print a batch of colored jobs in black, magenta, cyan, and yellow. After printing the finished product, the whole picture was dull and unclear. Customers are dissatisfied with this. What is the reason?

[Case Analysis] Obviously, the operator of the machine does not have enough knowledge about the gray balance. When arranging the color sequence in the four-color printing of the monochrome machine, the gray balance must be considered first. If the black plate is placed in the first color for printing on a monochrome offset press, the amount of ink directly affects the size of the cyan plate. If the amount of black plate ink changes, the gray balance will be destroyed. Secondly, the black plate placed in the first color printing will make it difficult to clean the water and ink rollers, resulting in a color shift after printing. In addition, if the amount of black ink is large and the absorption is too much, the gloss and density will be reduced, and the picture will also look dull.

Kindergarten Artificial Grass

Kindergarten Artificial Grass,Artificial Turf Grass,Range Artificial Grass,Artificial Lawn Grass

Changshu Keyuan Eco-friendly New Materials Co.,Ltd , https://www1.ailiqingky.com