Gray balance and dot enlargement principle (2)

Third, network expansion and outlet expansion curve

(1) Expanding outlets

Dot enlargement is a kind of dot size change phenomenon that occurs during the process of plate making and printing. It makes the actual dot area larger than the expected dot area.

(II) Types of outlet expansion

The process of the phenomenon of network expansion can be divided into two major categories, which are the network expansion in the process of plate making and the network expansion in the printing process.

1. Extending network points at the plate making stage

The dot enlargement in the process of plate making mainly exists in the two processes of film output and printing. The main factor affecting the dot enlargement when the film is output is the optical nonlinear effect of the photo-composing machine. This can be solved by the calibration of the collating machine. For the linearization of the image-setter, there are more factors affecting the dot enlargement during the printing process, which can only be compensated by controlling the process parameters. This dot enlargement cannot be reflected when setting the color separation parameters. When setting the color separation parameters, the dots to be expanded during the printing process should be considered.

2. Dot enlargement of printing process

Dot enlargement is an inevitable phenomenon in the printing process, and its impact on the quality of prints is crucial. Under the influence of printing pressure, the ink will expand around the dot. At the same time, due to the compressive deformation of the lining, the relative slippage between the printing plate and the paper (or plate and blanket), the rubber market and the paper is caused, and this slip will also cause the dot to enlarge. Obviously, dot gains due to expansion and slipping are always unavoidable.

In addition, the double reflectance of light on paper can also result in the expansion of dots. About 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 junction between the ink dot and the white paper. In this way, the human eye adds a circle of ink around the real outlet when watching the outlets. Although the outlets did not actually change, the information they felt was the expansion of the outlets.

(III) Expanding the network of different output devices

The development of computer peripheral equipment technology has brought about diversification of output devices, and also allows the output of images to have a variety of options. Different types of output devices have different network expansion rules, and even different devices may produce different network points. For example, black-and-white laser printers have dot gain values ​​much higher than those of offset presses, where the halftone dot gain value is usually around 35%, and inkjet printer dot gains are lower.

Therefore, the control of dot gain is not only significant for printing on the press, it is also important for outputting to the printer. Although printers in general do not belong to Post script printers, they cannot identify separation settings parameters, but trying to control their dot gain values ​​is very important for obtaining accurate image output.

Dot gain controls for non-Post script laser printers or other printers cannot be achieved through separation settings, but can be adjusted with certain color adjustment commands, such as using the Brightness/Contrast command to brighten the image to make the printed image Do not look very dark.

(d) Midtone dot enlargement values ​​for different ink paper combinations

Each ink sheet combination in the Ink Colors list has a specific set of midtone dot enlargement parameters. This set of parameters is an estimate of Adobe's midtone dot enlargement values ​​for each color under different combinations of ink sheets. The company's practical experience with each type of ink paper combination. For users who do not have professional knowledge, these parameters not only play a guiding role, but in most cases can give good separation results.

For the reader's convenience, Table 4-4-1 lists the midtone dot gain estimates for all ink paper combinations in Photoshop.

Table 4-4-1 Different Ink Paper Combinations

No. Ink and paper combination name Color version Midtone dot enlargement estimate/%
Qing Edition Yellow Edition Magenta Edition Black Edition
1 AD-LITUO(Newsprint) 33 30 30 30
2 Dainippon Ink 12 8 8 8
3 Eurostandard(Coated) 13 9 9 9
4 Eurostandard(Newsprint) 33 30 30 30
5 Eurostandard (Uncoated) 19 15 15 15
6 SWOP(Coated) 24 20 20 20
7 SWOP(Newsprint) 33 30 30 30
8 SWOP (Uncoated) 29 25 25 25
9 Toyo Inks (Coated Web Offset) 16 12 12 12
10 Toyo Inks(Coated) 12 8 8 8
11 Toyo Inks(Dull Coated) 12 8 8 8
12 Toyo Inks(Uncoated) 19 15 15 15

From the table data can be summarized as follows:

1 All inks and newsprint combinations use the same dot gain value, with the largest dot gain value;

2 In the four color versions, the cyan version has the largest dot gain value, and the other three color versions have the same dot gain value.

3 The difference between the dot gain value of the Cyan version of the newspaper printing and that of the other halftone dots is 3%, and the dot difference between the Cyan version and the other three external color versions of the other ink paper combinations is 4%;

4 Copperplate paper (coated paper) has a smaller dot gain than offset paper.

(E) Standard dot expansion curve

Photoshop has a default dot gain value built into the ink and paper combinations listed in the Ink Colors list. This means that each ink paper combination has an estimate of the midtone dot enlargement for each color version. The software uses this value to create a variety of colors. Version of the dot expansion curve, and the whole image of the dot expansion adjustment. Such dot gain curves are called standard dot gain curves and they are provided by software.