The effect of dot correction on the output

Summary

The main purpose of this experiment was to discuss the effect of the outlet correction with PhotoShop on the output, and to find out the timing and scope of the correction by the experimental method. The following conclusions were obtained in this experiment:

1. Correct the dot with the Dot Gain setting in CMYK Setup. Only the dot correction effect is available when the color gamut is changed. No effect on output.

2. With Transfer Function to correct the dot, no matter what color gamut mode the source file is, it will have an effect.

3. Use Transfer Function to correct the dot, you can control the dot gain within 15%~10%.

4. Between the two of PhotoShop 4 and PhotoShop 5, the processing of data at the time of output is different. Many past concepts have become unusable nowadays.

5. It is recommended that users print better with PhotoShop 4 when printing CMYK file.

6. The CMS support of PhotoShop 5 is more complete and direct. Therefore, there are many concepts in the software setup, which are different from the past versions. Therefore, we hope that the follow-up researchers can explore these settings in more depth for the gamut conversion. What is the impact of image output, etc., in order to provide better advice when printing the separation.

Preface

Research motivation

According to the study of the United States Printing Technology Foundation (GATF), the most important control variable in the change of color control in printing plants is Dot Gain. Dot enlargement is the main cause of color change. Severe network expansion will cause changes in the printing contrast, leading to the loss of details. Since there are many reasons for the expansion of outlets, the expansion of outlets has become the most difficult to understand and most difficult to control. Therefore, there are many scholars who study the expansion of outlets, regardless of the characteristics of the paper, the properties of the ink or the problems of the printer itself, all of which will result in the proliferation of outlets. During the printing process, a certain degree of dot enlargement is bound to exist. The important thing is how to control it. If we can understand the expansion value of the dot, we can effectively correct and control the compensation in the process of color separation.

In the past, most of the control of outlets was controlled by the length of time the print was adjusted during printing. However, in the course of CTP (Computer to press), there was no longer a process of netting, printing, etc., for outlets. Control, only to be completed in the separation, therefore, the separation of software control settings for outlets, it is very important. In recent years, due to technological advances in inkjet printing technology, the use of inkjet printing in proofing has also gradually been adopted by printing houses. As a result of analog printing by ink jet printing, dot gain control on inkjet printers is also important.

Research purposes

PhotoShop is a widely used image processing and color separation software currently used in printing factories. In its software settings, there are many settings that can be used to correct for dot gain. However, general printers are not very familiar with the settings of the PhotoShop software, so they will not change the original default values ​​of the software. However, each printer has its own characteristics, and not all printers are set up. Applicable, so this experiment hopes to reduce the problem of network expansion by adjusting these settings. It is also hoped that the problems caused by dot-compensation at the time of plate making will be reduced by the dot corrections that have been made beforehand in the output.

Research hypothesis

1. In CMYK mode, Dot Gain settings in PhotoShop will not affect the output.

2. Transfer function in PhotoShop can precisely control Dot Gain generation.

Second, research structure

叁, literature discussion

(I) What is the expansion of outlets?

Dot Gain occurs when a halftone image is created, and the dots created during copying are slightly different. It can be defined as the change of the size of the halftone dot from the halftone negative to the printing process. According to the reasons for network expansion, optical dot gain and mechanical dot gain can be divided into optical dot gain and mechanical dot gain. Network expansion is the sum of mechanical network expansion and optical network expansion.

1. Optical network expansion

The main reason for the optical dot gain is that light diffusion into the surface layer of the paper causes the observer to “visually” feel darker, similar to dot gain, when observing the observer or measuring with a reflective densitometer.

When light enters the surface of paper or ink, about 4% of the incident light is reflected on the surface of the ink layer before entering the ink layer. No matter how dark the ink itself is, if the surface is smooth, the reflected light will be in a fixed direction. Reflections do not enter the eyes, so the ink will feel darker.

The surface reflection occurs when light enters the paper from the ink ink layer, or when the paper enters the ink layer. This surface reflection can be ignored because the refractive index of the ink and the paper is almost the same. However, when light is reflected back to the paper through the ink layer and reflected back through the paper, it penetrates the ink and ink layer and enters the air light. Since the paper scatters the light, the light is composed of a high refractive index ink ink layer. Into the low refractive index of the air, so the light is scattered by the scattering of paper, but at various angles to reach the ink and air interface.

After the ink is printed on the paper, it is as though the paper is covered with a thin layer of color filter, and the ink layer that the light shines on the paper passes through the ink layer and enters the inside of the paper, but it does not pass from the light incident point. Come out as if the light is absorbed by the ink. The light that enters the paper surface near the dot, after reflection, comes out of the ink layer, and part of the light is absorbed by the ink, so that a circle of shadow is formed around the dot, which also increases the visual density of the dot. The concentration value measured by the concentration meter is also larger than the actual value. The dot area obtained through the formula is larger than it actually is. We call the situation optical point expansion. For optical dot gains, compensation has been made in the past.

2. Mechanical outlet expansion

Mechanical dot enlargement occurs during the film covering, plate making and printing of the film. The factors affecting mechanical dot gain include ink viscosity, tack, blanket characteristics, slur, double doubling, and adjustment of printing pressure. It is mainly caused by extrusion. It can be divided into directionality and non-directionality according to the presence or absence of directionality. For example, the squeezing and double shadowing are directional mechanical points, while the stuffing is a non-directional mechanical dot enlargement.

3. How to calculate the dot area

The dot area is calculated in the following ways: It can be read with a graduated high-power microscope, read with a scan, and calculated with the concentration of a mathematical formula. The calculation of concentration is currently the most common method. The formula used is mainly Murray-Davies Equation and Yule-Nielsen Equation.

Murray-Davies Equation

The basic assumption of the Murray-Davies Equation is that the paper will completely reflect the light back, so when using this formula to calculate the dot area, it is necessary to measure the white density first to correct the concentration. The formula is as follows: EDA: Effective Dot Area Effective Dot Area

Dt: Flat Screen Concentration

Ds: full plate density

Since this formula does not take into account multiple reflections of light inside the paper, the calculated dot area is not the actual dot area.

Yule-Nielsen Equation

Because Murray-Davies Equation does not consider the problem of light scattering, and the calculated area is not the actual dot area, Yule-Nielsen introduces the physical properties of the printed material in the formula, ie, the Murray-Davies Equation is introduced into an n. The value of the correction. The value of n is mainly related to the number of screen lines and scattering of light by the paper. The formula is as follows:

PDA: Physical Dot Area Actual Dot Area

Dt: Flat Screen Concentration

Ds: full plate density

n: Correction value (range: 1.6 to 1.7)

However, this formula is not very rigorous for the following reasons: (1) The surface reflection of paper plays an important role. (2) Paper does not completely diffuse the dot shape. (3) The ink thickness of the small dot is different from the full plate area.

4. The calculation of outlet expansion

The dot gain value is the difference between the effective dot area (EDA) and the film dot area (both FDA). The formula is as follows:

EDA (Effective Dot Area) is the effective dot area, which is the actual visual dot area after printing.

The film dot area of ​​the FDA (Film Dot Area) refers to the dot area originally on the film.

(B) Control settings for network expansion in PhotoShop

PhotoShop is a common image processing and color separation software currently used in printing plants. There are many color and color settings in the software settings, and the network expansion control settings can be performed from two places: the first is from CMYK. In Setup, the "Dot Gain" feature can be set for the ink property settings. The other is "Transfer functions" in the "Page Setup." In PhotoShop, these two types of settings have different timings and different ways of filling in values. However, no matter what mode is used to correct dot gains, we need to first know what the dot expansion value is.

a. Dot Gain's dot gain correction settings: Select File> Color Settings> CMYK Setup. In the Ink Option settings, there is Dot Gain settings.