The principle of gray balance and network expansion (2)

Three, network expansion and network expansion curve

(1) Expansion of outlets

Dot expansion is a phenomenon of dot size change in the process of plate making and printing. It makes the actual dot area larger than the expected dot area.

(2) Types of network expansion

The process according to the dot expansion phenomenon can be divided into two categories. They are the dot expansion in the process of plate making and the dot expansion in the printing process.

1. Expansion of outlets in the plate making stage

The dot expansion in the process of plate making mainly exists in the two processes of film output and printing. The main factor that affects the dot expansion when the film is output is the optical nonlinear effect of the imagesetter. This can be solved by comparing the calibration of the phototypesetting machine, commonly known as For the linearization of the imagesetter; there are more factors that affect the network expansion in the printing process, which can only be compensated by controlling the process parameters. This kind of dot expansion cannot be reflected when setting the color separation parameters. What needs to be considered when setting the color separation parameters is the dot expansion that occurs during the printing process.

2. Expansion of the printing process

Dot expansion is an inevitable phenomenon in the printing process, and its impact on the quality of printing is crucial. Under the action of printing pressure, the dots will expand to the surroundings of the dots. At the same time, due to the compression deformation of the liner, it causes a relative slip between the printing plate and the paper (or the printing plate and the blanket), the rubber market and the paper, and this slip will also cause the dot to expand. Obviously, network expansion due to expansion and slippage is always inevitable.

In addition, the double reflection of paper to light can also cause network expansion. Approximately 80% of the white light is reflected when the white light is irradiated on the white paper, but only 10% of the white light is reflected when the white light is irradiated to the boundary between the ink dot and the white paper. In this way, the human eye adds a circle of ink around the real dot when viewing the dot. Although the outlets have not actually changed, the information is that the outlets have expanded.

(3) Expansion of outlets of different output devices

The development of computer peripheral device technology has brought about the diversification of output devices, and also enables the output of images to have multiple options. Different types of output devices have different network expansion rules, and even the same type of equipment may produce different network expansions. For example, the dot expansion value of black-and-white laser printers is much higher than that of offset presses, the mid-tone dot expansion value is usually around 35%, and the dot expansion of inkjet printers is lower.

Therefore, the control of dot expansion is not only meaningful for printing on a printing press, it is also important for output to a printer. Although general printers are not Post Script printers, they cannot recognize color separation setting parameters, but trying to control their dot expansion values ​​is very important to obtain accurate image output.

The expansion of the dots of non-Post Script laser printers or other printers cannot be achieved by color separation settings, but can be adjusted with certain color adjustment commands, such as Brightness / Contrast commands to adjust the image to be brighter, so that the printed image It doesn't look so dark.

(4) Expanded value of halftone dots of different ink and paper combinations

Each ink and paper combination in the Ink Colors list has a specific set of midtone dot expansion parameters. This set of parameters is Adobe's estimate of the midtone dot expansion value of each color version under different ink and paper combinations. It comes from many printing The company's actual use experience of each type of ink and paper combination. For users without professional knowledge, these parameters not only play a guiding role, in most cases can get good color separation results.

For the convenience of readers, the following table lists the expanded estimate of the halftone dots of all the ink and paper combination color plates built into Photoshop.

Table The default value of mid-tone dot expansion for different ink and paper combination color plates â‘£The dot expansion value of coated paper (coated paper) should be less than that of offset paper.

The following rules can be summarized from the table data:

â‘  All ink and newsprint combinations use the same dot expansion value, and the dot expansion value is the largest;

â‘¡ Among the four color versions, the green version has the largest dot expansion value, and the other three color versions have the same dot expansion value;

③ The difference between the dot expansion value of the cyan version in newspaper printing and the dot expansion value of other color versions is 3%, and the difference between the dot expansion values ​​of the cyan version of other ink and paper combinations and the other three-color versions is 4%

(5) Standard network expansion curve

Photoshop has a set of default dot expansion values ​​for the ink and paper combinations listed in the Ink Colors list, that is, each ink and paper combination has an estimated value for the mid-tone dot expansion of each color version. The software uses this value to establish each color The dot expansion curve of the version, and the dot adjustment of the entire image. Such network expansion curves are called standard network expansion curves, and they are provided by software.

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