Reproduction of printed tone (1)

The reproduction of a color image of a printed product not only requires the reproduction of color, but also requires a level of contrast, which is also called the tone level or tone level. It is usually expressed in tone density (tone value).

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

(A) The relationship between outlets and tone

The definition of the 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 represented by the dot coverage (the ratio of the dot coverage area to the total area), that is, the percentage change of the dot. Assuming that the white part of the print is 100% reflective and 100% of the dot part is absorbed, the reflectivity of a certain area of ​​the print can be expressed by the following formula:
The coverage rate of R=1-aa for the dot can be seen from Table 1-5. When the dot coverage increases, it is obvious that the tone density is Dt=

Table 1-5 Relationship between dot coverage and tone density

In the reduction of image information, the tone is the changing law of dark and light sequences. Tonal density is a measure of the lightness or darkness of prints. The tone density increases in logarithmic relationship. Network coverage increased from 0% to 50%. The tone density increased by only 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 midtone portion, even if the dot coverage changes greatly, there is no obvious change in tone density. In the dark tone, the dot coverage slightly fluctuates. There is a large change in the tone density. This inhomogeneity indicates that the dots have undergone some small changes in the high and medium tone. There is little impact on the reproduction of the printed tones, but small changes in the dark tone have a greater impact on the reproduction of the tone. Therefore, when printing, the control of dark tone is more difficult than high and medium adjustment.

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Often when printed and shaded prints such as portrait portraits and bedroom furniture are more noticeable, people usually find the highlights slightly dirty, and the shadows are dirty (nets and pastes cause waste) but it is difficult to find.

When printing, dot coverage does not always have to change continuously. Taking a square dot as an example, there are discontinuous mutations in the following three sections.

Brightness adjustment: 0~5%
Middle tone part: 50% Shadow tone before and after: 97%~100%
For small outlets, due to the presence of wetting fluid, outlets below 3% are often lost.

In the dark part, due to the spread of the ink layer, more than 97% of dots cannot be reproduced. In the midtone part, when the network is suddenly changed from a disjointed state to a four-point network connection state, the dot coverage in this area produces mutations.

The reproduction of printed materials is based on the change of network coverage, and the non-uniformity of network coverage will inevitably lead to discontinuity in the adjustment of printed media. This is unavoidable under the current printing conditions. However, it is possible to reduce the extent of mutations of the tonality or to change the region of the tonal mutation. This can be achieved by using different dot shapes.

The tone of printed matter is realized by dot. The change of outlet coverage rate is the first condition for achieving the adjustment of printed media. In addition, the shape of the dot can change the discontinuous area and amplitude of the tune. The dot line number can determine the quantity of light and dark shades of the printed media. The angle of the cable can reflect the effect of the tone reduction after the prints are stacked.

When the original is to be copied, first, the correct dot line number, screen angle and dot shape should be selected according to the characteristics of the original, and the tone reproduction curve is determined according to the printing conditions in combination with the characteristics of the original. In the printing, it mainly controls the change of dots, controls the robustness of the dots, ensures the opening of the screen, controls the smoothness of the dots, maintains the sharpness of the screen, and controls the expansion of the dots to ensure high, medium and low tone. Correct reproduction. Obviously, the conditions for the reproduction of print mediation must begin by controlling the above basic elements.

(b) The relationship between length and printing and printing

The length of the original tone refers to the difference between the maximum dot coverage and the minimum dot coverage. The determination of the length of the original gradation is mainly based on the high light and dark tone network coverage that can be displayed in the printed matter, as well as the dot gain value at the time of printing. For example, print darkness can exhibit 98% dot coverage, and highlight areas can reproduce 30% dot. In the dark tone region, the dot gain value is estimated to be 15%, and the dot coverage rate at the deepest point on the original is 98%-15%=83%. For this reason, the original tone length is 83%-2%=81%.

The length of the original tone directly affects the quality of the printed matter. If the original scale is adjusted, the coverage rate at the deepest point on the original plus the dot gain value of the printing exceeds the dot coverage rate that can be represented by the dark tone of the print. This will cause the coverage of the dot of the region to be dark, which makes the entire picture darker. . If the original tone is too short, the dot coverage rate at the deepest point on the original plus the printed dot gain value cannot reach the dot coverage required by the print. At this time, by increasing the ink layer to generate a larger dot gain value or visual density forced to achieve, resulting in the loss of the entire picture level, giving a blunt feeling.

Thus, in order to ensure product quality, the dots must be expanded in a relatively stable state at the time of printing, adjustments to printing pressure, use of ink auxiliaries, and ink-water balance have certain operating specifications. Make the platemaking has higher accuracy and stability when determining the length of the original plate.