Color and proof (five)

Dot enlargement is a related feature that cannot be eliminated during the offset printing process.

However, each step of the replication process affects it. Therefore, it is very important to control network expansion. It can ensure that the final printed copy matches the original intention of the advertiser.

As total mesh expansion and field density affect the tone reproduction and gray balance in the replication process, it is also important to control each step in the workflow. The use of a densitometer to calculate the total dot gain is a fully automated process on some densitometers and can also be calculated manually. As long as there is a slight difference, different results will be measured, and even different densitometers will measure different results. Dot gain is a ratio, so it is relatively independent of the absolute density measurement. Not only must ensure that the value of network expansion is absolutely within the allowable deviation range, but also the various balances described in the previous paragraph to achieve the gray balance.

Here, the term "total dot enlargement/enhancement of tone values" is used to describe the apparent enlargement of the dot area from film or electronic documents to the printing process that often occurs in offset printing. This enlargement is formed by a combination of the physical changes in dot size (due to plate-making and ink transfer to the paper surface) and the significant change in paper optical properties (commonly referred to as "optical dot gain").

According to the SWOP parameter specification standard definition, the total dot gain should be the dot gain value at the 50% film or electronic document chroma value point. The term "increase in hue value" is the latest definition of domestic and international standards in terminology. Transparent dot area (Apparent
Dot Area, abbreviated as ADA) has been replaced by the term "tone value", so the total dot gain has also become the term "hue value increase" but is calculated in the same way. These terms no longer use the term "dot" and are not described by visual or measured values, whether or not dots are present. Instead, they use the overall density of hue and field.

An increase in the tone value occurs throughout the entire tone range because it is closely related to the perimeter size of the copy dot. The following schematic diagram shows the meaning of dot gain. The data in the chart was obtained in 1993 and recorded in ANSI/CGATS.6-1995.

The total network expansion curve is from the CGATS.6-1995 image technology - the printing process parameter standard - the first category. It represents the data information contained in the six SWOP accredited print samples produced in 1993, but it is used to demonstrate how the increase in hue values ​​occurs throughout the hue region. Dot gain is caused by density (ink film thickness). Therefore, comparisons can only be made when the field density is within the SWOP bias.

Apparent dot area (tone value) and total dot gain/tonality increase calculation and use

Although it can be assumed that the ADA on proofs and prints is an absolute value, the total dot gain can only be calculated on the premise of knowing the film or file dot area (used to create color patch samples). The SWOP states that these 50% tone blocks should be within 50% ± 2%. These patches can be measured and the appropriate values ​​added to the total dot gain calculation formula. If there is an electronic file, the 50% tone value in the file can be easily used in the calculation, but if you want to check the dot on the sample sheet, but there is no original color mark film or file, accurate real-time dot gain is not possible. measuring.

However, calculations, reports, and analysis of ADA in the 50% film area of ​​the proof color can still show total dot gain information, and if it is too high, problems will still arise because the color patches and dot gain on the film are beyond the specification and deviation range. .

How to calculate the total dot gain/hue value increase

The total network expansion/tonality increase is defined as a combination of physical and optical enlargement, and according to the SWOP parameter specification, measurements should be made on a 133-line screening and 50% of the dot area. The ADA can be measured and calculated by a density meter, according to the Murray-Davies formula, and the physical and optical dot gains are integrated.

The total outlet expansion is equal to the apparent outlet area minus the film or file outlet area.

The ADA in printing can be manually measured by the automatic functions of many densitometers or by the following density data:

The density of the printed background paper.

The density of solid patches of midtones (50%) near the target object.

The density of the printed midtone object (50%).

* Be sure to use a filter that matches the printing primaries to be measured.

The following formula is used to calculate the ADA value (that is, the Murray-Davies formula for measuring the equivalent dot area in CGATS.4):

ADA=(1-10-DT)/(1-10-DS) font-family: Times New Roman; mso-bidi-font-family:"Times New Roman"; mso-font-kerning:1.0pt;
Mso-ansi-language:EN-US;mso-fareast-language:ZH-CN;mso-bidi-language:AR-SA">×100%

In this formula:

DT = Density value of printed hue objects - Density of paper

DS = Density of Solid Patches - Paper Density
ADA = apparent outlet area (100%)

Note: If the densitometer is zeroed on paper, the paper density is not subtracted from the formula. The total dot area and density values ​​can be directly inserted into the formula. This should also be taken into account when directly measuring the total dot area by a densitometer.

There are many other methods for measuring and calculating total network expansion (eg Yule-Nielson,
Fineline Coarseline, etc.) The above method can also be used as a reference. If other methods are used, the operator must determine the relationship between the values ​​measured using this method and the values ​​measured using the reference method.

Traditional practice

If you want to compare the data in the proofing data and the description of this parameter, you must understand the following tradition (standard practice):

Only midtone measurements are necessary - 50% of the 133-line screen.

In the ADA, the total network expansion is an increasing number.

For example, the printed colorimetric value of 50% of the input dot area on the film is measured as 72% ADA (through the above process) - the defined total network point is expanded to 22%.